6.5: Observational Learning (Modeling)
Learning Objectives
- Define observational learning
- Discuss the steps in the modeling process
- Explain the prosocial and antisocial effects of observational learning
Previous sections of this chapter focused on classical and operant conditioning, which are forms of associative learning. In observational learning , we learn by watching others and then imitating, or modeling, what they do or say. The individuals performing the imitated behavior are called models . Research suggests that this imitative learning involves a specific type of neuron, called a mirror neuron (Hickock, 2010; Rizzolatti, Fadiga, Fogassi, & Gallese, 2002; Rizzolatti, Fogassi, & Gallese, 2006).
Humans and other animals are capable of observational learning. As you will see, the phrase “monkey see, monkey do” really is accurate. The same could be said about other animals. For example, in a study of social learning in chimpanzees, researchers gave juice boxes with straws to two groups of captive chimpanzees. The first group dipped the straw into the juice box, and then sucked on the small amount of juice at the end of the straw. The second group sucked through the straw directly, getting much more juice. When the first group, the “dippers,” observed the second group, “the suckers,” what do you think happened? All of the “dippers” in the first group switched to sucking through the straws directly. By simply observing the other chimps and modeling their behavior, they learned that this was a more efficient method of getting juice (Yamamoto, Humle, and Tanaka, 2013).
Imitation is much more obvious in humans, but is imitation really the sincerest form of flattery? Consider Claire’s experience with observational learning. Claire’s nine-year-old son, Jay, was getting into trouble at school and was defiant at home. Claire feared that Jay would end up like her brothers, two of whom were in prison. One day, after yet another bad day at school and another negative note from the teacher, Claire, at her wit’s end, beat her son with a belt to get him to behave. Later that night, as she put her children to bed, Claire witnessed her four-year-old daughter, Anna, take a belt to her teddy bear and whip it. Claire was horrified, realizing that Anna was imitating her mother. It was then that Claire knew she wanted to discipline her children in a different manner.
Like Tolman, whose experiments with rats suggested a cognitive component to learning, psychologist Albert Bandura’s ideas about learning were different from those of strict behaviorists. Bandura and other researchers proposed a brand of behaviorism called social learning theory, which took cognitive processes into account. According to Bandura , pure behaviorism could not explain why learning can take place in the absence of external reinforcement. He felt that internal mental states must also have a role in learning and that observational learning involves much more than imitation. In imitation, a person simply copies what the model does. Observational learning is much more complex. According to Lefrançois (2012) there are several ways that observational learning can occur:
- You learn a new response. After watching your coworker get chewed out by your boss for coming in late, you start leaving home 10 minutes earlier so that you won’t be late.
- You choose whether or not to imitate the model depending on what you saw happen to the model. Remember Julian and his father? When learning to surf, Julian might watch how his father pops up successfully on his surfboard and then attempt to do the same thing. On the other hand, Julian might learn not to touch a hot stove after watching his father get burned on a stove.
- You learn a general rule that you can apply to other situations.
Bandura identified three kinds of models: live, verbal, and symbolic. A live model demonstrates a behavior in person, as when Ben stood up on his surfboard so that Julian could see how he did it. A verbal instructional model does not perform the behavior, but instead explains or describes the behavior, as when a soccer coach tells his young players to kick the ball with the side of the foot, not with the toe. A symbolic model can be fictional characters or real people who demonstrate behaviors in books, movies, television shows, video games, or Internet sources.
Steps in the Modeling Process
Of course, we don’t learn a behavior simply by observing a model. Bandura described specific steps in the process of modeling that must be followed if learning is to be successful: attention, retention, reproduction, and motivation. First, you must be focused on what the model is doing—you have to pay attention. Next, you must be able to retain, or remember, what you observed; this is retention. Then, you must be able to perform the behavior that you observed and committed to memory; this is reproduction. Finally, you must have motivation. You need to want to copy the behavior, and whether or not you are motivated depends on what happened to the model. If you saw that the model was reinforced for her behavior, you will be more motivated to copy her. This is known as vicarious reinforcement . On the other hand, if you observed the model being punished, you would be less motivated to copy her. This is called vicarious punishment . For example, imagine that four-year-old Allison watched her older sister Kaitlyn playing in their mother’s makeup, and then saw Kaitlyn get a time out when their mother came in. After their mother left the room, Allison was tempted to play in the make-up, but she did not want to get a time-out from her mother. What do you think she did? Once you actually demonstrate the new behavior, the reinforcement you receive plays a part in whether or not you will repeat the behavior.
Bandura researched modeling behavior, particularly children’s modeling of adults’ aggressive and violent behaviors (Bandura, Ross, & Ross, 1961). He conducted an experiment with a five-foot inflatable doll that he called a Bobo doll. In the experiment, children’s aggressive behavior was influenced by whether the teacher was punished for her behavior. In one scenario, a teacher acted aggressively with the doll, hitting, throwing, and even punching the doll, while a child watched. There were two types of responses by the children to the teacher’s behavior. When the teacher was punished for her bad behavior, the children decreased their tendency to act as she had. When the teacher was praised or ignored (and not punished for her behavior), the children imitated what she did, and even what she said. They punched, kicked, and yelled at the doll.
What are the implications of this study? Bandura concluded that we watch and learn, and that this learning can have both prosocial and antisocial effects. Prosocial (positive) models can be used to encourage socially acceptable behavior. Parents in particular should take note of this finding. If you want your children to read, then read to them. Let them see you reading. Keep books in your home. Talk about your favorite books. If you want your children to be healthy, then let them see you eat right and exercise, and spend time engaging in physical fitness activities together. The same holds true for qualities like kindness, courtesy, and honesty. The main idea is that children observe and learn from their parents, even their parents’ morals, so be consistent and toss out the old adage “Do as I say, not as I do,” because children tend to copy what you do instead of what you say. Besides parents, many public figures, such as Martin Luther King, Jr. and Mahatma Gandhi, are viewed as prosocial models who are able to inspire global social change. Can you think of someone who has been a prosocial model in your life?
The antisocial effects of observational learning are also worth mentioning. As you saw from the example of Claire at the beginning of this section, her daughter viewed Claire’s aggressive behavior and copied it. Research suggests that this may help to explain why abused children often grow up to be abusers themselves (Murrell, Christoff, & Henning, 2007). In fact, about \(30\%\) of abused children become abusive parents (U.S. Department of Health & Human Services, 2013). We tend to do what we know. Abused children, who grow up witnessing their parents deal with anger and frustration through violent and aggressive acts, often learn to behave in that manner themselves. Sadly, it’s a vicious cycle that’s difficult to break.
Some studies suggest that violent television shows, movies, and video games may also have antisocial effects (see figure below), although further research needs to be done to understand the correlational and causational aspects of media violence and behavior. Some studies have found a link between viewing violence and aggression seen in children (Anderson & Gentile, 2008; Kirsch, 2010; Miller, Grabell, Thomas, Bermann, & Graham-Bermann, 2012). These findings may not be surprising, given that a child graduating from high school has been exposed to around 200,000 violent acts including murder, robbery, torture, bombings, beatings, and rape through various forms of media (Huston et al., 1992). Not only might viewing media violence affect aggressive behavior by teaching people to act that way in real life situations, but it has also been suggested that repeated exposure to violent acts also desensitizes people to it. Psychologists are working to understand this dynamic.
Summary
According to Bandura, learning can occur by watching others and then modeling what they do or say. This is known as observational learning. There are specific steps in the process of modeling that must be followed if learning is to be successful. These steps include attention, retention, reproduction, and motivation. Through modeling, Bandura has shown that children learn many things both good and bad simply by watching their parents, siblings, and others.
Glossary
- model
- person who performs a behavior that serves as an example (in observational learning)
- observational learning
- type of learning that occurs by watching others
- vicarious punishment
- process where the observer sees the model punished, making the observer less likely to imitate the model’s behavior
- vicarious reinforcement
- process where the observer sees the model rewarded, making the observer more likely to imitate the model’s behavior
Contributors and Attributions
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Rose M. Spielman with many significant contributors. The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the creative commons license and may not be reproduced without the prior and express written consent of Rice University. For questions regarding this license, please contact<EMAIL_ADDRESS>.Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at<EMAIL_ADDRESS>.


8.15: Exercise- Visualizing the Eye Movements
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Now that you know about the ERP CORE N2pc paradigm, let’s take a look at the eye movements. As you can imagine, it’s difficult to maintain gaze on the central fixation point and not look toward the target. But as described above, it’s important to make sure that the N2pc results aren’t impacted by eye movements, which will change the location of the target relative to the center of gaze and also create a negative voltage over the contralateral hemisphere.
In this exercise, we’ll focus on the data from Subject 15. If you look in the Chapter_8 folder, you’ll see two datasets from this participant, one containing the continuous data ( 15_N2pc_ICA_preprocessed.set ) and one containing the epoched data ( 15_N2pc_ICA_preprocessed_epoched.set ). To make this exercise simpler, ICA-based artifact correction was already applied to these datasets to eliminate blinks. However, the datasets include a VEOG-uncorrected channel that contains the original uncorrected VEOG-bipolar signal. We like to keep this uncorrected channel so that we can reject any trials on which the participant blinked during the time period of the stimulus and therefore could not see the stimulus. This particular participant never blinked during that time period, however, so we don’t need to worry about rejecting those epochs in this exercise. I also applied several other preprocessing operations, including filtering (bandpass 0.1–30 Hz, 12 dB/octave). The EEG channels and the HEOG-left and HEOG-right channels have all been referenced to the average of P9 and P10, and there is also an HEOG-bipolar channel (HEOG-left minus HEOG-right).
Before we get started on the exercise, quit and restart EEGLAB, load the continuous dataset ( 15_N2pc_ICA_preprocessed .set ), and scroll through the data. You should always scroll through the continuous data as the first step so that you know what’s in the file (as we did in the video demonstration in the previous chapter). Subject 15 has some weird stuff in the F7 channel near the end of the session, but we won’t worry about that for this exercise.
Now load the epoched dataset ( 15_N2pc_ICA_preprocessed_epoched.set ) and scroll through the data. On my widescreen desktop monitor, I like to show 15 epochs per screen. And to focus on the eye movements, I like to display only 6 channels, with a vertical scale of 100 or 150. This participant did a good job of following the fixation instructions initially, but you’ll start to see a fair number of eye movements beginning at Epoch 70.
If you look at the event codes and the polarity of the HEOG deflections, you’ll see that most of the clear eye movements are toward the side containing the target. Remember that the HEOG-bipolar channel was calculated as HEOG-left minus HEOG-right, and the dipole is positive at the front of the eyes, so a leftward eye movement produces a positive deflection and a rightward eye movement produces a negative deflection. For example, Epoch 70 has a negative deflection, indicating a rightward eye movement. You can see that the time-locking event code for this epoch is labeled “B2(121)”, indicating that the event code was 121 (attend-blue, target on the right, gap on the top) and was assigned to Bin 2 (right-side targets). This is important because it indicates that we had a rightward eye movement on a trial with a right-side target.
When participants are trying to maintain central fixation, the eyes will typically move away from the fixation point rapidly, stay in a new location for a few hundred milliseconds, and then “snap back” to the fixation point. This leads to a “boxcar” shape in the HEOG signal (a flat signal at one voltage level corresponding to the location of the fixation point, a sudden change to a different voltage level for 100–500 milliseconds, and then a return back to the original voltage level corresponding to the fixation point). You can see this pattern in Epoch 70. The voltage level changes suddenly at approximately 285 ms after the time-locking event and then back to the original level approximately 180 ms later. (You can see the latencies by hovering the mouse pointer over the relevant part of the waveform and looking at the Time value near the bottom of the plotting window.)
If you look at epoch 73, you’ll see a voltage deviation in the HEOG-bipolar channel, but it isn’t a saccadic eye movement. It doesn’t have the classic boxcar shape. Instead, it’s a little bit of blink voltage that has leaked through to the HEOG electrodes.
There’s a small leftward (positive) eye movement at approximately 570 ms after the time-locking event in Epoch 78, which had a left-side target. Epoch 81, with a right-side target, has a somewhat complicated pattern that looks like a brief leftward movement followed by a clearer rightward movement. Epoch 82, also with a right-side target, has a clear rightward eye movement.
There’s also a brief spike in voltage at the onset of the eye movement in Epoch 82. That’s probably an EMG burst coming from the muscles that produce the eye movement. Ordinarily, the muscle contraction that produces an eye movement occurs briefly at the start of the eye movement (to overcome the inertia in eye position) but then becomes too small to see as the eyes maintain their new location (which requires very little muscle activity). The present data have been low-pass filtered, so this spike potential isn’t very clear. Without the filtering, the spike potential is quite large in some participants. It’s fairly localized to frontal electrode sites, and it’s easy to filter out, so I don’t usually worry about it as an artifact. However, if you perform time-frequency analyses, it’s easy to mistake this EMG burst for gamma-band EEG activity (Yuval-Greenberg et al., 2008), so be cautious when someone says they’re seeing gamma-band EEG oscillations at frontal electrode sites.
If you keep scrolling through the data, you’ll see quite a few eye movements (mainly in the direction of the target, and mainly starting around 200 ms after stimulus onset). We clearly have a lot of work to do to make sure that these eye movements don’t confound our N2pc data!


8.1.1C: Power
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Power refers to someone’s ability to get others to do his or her will, regardless of whether or not they want to.
Learning Objectives
- Compare the positives and negatives associated with the use of power and how power operates in society
Key Points
- Legitimate power, power given to individuals willingly by others, is called ” authority “.
- Illegitimate power, power taken by force or the threat of force, is called ” coercion “.
- Coercion is power taken by force or the threat of force.
- Influence, in contrast to coercion, refers to the means by which power is used.
- Because power operates both relationally and reciprocally, sociologists speak of the balance of power between parties to a relationship. All parties to all relationships have some power.
Key Terms
- coercion : Actual or threatened force for the purpose of compelling action by another person; the act of coercing.
- influence : An action exerted by a person or thing with such power on another to cause change.
Power refers to someone’s ability to get others to do his or her will, regardless of whether or not they want to. It is also a measurement of an entity’s ability to control its environment, including the behavior of other entities. Legitimate power, power given to individuals willingly by others, is called “authority;” illegitimate power, power taken by force or the threat of force, is called “coercion. ” In the corporate environment, power is often expressed as upward or downward. With downward power, a company’s superior influences subordinates. When a company exerts upward power, it is the subordinates who influence the decisions of the leader. Often, the study of power in a society is referred to as “politics. ”
Power can be seen as evil or unjust, but the exercise of power is accepted as endemic to humans as social beings. The use of power need not involve coercion (force or the threat of force). At one extreme, it more closely resembles what everyday English-speakers call “influence,” or the means by which power is used. Although power can be seen as various forms of constraint on human action, it can also be understood as that which makes action possible, although in a limited scope.
Because power operates both relationally and reciprocally, sociologists speak of the balance of power between parties to a relationship. All parties to all relationships have some power. The sociological examination of power concerns itself with discovering and describing the relative strengths–equal or unequal, stable, or subject to periodic change. Sociologists usually analyze relationships in which the parties have relatively equal or nearly equal power in terms of constraint rather than of power.


14: The Big Bang
In previous chapters, we explored the contents of the universe—planets, stars, and galaxies—and learned about how these objects change with time. But what about the universe as a whole? How old is it? What did it look like in the beginning? How has it changed since then? What will be its fate?
Cosmology is the study of the universe as a whole and is the subject of this chapter. The story of observational cosmology really begins in 1929 when Edwin Hubble published observations of redshifts and distances for a small sample of galaxies and showed the then-revolutionary result that we live in an expanding universe—one which in the past was denser, hotter, and smoother. From this early discovery, astronomers developed many predictions about the origin and evolution of the universe and then tested those predictions with observations. In this chapter, we will describe what we already know about the history of our dynamic universe and highlight some of the mysteries that remain.
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- 14.1: The Age of the Universe
- Cosmology is the study of the organization and evolution of the universe. The universe is expanding, and this is one of the key observational starting points for modern cosmological theories. Modern observations show that the rate of expansion has not been constant throughout the life of the universe. Initially, when galaxies were close together, the effects of gravity were stronger than the effects of dark energy, and the expansion rate gradually slowed.
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- 14.2: A Model of the Universe
- A model that is isotropic and homogeneous (same everywhere) is a pretty good approximation of reality. The universe is expanding, which means that the universe undergoes a change in scale with time; space stretches and distances grow larger by the same factor everywhere at a given time. Observations show that the mass density of the universe is less than the critical density. In other words, there is not enough matter in the universe to stop the expansion.
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- 14.3: The Beginning of the Universe
- The universe cools as it expands. The energy of photons is determined by their temperature, and calculations show that in the hot, early universe, photons had so much energy that when they collided with one another, they could produce material particles. As the universe expanded and cooled, protons and neutrons formed first, then came electrons and positrons. Next, fusion reactions produced deuterium, helium, and lithium nuclei.
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- 14.4: The Cosmic Microwave Background
- When the universe became cool enough to form neutral hydrogen atoms, the universe became transparent to radiation. Scientists have detected the cosmic microwave background (CMB) radiation from this time during the hot, early universe. Measurements with the COBE satellite show that the CMB acts like a blackbody with a temperature of 2.73 K. Tiny fluctuations in the CMB show us the seeds of large-scale structures in the universe.
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- 14.5: What Is the Universe Really Made Of?
- Twenty-seven percent of the critical density of the universe is composed of dark matter. To explain so much dark matter, some physics theories predict that additional types of particles should exist. One type has been given the name of WIMPs (weakly interacting massive particles), and scientists are now conducting experiments to try to detect them in the laboratory. Dark matter plays an essential role in forming galaxies.
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- 14.6: The Inflationary Universe
- The Big Bang model does not explain why the CMB has the same temperature in all directions. Neither does it explain why the density of the universe is so close to critical density. These observations can be explained if the universe experienced a period of rapid expansion, which scientists call inflation, about 10^–35 seconds after the Big Bang. New grand unified theories (GUTs) are being developed to describe physical processes in the universe before and at the time that inflation occurred.
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- 14.7: The Anthropic Principle
- Recently, many cosmologists have noted that the existence of humans depends on the fact that many properties of the universe—the size of density fluctuations in the early universe, the strength of gravity, the structure of atoms—were just right. The idea that physical laws must be the way they are because otherwise we could not be here to measure them is called the anthropic principle. Some scientists speculate that there may be a multiverse of universes, in which ours is just one.
Thumbnail: This drawing shows the James Webb Space Telescope, which is currently planned for launch in 2018. The silver sunshade shadows the primary mirror and science instruments. The primary mirror is 6.5 meters (21 feet) in diameter. Before and during launch, the mirror will be folded up. After the telescope is placed in its orbit, ground controllers will command it to unfold the mirror petals. To see distant galaxies whose light has been shifted to long wavelengths, the telescope will carry several instruments for taking infrared images and spectra. (credit: modification of work by NASA).
Contributors and Attributions
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Andrew Fraknoi (Foothill College), David Morrison (NASA Ames Research Center), Sidney C. Wolff (National Optical Astronomy Observatory) with many contributing authors. Textbook content produced by OpenStax College is licensed under a Creative Commons Attribution License 4.0 license. Download for free at https://openstax.org/details/books/astronomy ).


3.3: Terrestrial Biomes
Figure \(\PageIndex{1}\). Each of the world’s eight major biomes is distinguished by characteristic temperatures and amount of precipitation. Polar ice caps and mountains are also shown.
There are eight major terrestrial biomes: tropical rainforests, savannas, subtropical deserts, chaparral, temperate grasslands, temperate forests, boreal forests, and Arctic tundra. Biomes are large-scale environments that are distinguished by characteristic temperature ranges and amounts of precipitation. These two variables affect the types of vegetation and animal life that can exist in those areas. Because each biome is defined by climate, the same biome can occur in geographically distinct areas with similar climates (Figures 1 and 2).
Tropical rainforests are found in equatorial regions (Figure \(\PageIndex{1}\)) are the most biodiverse terrestrial biome. This biodiversity is under extraordinary threat primarily through logging and deforestation for agriculture. Tropical rainforests have also been described as nature’s pharmacy because of the potential for new drugs that is largely hidden in the chemicals produced by the huge diversity of plants, animals, and other organisms. The vegetation is characterized by plants with spreading roots and broad leaves that fall off throughout the year, unlike the trees of deciduous forests that lose their leaves in one season.
The temperature and sunlight profiles of tropical rainforests are stable in comparison to other terrestrial biomes, with average temperatures ranging from 20 o C to 34 o C (68 o F to 93 o F). Month-to-month temperatures are relatively constant in tropical rainforests, in contrast to forests farther from the equator. This lack of temperature seasonality leads to year-round plant growth rather than just seasonal growth. In contrast to other ecosystems, a consistent daily amount of sunlight (11–12 hours per day year-round) provides more solar radiation and therefore more opportunity for primary productivity.
The annual rainfall in tropical rainforests ranges from 125 to 660 cm (50–200 in) with considerable seasonal variation. Tropical rainforests have wet months in which there can be more than 30 cm (11–12 in) of precipitation, as well as dry months in which there are fewer than 10 cm (3.5 in) of rainfall. However, the driest month of a tropical rainforest can still exceed the annual rainfall of some other biomes, such as deserts.Tropical rainforests have high net primary productivity because the annual temperatures and precipitation values support rapid plant growth. However, the high amounts of rainfall leaches nutrients from the soils of these forests.
Tropical rainforests are characterized by vertical layering of vegetation and the formation of distinct habitats for animals within each layer. On the forest floor is a sparse layer of plants and decaying plant matter. Above that is an understory of short, shrubby foliage. A layer of trees rises above this understory and is topped by a closed upper canopy—the uppermost overhead layer of branches and leaves. Some additional trees emerge through this closed upper canopy. These layers provide diverse and complex habitats for the variety of plants, animals, and other organisms. Many species of animals use the variety of plants and the complex structure of the tropical wet forests for food and shelter. Some organisms live several meters above ground, rarely descending to the forest floor.
Figure \(\PageIndex{4}\). A MinuteEarth video about how trees create rainfall, and vice versa.
Subtropical deserts exist between 15 o and 30 o north and south latitude and are centered on the Tropic of Cancer and the Tropic of Capricorn (Figure \(\PageIndex{6}\) below). Deserts are frequently located on the downwind or lee side of mountain ranges, which create a rain shadow after prevailing winds drop their water content on the mountains. This is typical of the North American deserts, such as the Mohave and Sonoran deserts. Deserts in other regions, such as the Sahara Desert in northern Africa or the Namib Desert in southwestern Africa are dry because of the high-pressure, dry air descending at those latitudes. Subtropical deserts are very dry; evaporation typically exceeds precipitation. Subtropical hot deserts can have daytime soil surface temperatures above 60 o C (140 o F) and nighttime temperatures approaching 0 o C (32 o F). Subtropical deserts are characterized by low annual precipitation of fewer than 30 cm (12 in) with little monthly variation and lack of predictability in rainfall. Some years may receive tiny amounts of rainfall, while others receive more. In some cases, the annual rainfall can be as low as 2 cm (0.8 in) in subtropical deserts located in central Australia (“the Outback”) and northern Africa.
Figure \(\PageIndex{6}\). A MinuteEarth video about the global climate patterns which lead to subtropical deserts.
The low species diversity of this biome is closely related to its low and unpredictable precipitation. Despite the relatively low diversity, desert species exhibit fascinating adaptations to the harshness of their environment. Very dry deserts lack perennial vegetation that lives from one year to the next; instead, many plants are annuals that grow quickly and reproduce when rainfall does occur, then they die. Perennial plants in deserts are characterized by adaptations that conserve water: deep roots, reduced foliage, and water-storing stems (Figure \(\PageIndex{6}\) below). Seed plants in the desert produce seeds that can lie dormant for extended periods between rains. Most animal life in subtropical deserts has adapted to a nocturnal life, spending the hot daytime hours beneath the ground. The Namib Desert is the oldest on the planet, and has probably been dry for more than 55 million years. It supports a number of endemic species (species found only there) because of this great age. For example, the unusual gymnosperm Welwitschia mirabilis is the only extant species of an entire order of plants. There are also five species of reptiles considered endemic to the Namib.
In addition to subtropical deserts there are cold deserts that experience freezing temperatures during the winter and any precipitation is in the form of snowfall. The largest of these deserts are the Gobi Desert in northern China and southern Mongolia, the Taklimakan Desert in western China, the Turkestan Desert, and the Great Basin Desert of the United States.
The chaparral is also called scrub forest and is found in California, along the Mediterranean Sea, and along the southern coast of Australia (Figure \(\PageIndex{7}\) below). The annual rainfall in this biome ranges from 65 cm to 75 cm (25.6–29.5 in) and the majority of the rain falls in the winter. Summers are very dry and many chaparral plants are dormant during the summertime. The chaparral vegetation is dominated by shrubs and is adapted to periodic fires, with some plants producing seeds that germinate only after a hot fire. The ashes left behind after a fire are rich in nutrients like nitrogen and fertilize the soil, promoting plant regrowth. Fire is a natural part of the maintenance of this biome.
Temperate grasslands are found throughout central North America, where they are also known as prairies, and in Eurasia, where they are known as steppes (Figure \(\PageIndex{8}\) below). Temperate grasslands have pronounced annual fluctuations in temperature with hot summers and cold winters. The annual temperature variation produces specific growing seasons for plants. Plant growth is possible when temperatures are warm enough to sustain plant growth, which occurs in the spring, summer, and fall.
Annual precipitation ranges from 25.4 cm to 88.9 cm (10–35 in). Temperate grasslands have few trees except for those found growing along rivers or streams. The dominant vegetation tends to consist of grasses. The treeless condition is maintained by low precipitation, frequent fires, and grazing. The vegetation is very dense and the soils are fertile because the subsurface of the soil is packed with the roots and rhizomes (underground stems) of these grasses. The roots and rhizomes act to anchor plants into the ground and replenish the organic material (humus) in the soil when they die and decay.
Fires, which are a natural disturbance in temperate grasslands, can be ignited by lightning strikes. It also appears that the lightning-caused fire regime in North American grasslands was enhanced by intentional burning by humans. When fire is suppressed in temperate grasslands, the vegetation eventually converts to scrub and dense forests. Often, the restoration or management of temperate grasslands requires the use of controlled burns to suppress the growth of trees and maintain the grasses.
Temperate forests are the most common biome in eastern North America, Western Europe, Eastern Asia, Chile, and New Zealand (Figure \(\PageIndex{9}\) below). This biome is found throughout mid-latitude regions. Temperatures range between –30 o C and 30 o C (–22 o F to 86 o F) and drop to below freezing on an annual basis. These temperatures mean that temperate forests have defined growing seasons during the spring, summer, and early fall. Precipitation is relatively constant throughout the year and ranges between 75 cm and 150 cm (29.5–59 in).
Deciduous trees are the dominant plant in this biome with fewer evergreen conifers. Deciduous trees lose their leaves each fall and remain leafless in the winter. Thus, little photosynthesis occurs during the dormant winter period. Each spring, new leaves appear as temperature increases. Because of the dormant period, the net primary productivity of temperate forests is less than that of tropical rainforests. In addition, temperate forests show far less diversity of tree species than tropical rainforest biomes.
The trees of the temperate forests leaf out and shade much of the ground. However, more sunlight reaches the ground in this biome than in tropical rainforests because trees in temperate forests do not grow as tall as the trees in tropical rainforests. The soils of the temperate forests are rich in inorganic and organic nutrients compared to tropical rainforests. This is because of the thick layer of leaf litter on forest floors and reduced leaching of nutrients by rainfall. As this leaf litter decays, nutrients are returned to the soil. The leaf litter also protects soil from erosion, insulates the ground, and provides habitats for invertebrates and their predators.
The boreal forest, also known as taiga or coniferous forest , is found roughly between 50 o and 60 o north latitude across most of Canada, Alaska, Russia, and northern Europe (Figure \(\PageIndex{10}\) below). Boreal forests are also found above a certain elevation (and below high elevations where trees cannot grow) in mountain ranges throughout the Northern Hemisphere. This biome has cold, dry winters and short, cool, wet summers. The annual precipitation is from 40 cm to 100 cm (15.7–39 in) and usually takes the form of snow; relatively little evaporation occurs because of the cool temperatures.
The long and cold winters in the boreal forest have led to the predominance of cold-tolerant cone-bearing plants. These are evergreen coniferous trees like pines, spruce, and fir, which retain their needle-shaped leaves year-round. Evergreen trees can photosynthesize earlier in the spring than deciduous trees because less energy from the Sun is required to warm a needle-like leaf than a broad leaf. Evergreen trees grow faster than deciduous trees in the boreal forest. In addition, soils in boreal forest regions tend to be acidic with little available nitrogen. Leaves are a nitrogen-rich structure and deciduous trees must produce a new set of these nitrogen-rich structures each year. Therefore, coniferous trees that retain nitrogen-rich needles in a nitrogen limiting environment may have had a competitive advantage over the broad-leafed deciduous trees.
The net primary productivity of boreal forests is lower than that of temperate forests and tropical wet forests. The aboveground biomass of boreal forests is high because these slow-growing tree species are long-lived and accumulate standing biomass over time. Species diversity is less than that seen in temperate forests and tropical rainforests. Boreal forests lack the layered forest structure seen in tropical rainforests or, to a lesser degree, temperate forests. The structure of a boreal forest is often only a tree layer and a ground layer. When conifer needles are dropped, they decompose more slowly than broad leaves; therefore, fewer nutrients are returned to the soil to fuel plant growth.
The Arctic tundra lies north of the subarctic boreal forests and is located throughout the Arctic regions of the Northern Hemisphere. Tundra also exists at elevations above the tree line on mountains. The average winter temperature is –34°C (–29.2°F) and the average summer temperature is 3°C–12°C (37°F –52°F). Plants in the Arctic tundra have a short growing season of approximately 50–60 days. However, during this time, there are almost 24 hours of daylight and plant growth is rapid. The annual precipitation of the Arctic tundra is low (15–25 cm or 6–10 in) with little annual variation in precipitation. And, as in the boreal forests, there is little evaporation because of the cold temperatures.
Plants in the Arctic tundra are generally low to the ground and include low shrubs, grasses, lichens, and small flowering plants (Figure \(\PageIndex{11}\) below). There is little species diversity, low net primary productivity, and low above-ground biomass. The soils of the Arctic tundra may remain in a perennially frozen state referred to as permafrost. The permafrost makes it impossible for roots to penetrate far into the soil and slows the decay of organic matter, which inhibits the release of nutrients from organic matter. The melting of the permafrost in the brief summer provides water for a burst of productivity while temperatures and long days permit it. During the growing season, the ground of the Arctic tundra can be completely covered with plants or lichens.
Suggested Supplementary Reading
HHMI. 2018. Biome Viewer . [Interactive Website]. Howard Hughes Medical Institute. < https://www.hhmi.org/biointeractive/biomeviewer >


3.7.9: Margaret Sanger, Title X, Abortion, and Birth Control
Margaret Sanger was one of 11 children born to her parents in 1879. Her mother was frail due having experienced 18 pregnancies in total - which she believed contributed to her mother’s death from tuberculosis at the age of 50. This galvanized Sanger’s mission to become a nurse and provide women access to birth control. In 1921 she founded the American Birth Control League, which would eventually become the Planned Parenthood Federation. At the time, there were only two barrier methods of birth control - the rubber condom and the diaphragm - and neither were widely used. By the 1950s, Sanger turned her focus toward promoting research and development of a contraceptive pill. Funded by her friend, Katherine McCormick - who had inherited a fortune from the International Harvester company - Sanger supported research conducted by two doctors: Gregory Pincus in developing the pill, and John Rock in testing it. The researchers used progesterone derived from certain wild yams which had been traditionally used by the Aztecs for birth control and menstrual cramping. The pill went through clinical trials with women in Puerto Rico as subjects (since it would not have gotten approval in other states) and was approved by the FDA in 1960. Since then, it has been discovered that much lower doses of estrogen and progesterone are just as effective at preventing pregnancy and cause much fewer side effects (Planned Parenthood Federation of America, Inc., 2015).
The importance of sexual and reproductive health as a public health service was recognized in the next decade. Title X was signed into law by President Nixon in 1970, and provided funding for family planning clinics. These clinics could provide contraceptives and reproductive health services to people who could not afford them. Screening services for sexually transmitted infections, reproductive cancers could also be provided, as well as education on preventing HIV and other STDs - but importantly, funding by Title X is prohibited from being used to provide abortion services. Further regulations subsequently added to Title X during conservative presidencies made it difficult for healthcare providers to even discuss abortion as an option for reproductive healthcare (Seabert et al., 2021).
Two key court cases solidified access to both contraceptives and abortions for women in the 60’s and 70’s. Griswold v. Connecticut in 1965 prevented states from banning contraceptives, citing the right to privacy derived from several constitutional amendments but most notably the 14th amendment (LII, n.d.). In 1973, Roe v, Wade struck down a Texas law which criminalized helping a woman get an abortion, again citing the right to privacy. This effectively legalized abortion federally, but only up to the age of fetal viability (the age at which the fetus could possibly survive outside the uterus), with exceptions beyond if there is risk to the mother’s health. It is important to note that the age of fetal viability is influenced by a variety of factors including sex, weight, availability of medical support, etc. and the current consensus places fetal viability between 23-25 weeks of pregnancy (The American College of Obstetrics and Gynecologists, 2023). In 2022, the Roe v. Wade ruling was overturned in Dobbs v. Jackson , allowing the conservative legislature in many states to enact abortion bans between 6 and 15 weeks of pregnancy (Contributors to Wikimedia projects, Griswold v. Connecticut, 2023b). Within the same year, the ripple effects of these laws was apparent - exemplified by a lawsuit against the state of Texas where several women testified that they were denied abortions for pregnancies risking their health, and nonviable pregnancies (including fatal fetal growth complications) - resulting in traumatic illness and birth experiences (Varney, 2023). Fears around lawsuits and navigating obstetric care have also been cited as reasons for some hospitals to close their maternity wards (Kekatos, 2023). This is a contemporary example of how legislative decisions can have immediate effects on medical care.


As difficult as it may be to define personality, it is important to know something about it. Personality is probably the most important field in psychology. Understanding who we are as individuals, and why we think certain thoughts and do certain things is the starting point for addressing clinical issues, abnormal psychology, and health psychology, it is the ultimate goal of studying human development, and it is the point from which we begin to address social psychology. Without an appreciation of the individual, without concern for each person, these other areas of psychology become little more than academic subjects.
Personality as a Common Thread in the History of Psychology
Most historians identify the starting point of the modern field of psychology with the experimental psychologists, particularly the establishment of Wilhelm Wundt’s laboratory in Leipzig, Germany in 1879. Personality theorists, however, were not far behind. Freud, Adler, and Jung were all beginning to work in the field of psychiatry (they were medical doctors) in the late 1800s as well. If one wants to put an official date on the start of modern personality theory, it would most likely be 1900, the year in which Freud published “
The Interpretation of Dreams
(Freud, 1900/1995).” Since Freud’s theories were based on his work with patients, this date is also the beginning of a relationship between personality theory, abnormal psychology, and psychotherapy, a relationship that continues today. Early psychodynamic theorists were also influential in developmental psychology and school psychology. Freud began his theory of personality with a proposed series of developmental stages: the five psychosexual stages. Erik Erikson, another well known psychodynamic theorist, is probably better known as a developmental psychologist for his eight-stage theory of psychosocial crises. Adler considered the early years of childhood so important that he felt parents and teachers should have access to training and counseling in the schools. With the endorsement of the minister of education, Adler and his colleagues established child guidance centers in many public schools.
As psychodynamic theory dominated the European scene in the early 1900s, America was largely influenced by behaviorism. Behavioral theorists also considered personality within their domain, suggesting that personality is learned. John B. Watson boasted that he could use behavioral principles to direct any child into any given career path. B.F. Skinner constructed an advanced version of his famous “Skinner Box,” which was typically used to train rats or pigeons, to raise and care for children during the early years of life. As behaviorism continued to develop, social learning theorists focused on a rich mixture of imitation, observation of the rewards and punishment experienced by others, expectations, and personal assessment of the value of potential rewards and punishers.
Against this backdrop of psychodynamic theory and behaviorism, Rogers and Maslow became the leading advocates of a new and openly positive view of human development, referred to most commonly as humanistic psychology. This new field emphasized self-actualization, though self-actualization itself does not appear to have been a new concept. It closely resembles the enlightenment described by Yoga and Buddhism (each of which is thousands of years old), though Yoga and Buddhism ultimately reject the existence of the self. Is there a relationship between Eastern philosophical perspectives and humanistic psychology? Rogers had traveled throughout Asia, particularly in China, and Maslow had studied with renowned psychodynamic theorists who were fascinated by Buddhism (such as Horney), so both were well acquainted with the basics of Eastern philosophical thought.
Closely related to the behavioral perspectives, cognitive theories of personality are also prominent in psychology. Today, the use of non-invasive brain imaging techniques (e.g., functional magnetic resonance imaging), which function in real time, have made the study of cognitive processes one of the most exciting areas of psychological research. Studying cognition is hardly new, however, since the earliest studies of consciousness can be traced to William James in the late 1800s. Still, what James was able to study over 100 years ago is completely different than what modern cognitive neuroscientists are able to study today. Nonetheless, if we can find something in common between the studies of over a century ago and the research of today, perhaps we will really begin to understand the complexity and diversity of personality.
Positive Psychology and Spirituality: New Directions in the Field of Personality
Figure \(\PageIndex{1}\)
Young children enjoy helping even younger children. Positive psychology encourages us all to study and promote such behavior.
In 1998, Martin Seligman, then president of APA and author of
What You Can Change & What You Can’t
(1994), urged psychologists to rediscover the forgotten mission of psychology: to build human strength and nurture genius. Seligman called this new area Positive Psychology (for thorough overviews see Compton, 2005 or Peterson, 2006). In 2000,
American Psychologist
published a special edition on happiness, excellence, and optimal human functioning (
American Psychologist, Vol. 55
, Number 1, 2000; with an introduction by Seligman & Csikszentmihalyi, 2000). The general goal of positive psychology is to find ways in which psychological research can help people to be happier, and to lead more fulfilling lives. Positive psychology can also serve as a focus for psychologists to become more appreciative of not only human nature, but also of the potential for the field of psychology itself to benefit all people (Sheldon & King, 2001). Table \(\PageIndex{1}\) offers a sampling of the wide range of interest in positive psychology that exists today.
Closely related to positive psychology is the concept of resilience. Many individuals face difficult or traumatic challenges in life, and yet some manage to maintain stability in their lives in spite of these unfortunate circumstances. How exactly these individuals maintain stability and a positive direction in their lives is not always clear, and there may be a variety of different ways that individuals respond to such extreme stress (Bonanno, 2004, 2005a; Masten, 2001; for commentary on the first article see also Bonanno, 2005b; Kelley, 2005; Linley & Joseph, 2005; Litz, 2005; Maddi, 2005; Roisman, 2005). Among the important factors, particularly for our perspective here, is the ability to maintain positive emotions and to pursue self-enhancement (Bonanno, 2004, 2005; Masten, 2001). Throughout history, a variety of cultures have given rise to spiritual pursuits that help to guide the development of individuals in positive directions. We will cover some of these spiritual paths in the last section of this book, taking just a brief look here at the relationship between spirituality, positive psychology, and personality.
|
Table \(PageIndex{1}\): Selection of Books and Articles Related to Positive
Psychology Published Since the Year 2000
|
|
|
Books
on Positive Psychology
|
Compton, 2005; Snyder & Lopez, 2005
|
|
Human Strengths and Virtues
|
Aspinwall & Staudinger, 2003; Fowers, 2005
|
|
Happiness and Well-Being
|
Cloninger, 2004; Hampson, 2008; Hsee et al., 2008; Huppert, 2009; Inglehart et al., 2008; Molden et al., 2009; Seligman, 2002; Siegel, 2007
|
|
Articles
on Academic Excellence and Creativity
|
Lubinski & Benbow, 2000; Simonton, 2000; Winner, 2000
|
|
Cognition and Motivation
|
Baltes & Staudinger, 2000; Lyubomirsky, 2001; Peterson, 2000
|
|
Coping
|
Folkman & Moskowitz; 2000; Vaillant, 2000
|
|
Economics
|
Diener, 2000; Diener & Seligman, 2004; Myers, 2000; Smith et al., 2005
|
|
Emotions, Happiness, Well-Being
|
Dolan & White, 2007; Ekman et al., 2005; Fredrickson, 2001; Fredrickson & Joiner, 2002; Fredrickson & Losada, 2005; Kesebir & Diener, 2008; Kim & Moen, 2001; Napier & Jost, 2008; Oishi et al., 2007; Robinson et al., 2004; Weiss et al., 2008
|
|
Enjoying a Good Life
|
Bauer & McAdams, 2004a,b; Bauer et al., 2005a,b; Schneider, 2001
|
|
Evolution
|
Buss, 2000a; Massimini & Fave, 2000
|
|
Physical Health
|
Cohen & Pressman, 2006; Ray, 2004; Salovey et al., 2000; Taylor et al., 2000
|
|
Relationships
|
Myers, 2000
|
|
Self-Determination
|
Ryan & Deci, 2000; Schwartz, 2000
|
|
Therapeutic Intervention
|
Ahmed & Boisvert, 2006; Joseph & Linley, 2006; Seligman et al., 2005
|
|
Youth Development
|
Larson, 2000
|
It appears that spirituality is an essential attribute of human nature. It has been recognized for some time that religious ritual is a cultural universal (Murdock, 1945; see also Ferraro, 2006a). More than simply a cultural universal, however, spirituality appears to be a natural consequence of child development. Deborah Kelemen (2004) brought together a number of different theories, and was able to demonstrate that young children, around
the age of 5 years old, have both the ability and the inclination to explain the world around them in terms of an intentional act by a supernatural being. Thus, Kelemen suggests that young children are what she calls “intuitive theists.” Surprisingly, this tendency appears to continue into adulthood, since even college students studying evolution exhibit a tendency to think of evolution as a purposeful agent itself, an agent that guides further evolution according to a thoughtful plan (Kelemen, 2004).
The relationship between psychology and religious/spiritual pursuits has a long and interesting history. One of William James’ most famous books is
The Varieties of Religious Experience: A Study in Human Nature
(James, 1902/1987), and around the turn of the century in 1900 psychologists of the day actually used religion in the popular press to help engender respect for the new field of psychology (Pickren, 2000). Since the more recent turn of the century there have been a number of books and articles published connecting psychology, spirituality, religion, and psychotherapy (see Table \(\PageIndex{2}\)). Thus, a topic that was viewed as important at the beginning of the field of psychology, but was then pushed aside as unscientific, is once again become an area of interest and importance. Although spirituality is certainly not synonymous with positive psychology, it does appear to be an important factor in positive psychology.
|
Table \(\PageIndex{2}\): Selection of Recent Books and Articles
Relating Spirituality to Psychology
|
|
|
Books
on psychology seen through the eyes of faith
|
Myers & Jeeves, 2003
|
|
Relationships between meditation, mindfulness, Buddhism and psychology
|
Brantley, 2003; Dockett et al., 2003; Helminiak, 2005; McQuaid & Carmona, 2004
|
|
Spiritual counseling from a variety of perspectives
|
Brach, 2003; Brazier, 1995; Mruk & Hartzell, 2003; Richards & Bergin, 2000, 2004, 2005; Sperry & Shafranske, 2005; Weiner et al., 2005; Williams et al., 2007
|
|
Multicultural clinical assessment
|
Dana, 2000; Suzuki et al., 2001
|
|
Articles
on the religious roots of individualism vs. collectivism
|
Burston, 2001; Lynch, 2001; Lynch Jr., 2001; Margolis, 2001; Sampson, 2000
|
|
Childhood Development of Faith
|
Kelemen, 2004
|
|
Shamanism
|
Krippner, 2002
|
|
The relationships between spirituality, religion, and health
|
Ginges et al., 2009; Hill & Pargament, 2003; Kier & Davenport, 2004; McCormick, 2004; Miller & Thoresen, 2003; Miller & Thoresen, 2004; Powell et al., 2003; Rayburn, 2004; Richmond, 2004; Seeman et al., 2003; Seybold & Hill, 2001; Wallace & Shapiro, 2006
|
Numerous studies have shown that individuals who are actively spiritual have higher levels of well-being and fewer serious problems in their lives (see Compton, 2005; Myers, 2000; Seligman, 2002; Seligman & Csikszentmihalyi, 2000). The recently published
Handbook of Positive Psychology
has two chapters devoted specifically to spiritual pursuits and their benefits (Pargament & Mahoney, 2005; Shapiro et al., 2005). Peterson and Seligman (2004) have identified spirituality as one of the twenty-four specific character strengths that have consistently emerged across history and culture. Indeed, they believe that spirituality “is the most human of the character strengths as well as the most sublime…People with this strength have a theory about the ultimate meaning of life that shapes their conduct and provides comfort to them” (pg. 533; Peterson & Seligman, 2004). In the last section of this book we will examine a number of spiritual approaches to life, each of which suggests a path for positive development. Despite being associated with very different religions, which range from 1,400 years old to perhaps more than 5,000 years old, these spiritual paths have much in common. Perhaps this should not be surprising, as it may help to explain the inherent nature of children to be “intuitive theists” and the universality of religious ritual in human culture.
Discussion Question:
Do you believe that psychology should work to develop itself as a field that focuses on helping people to develop in positive ways? Can spirituality or religion be helpful, or might they present more problems?


 The next day she confides in you that she was concerned that the stars in the Big Dipper (her favorite star pattern) might be the next ones to go. How would you put her mind at ease?
In what ways are meteorites different from meteors? What is the probable origin of each?
How are comets related to meteor showers?
What do we mean by primitive material? How can we tell if a meteorite is primitive?
Describe the solar nebula, and outline the sequence of events within the nebula that gave rise to the planetesimals.
Why do the giant planets and their moons have compositions different from those of the terrestrial planets?
How do the planets discovered so far around other stars differ from those in our own solar system? List at least two ways.
Explain the role of impacts in planetary evolution, including both giant impacts and more modest ones.
Why are some planets and moons more geologically active than others?
Summarize the origin and evolution of the atmospheres of Venus, Earth, and Mars.
Why do meteors in a meteor shower appear to come from just one point in the sky?


6.3: Energy and Metabolism - Metabolic Pathways
- Describe the two major types of metabolic pathways
Metabolic Pathways
The processes of making and breaking down carbohydrate molecules illustrate two types of metabolic pathways. A metabolic pathway is a step-by-step series of interconnected biochemical reactions that convert a substrate molecule or molecules through a series of metabolic intermediates, eventually yielding a final product or products. For example, one metabolic pathway for carbohydrates breaks large molecules down into glucose. Another metabolic pathway might build glucose into large carbohydrate molecules for storage. The first of these processes requires energy and is referred to as anabolic. The second process produces energy and is referred to as catabolic. Consequently, metabolism is composed of these two opposite pathways:
- Anabolism (building molecules)
- Catabolism (breaking down molecules)
Anabolic Pathways
Anabolic pathways require an input of energy to synthesize complex molecules from simpler ones. One example of an anabolic pathway is the synthesis of sugar from CO 2 . Other examples include the synthesis of large proteins from amino acid building blocks and the synthesis of new DNA strands from nucleic acid building blocks. These processes are critical to the life of the cell, take place constantly, and demand energy provided by ATP and other high-energy molecules like NADH (nicotinamide adenine dinucleotide) and NADPH.
Catabolic Pathways
Catabolic pathways involve the degradation of complex molecules into simpler ones, releasing the chemical energy stored in the bonds of those molecules. Some catabolic pathways can capture that energy to produce ATP, the molecule used to power all cellular processes. Other energy-storing molecules, such as lipids, are also broken down through similar catabolic reactions to release energy and make ATP.
Importance of Enzymes
Chemical reactions in metabolic pathways rarely take place spontaneously. Each reaction step is facilitated, or catalyzed, by a protein called an enzyme. Enzymes are important for catalyzing all types of biological reactions: those that require energy as well as those that release energy.
Key Points
- A metabolic pathway is a series of chemical reactions in a cell that build and breakdown molecules for cellular processes.
- Anabolic pathways synthesize molecules and require energy.
- Catabolic pathways break down molecules and produce energy.
- Because almost all metabolic reactions take place non-spontaneously, proteins called enzymes help facilitate those chemical reactions.
Key Terms
- catabolism : destructive metabolism, usually including the release of energy and breakdown of materials
- enzyme : a globular protein that catalyses a biological chemical reaction
- anabolism : the constructive metabolism of the body, as distinguished from catabolism


44.1: The Scope of Ecology
Skills to Develop
- Define ecology and the four levels of ecological research
- Describe examples of the ways in which ecology requires the integration of different scientific disciplines
- Distinguish between abiotic and biotic components of the environment
- Recognize the relationship between abiotic and biotic components of the environment
Ecology is the study of the interactions of living organisms with their environment. One core goal of ecology is to understand the distribution and abundance of living things in the physical environment. Attainment of this goal requires the integration of scientific disciplines inside and outside of biology, such as biochemistry, physiology, evolution, biodiversity, molecular biology, geology, and climatology. Some ecological research also applies aspects of chemistry and physics, and it frequently uses mathematical models.
Link to Learning
Climate change can alter where organisms live, which can sometimes directly affect human health. Watch the PBS video “Feeling the Effects of Climate Change” in which researchers discover a pathogenic organism living far outside of its normal range.
Levels of Ecological Study
When a discipline such as biology is studied, it is often helpful to subdivide it into smaller, related areas. For instance, cell biologists interested in cell signaling need to understand the chemistry of the signal molecules (which are usually proteins) as well as the result of cell signaling. Ecologists interested in the factors that influence the survival of an endangered species might use mathematical models to predict how current conservation efforts affect endangered organisms. To produce a sound set of management options, a conservation biologist needs to collect accurate data, including current population size, factors affecting reproduction (like physiology and behavior), habitat requirements (such as plants and soils), and potential human influences on the endangered population and its habitat (which might be derived through studies in sociology and urban ecology). Within the discipline of ecology, researchers work at four specific levels, sometimes discretely and sometimes with overlap: organism, population, community, and ecosystem (Figure \(\PageIndex{1}\)).
Organismal Ecology
Researchers studying ecology at the organismal level are interested in the adaptations that enable individuals to live in specific habitats. These adaptations can be morphological, physiological, and behavioral. For instance, the Karner blue butterfly ( Lycaeides melissa samuelis ) (Figure \(\PageIndex{2}\)) is considered a specialist because the females preferentially oviposit (that is, lay eggs) on wild lupine. This preferential adaptation means that the Karner blue butterfly is highly dependent on the presence of wild lupine plants for its continued survival.
After hatching, the larval caterpillars emerge and spend four to six weeks feeding solely on wild lupine (Figure \(\PageIndex{3}\)). The caterpillars pupate (undergo metamorphosis) and emerge as butterflies after about four weeks. The adult butterflies feed on the nectar of flowers of wild lupine and other plant species. A researcher interested in studying Karner blue butterflies at the organismal level might, in addition to asking questions about egg laying, ask questions about the butterflies’ preferred temperature (a physiological question) or the behavior of the caterpillars when they are at different larval stages (a behavioral question).
Population Ecology
A population is a group of interbreeding organisms that are members of the same species living in the same area at the same time. (Organisms that are all members of the same species are called conspecifics .) A population is identified, in part, by where it lives, and its area of population may have natural or artificial boundaries: natural boundaries might be rivers, mountains, or deserts, while examples of artificial boundaries include mowed grass, manmade structures, or roads. The study of population ecology focuses on the number of individuals in an area and how and why population size changes over time. Population ecologists are particularly interested in counting the Karner blue butterfly, for example, because it is classified as federally endangered. However, the distribution and density of this species is highly influenced by the distribution and abundance of wild lupine. Researchers might ask questions about the factors leading to the decline of wild lupine and how these affect Karner blue butterflies. For example, ecologists know that wild lupine thrives in open areas where trees and shrubs are largely absent. In natural settings, intermittent wildfires regularly remove trees and shrubs, helping to maintain the open areas that wild lupine requires. Mathematical models can be used to understand how wildfire suppression by humans has led to the decline of this important plant for the Karner blue butterfly.
Community Ecology
A biological community consists of the different species within an area, typically a three-dimensional space, and the interactions within and among these species. Community ecologists are interested in the processes driving these interactions and their consequences. Questions about conspecific interactions often focus on competition among members of the same species for a limited resource. Ecologists also study interactions among various species; members of different species are called heterospecifics . Examples of heterospecific interactions include predation, parasitism, herbivory, competition, and pollination. These interactions can have regulating effects on population sizes and can impact ecological and evolutionary processes affecting diversity.
For example, Karner blue butterfly larvae form mutualistic relationships with ants. Mutualism is a form of a long-term relationship that has coevolved between two species and from which each species benefits. For mutualism to exist between individual organisms, each species must receive some benefit from the other as a consequence of the relationship. Researchers have shown that there is an increase in the probability of survival when Karner blue butterfly larvae (caterpillars) are tended by ants. This might be because the larvae spend less time in each life stage when tended by ants, which provides an advantage for the larvae. Meanwhile, the Karner blue butterfly larvae secrete a carbohydrate-rich substance that is an important energy source for the ants. Both the Karner blue larvae and the ants benefit from their interaction.
Ecosystem Ecology
Ecosystem ecology is an extension of organismal, population, and community ecology. The ecosystem is composed of all the biotic components (living things) in an area along with the abiotic components (non-living things) of that area. Some of the abiotic components include air, water, and soil. Ecosystem biologists ask questions about how nutrients and energy are stored and how they move among organisms and the surrounding atmosphere, soil, and water.
The Karner blue butterflies and the wild lupine live in an oak-pine barren habitat. This habitat is characterized by natural disturbance and nutrient-poor soils that are low in nitrogen. The availability of nutrients is an important factor in the distribution of the plants that live in this habitat. Researchers interested in ecosystem ecology could ask questions about the importance of limited resources and the movement of resources, such as nutrients, though the biotic and abiotic portions of the ecosystem.
Career Connection: Ecologist
A career in ecology contributes to many facets of human society. Understanding ecological issues can help society meet the basic human needs of food, shelter, and health care. Ecologists can conduct their research in the laboratory and outside in natural environments (Figure \(\PageIndex{4}\)). These natural environments can be as close to home as the stream running through your campus or as far away as the hydrothermal vents at the bottom of the Pacific Ocean. Ecologists manage natural resources such as white-tailed deer populations ( Odocoileus virginianus ) for hunting or aspen ( Populus spp.) timber stands for paper production. Ecologists also work as educators who teach children and adults at various institutions including universities, high schools, museums, and nature centers. Ecologists may also work in advisory positions assisting local, state, and federal policymakers to develop laws that are ecologically sound, or they may develop those policies and legislation themselves. To become an ecologist requires an undergraduate degree, usually in a natural science. The undergraduate degree is often followed by specialized training or an advanced degree, depending on the area of ecology selected. Ecologists should also have a broad background in the physical sciences, as well as a sound foundation in mathematics and statistics.
Link to Learning
Visit this site to see Stephen Wing, a marine ecologist from the University of Otago, discuss the role of an ecologist and the types of issues ecologists explore.
Summary
Ecology is the study of the interactions of living things with their environment. Ecologists ask questions across four levels of biological organization—organismal, population, community, and ecosystem.  At the population and community levels, ecologists explore, respectively, how a population of organisms changes over time and the ways in which that population interacts with other species in the community. Ecologists studying an ecosystem examine the living species (the biotic components) of the ecosystem as well as the nonliving portions (the abiotic components), such as air, water, and soil, of the environment.
Glossary
- abiotic
- nonliving components of the environment
- biotic
- living components of the environment
- conspecifics
- individuals that are members of the same species
- ecology
- study of interaction between living things and their environment
- heterospecifics
- individuals that are members of different species


2: Disease and Epidemiology (Part A)
In the United States and other developed nations, public health is a key function of government. A healthy citizenry is more productive, content, and prosperous; high rates of death and disease, on the other hand, can severely hamper economic productivity and foster social and political instability. The burden of disease makes it difficult for citizens to work consistently, maintain employment, and accumulate wealth to better their lives and support a growing economy.
In this chapter, we will explore the intersections between microbiology and epidemiology, the science that underlies public health.
Chapter Outline
-
- 2.1: Tracking Infectious Diseases
- Some important researchers, such as Florence Nightingale, subscribed to the miasma hypothesis. The transition to acceptance of the germ theory during the 19th century provided a solid mechanistic grounding to the study of disease patterns. The studies of 19th century physicians and researchers such as John Snow, Florence Nightingale, Ignaz Semmelweis, Joseph Lister, Robert Koch, Louis Pasteur, and others sowed the seeds of modern epidemiology.


5.E: Trekking through Spacetime (Exercises)
PRACTICE
5-1 more is less
The spacetime diagram shows two alternative worldlines from event \(A\) to event \(D\) . The table shows coordinates of numbered events in this frame. Time and space are measured in years.
-
One traveler moves along the solid segmented worldline from event
\(A\)
to events
\(B, C\)
, and
\(D\)
. Calculate the time increase on his wristwatch (proper clock)
(1) between event \(A\) and event \(B\).
(2) between event \(B\) and event \(C\) .
(3) between event \(C\) and event \(D\) .
(4) Also calculate the total proper time along worldline \(A, B, C, D\). - His twin sister moves along the straight dotted worldline from event \(A\) directly to event \(D\) . Calculate the time increase on her wristwatch between events \(A\) and \(D\) .
- Which twin (solid-line or dotted-line traveler) is younger when they rejoin at event \(D\) ?
5-2 transforming worldlines
The laboratory spacetime diagram in the figure shows two worldlines. One, the vertical line labeled B, is the worldline of an object that is at rest in this frame. The other, the segmented line that connects events 0, 1, 2, and 3, is the worldline of an object that moves at different speeds at different times in this frame. The proper time is written on each segment and invariant hyperbolas are drawn through events 1,2, and 3. The event table shows the space and time locations in this frame of the four events 0, 1, 2, and 3.
- Trace the axes and hyperbolas onto a blank piece of paper. Sketch a qualitatively correct spacetime diagram for the same pair of worldlines observed in a frame in which the particle on the segmented worldline has zero velocity between event 1 and event 2.
- What is the velocity, in this new frame, of the particle moving along worldline \(B\) ?
- On each straight portion of the segmented worldline for this new frame write the numerical value of the interval between the two connected events.
5-3 mapmaking in spacetime
Note: Recall Exercise 1-6, the corresponding mapmaking exercise in Chapter 1 .
Here is a table of timelike intervals between events, in meters. The events occur in the time sequence \(A B C D\) in all frames and along a single line in space in all frames. (They do not occur along a single line on the spacetime map.)
| INTERVAL | to event | |||
|---|---|---|---|---|
| \(A\) | \(B\) | \(C\) | \(D\) | |
| from event | ||||
| \(A\) | 0 | \(1.0\) | \(3.161\) | \(5.196\) |
| \(B\) | 0 | \(2.0\) | \(4.0\) | |
| \(C\) | 0 | \(2.0\) | ||
| \(D\) | 0 |
-
Use a ruler and the hyperbola graph to construct a spacetime map of these events. Draw this map on thin paper so you can lay it over the hyperbola graph and see the hyperbolas.
Discussion: How to start? With three arbitrary decisions! (1) Choose event \(A\) to be at the origin of the spacetime map. (2) Choose event \(B\) to occur at the same place as event \(A\) . That is, event point \(B\) is located on the positive time axis with respect to event point \(A\) . After plotting \(B\) , use your ruler to draw this straight time axis through event points \(A\) and \(B\) . Keep this line parallel to the vertical lines on the hyperbola graph in all later constructions. (3) Even with these choices, there are two spacetime locations \((x, t)\) at which you can locate the event point \(C\) ; choose either of these two spacetime locations arbitrarily. Then go on to plot event \(D\) .
Analogy to surveying: In surveying (using Euclidean geometry) you locate all points a given distance from some stake by using that stake as origin and drawing a circle of radius equal to the desired distance. In a spacetime map (using Lorentz geometry) you locate all event points a given interval from some event by using that event point as origin and drawing a hyperbola with nearest point equal to the desired interval. - Now take a new piece of paper and draw a spacetime map for another reference frame. Choose event \(D\) to be at the origin of the spacetime map. This means that all other events occur before \(D\) . Hence turn the hyperbola plot upside down, so that the hyperbolas open downward. Choose event \(B\) to occur at the same place as \(D\) . Now find the locations of \(A\) and \(C\) using the same strategy as in part a .
- Find an approximate value for the relative speed of the two frames for which you have made spacetime plots.
- Hold one of your spacetime maps up to the light with the marks on the side of the paper facing the light. Does the map you see from the back also satisfy the table entries?
PROBLEMS
5-4 the pole and barn paradox
A worried student writes, "Relativity must be wrong. Consider a 20-meter pole carried so fast in the direction of its length that it appears to be only 10 meters long in the laboratory frame of reference. Let the runner who carries the pole enter a barn 10 meters long, as shown in the figure. At some instant the farmer can close the front door and the pole will be entirely enclosed in the barn. However, look at the same situation from the frame of reference of the runner. To him the barn appears to be contracted to half its length. How can a 20 -meter pole possibly fit into a 5-meter barn? Does not this unbelievable conclusion prove that relativity contains somewhere a fundamental logical inconsistency?"
Write a reply to the worried student explaining clearly and carefully how the pole and barn are treated by relativity without internal contradiction. Use the following outline or some other method.
- Make two carefully labeled spacetime diagrams, one an \(x t\) diagram for the barn rest frame, the other an \(x^{\prime} t^{\prime}\) diagram for the runner rest frame. Referring to the figure, take the event "\(Q\) coincides with \(A\)" to be at the origin of both diagrams. In both plot the worldlines of \(A, B, P\) , and \(Q\) . Pay attention to the scale of both diagrams. Label both diagrams with the time (in meters) of the event " \(Q\) coincides with \(B\) " (derived from Lorentz transformation equations or otherwise). Do the same for the times of events " \(P\) coincides with \(A\) " and " \(P\) coincides with \(B\) ."
- Discussion question: Suppose the barn has no back door but rather a back wall of steel-reinforced concrete. What happens after the farmer closes the front door on the pole?
- Replace the pole with a line of ten tennis balls the same length as the pole and moving together with the same velocity as the pole. The farmer's ten children line up inside the barn, and each catches and stops one tennis ball at the same time as the farmer closes the front door of the barn. Describe the stopping events as recorded by the observer riding on the last tennis ball. Plot them on your two diagrams.
5-5 radar speed trap
A highway patrolman aims a stationary radar transmitter backward along the highway toward oncoming traffic. A detector mounted next to the transmitter analyzes the radar wave reflected from an approaching car. An internal computer uses the shift in frequency of the reflected wave to reckon and display the car's speed. Analyze this shift in frequency as in parts a-e or with some other method. Treat the car as a simple mirror and assume that the radar signals move back and forth along one line on the highway. Radar is an electromagnetic wave that moves with the speed of light.
The figure shows the worldline of the car, worldlines of two adjacent maxima of the radar wave, and the wavelength \(\lambda\) of incident and reflected waves.
-
From the 45-degree right triangle
\(A B C\)
, show that
\[\Delta t=v \Delta t+\lambda_{\text {reflected }} \nonumber\]
From the 45-degree right triangle \(D E F\) , show that
\[\Delta t=\lambda_{\text {incident }}-v \Delta t \nonumber\]
Eliminate \(\Delta t\) from these two equations to find an expression for \(\lambda_{\text {reflected }}\) in terms of \(\lambda_{\text {incident }}\) and the automobile speed \(v\) . -
The frequency
\(f\)
of radar (in cycles/second) is related to its wavelength
\(\lambda\)
in a vacuum by the formula
\(f=c / \lambda\)
, where
\(c\)
is the speed of light
\((\approx\)
the speed of radar waves in air). Derive an expression or frequency
\(f_{\text {reflected }}\)
of the reflected radar signal in terms of frequency
\(f_{\text {incident }}\)
of the incident wave and the speed
\(v\)
of the oncoming automobile. Show that the result is
\[f_{\text {reflected }}=\left(\frac{1+v}{1-v}\right) f_{\text {incident }} \nonumber\] -
For an automobile moving at a speed
\(v=\)
\(v_{\text {conv }} / c\)
that is a small fraction of the speed of light, assume that the fractional change in frequency of reflected radar is small. Under this assumption, use the first two terms of the binomial expansion
\[(1-z)^{n} \approx 1-n z \text { for }|z|<<1 \nonumber\]
to show that the fractional change of frequency is given by the approximate expression
\[\frac{\Delta f}{f} \approx 2 v \nonumber\]
Substitute the speed of a car moving at 100 kilometers/hour ( \(=27.8\) meters/second \(\approx 60\) miles/hour) and show that your assumption about the small fractional change is justified. - One radar gun used by the Massachusetts Highway Patrol operates at a frequency of \(10.525 \times 10^{9}\) cycles/second. By how many cycles/second is the reflected beam shifted in frequency when reflected from a car approaching at 100 kilometers/ hour?
- What discrimination between different frequency shifts must the unit have if it can distinguish the speed of a car moving at 100 kilometers/hour from the speed of one moving at 101 kilometers/ hour?
Reference: T . M. Kalotas and A. R. Lee, American Journal of Physics , Volume 58, pages 187-188 (February 1990).
5-6 a summer evening's fantasy
You are standing alone outdoors at dusk on the first day of summer. You see Sun setting due west and the planet Venus in the same direction. On the opposite horizon the full Moon is rising due east. An alien ship approaches from the east and lands beside you. The occupants inform you that they are from Proxima Centauri, which lies due east beyond the rising Moon. They say they have been traveling straight to Earth and that their reduced approach speed within the solar system was such that the time stretch factor gamma during the approach was \(5 / 3\) .
At the same instant that the aliens land, you see Sun explode. The aliens admit to you that earlier, on their way to Earth, they shot a laser light pulse at Sun, which caused this explosion. They warn that Sun's explosion emitted an immense pulse of particles moving at half the speed of light that will blow away Earth's atmosphere. In confirmation, shortly after the aliens land you notice that the planet Venus, lying in the direction of Sun, suddenly changes color.
 They agree and set the dials to flee in an easterly direction away from Sun at top speed, with time stretch factor gamma of \(25 / 7\) . The takeoff is to be 7 minutes after the alien landing on Earth.
Do you make it?
Draw a detailed Earth spacetime diagram showing the events and worldlines of this story. Use the following information.
- Sun is 8 light-minutes from Earth.
- Venus is 2 light-minutes from Earth.
- Assume that Sun, Venus, Earth, and Moon all lie along a single direction in space and are relatively at rest during this short story. The incoming and outgoing paths of the alien ship lie along this same line in space.
- All takeoffs and landings involve instantaneous changes from initial to final speed.
- \(5^{2}-3^{2}=4^{2}\) and \((25)^{2}-(7)^{2}=(24)^{2}\)
-
Plot EVENTS labeled with the following NUMBERS.
0. your location when the aliens land (at the origin)
1. Sun explodes
2. light from Sun explosion reaches you
3. Venus's atmosphere blown away
4. light from event 3 reaches you
5. you and aliens depart Earth (you hope!)
6. Earth atmosphere blown away -
Plot WORLDLINES labeled with the following CAPITAL LETTERS.
A. your worldline
B. worldline of Earth
C. aliens' worldline
D. worldline of Sun
E. worldline of Venus
F. worldline of light from Sun's explosion
G. worldline of the "speed-one-half" pulse of particles from Sun's explosion
H. worldline of light emitted when Venus loses atmosphere
J. terminal part of the worldline of the laser cannon pulse fired at Sun by the aliens - Write numerical values for the speed \(v=\) \(v_{\text {conv }} / c\) on every segment of all worldlines.
5-7 the runner on the train paradox
A letter sent to the Massachusetts Institute of Technology by Hsien-Yen Tsao of Los Angeles poses the following paradox, which he asserts disproves the theory of relativity. The Chairman of the Physics Department sends the inquiry along to you, asking you to respond to Mr. Tsao. You determine to make the answer clear, concise, decisive, and polite-a personal test of your diplomacy and grasp of relativity.
The setting: A train travels at high speed. A runner on the train sprints toward the back of the train with the same speed (with respect to the train) as the train moves forward (with respect to Earth). Therefore the runner is not moving with respect to Earth.
The paradox: We know that, crudely speaking, clocks on the train run "slow" compared to the Earth clock. We also know that the runner's clock runs "slow" compared to the train clocks. Therefore the runner's clock should run "doubly slow" with respect to the Earth clock. But the runner is not moving with respect to Earth! 'Therefore the runner's clock must run at the same rate as the Earth clock. How can it possibly be that the runner's clock runs "doubly slow" with respect to the Earth clock and also runs at the same rate as the Earth clock?
5-8 the twin paradox put to rest - a worked example
Motto: The swinging line of simultaneity tells all!
Combine the Lorentz transformation with the spacetime diagram to clear up - once and for all! - the solution to the Twin Paradox. An astronaut travels from Earth to Canopus (Chapter 4) at speed \(v_{\text {rel }}=\) \(99 / 101\) , arriving at Canopus \(t^{\prime}=20\) years later according to her rocket clock, \(t=101\) years later according to Earth-linked clocks - which means that the stretch factor \(\gamma\) has the value 101/20.
The key idea is "lines of simultaneity" (boxed labels in the figure). A line of simultaneity connects events that occur "at the same time." But events simultaneous in the Earth ("laboratory") frame are typically not simultaneous in the rocket frame (Section 3.4). Horizontal is the line of simultaneity on the Earth ("laboratory") spacetime map that connects events occurring at the same time in the Earth frame. Totally different 一 not a horizontal line! 一is a line of simultaneity on the Earth spacetime map that connects events simultaneous in the outgoing astronaut frame. To draw this line of outgoing-astronaut simultaneity, start with the inverse Lorentz transformation equation for time:
\[t^{\prime}=-v_{\mathrm{rel}} \gamma_{x}+\gamma t \nonumber\]
For the outgoing astronaut, \(v_{\text {rel }}=99 / 101\) and \(\gamma=\) \(101 / 20\) . We want the line of simultaneity that passes through turnaround event \(T\) . So let \(t^{\prime}=20\) years. Then:
\[20=-(99 / 101)(101 / 20) x+(101 / 20) t \nonumber\]
Multiply through by \(20 / 101\) :
\[400 / 101=-(99 / 101) x+t \nonumber\]
which yields
\[t=0.980 x+3.96 \nonumber\]
This is the equation for a straight line passing through event points \(A\) and \(T\) in the spacetime diagram. It is the line of simultaneity for the outgoing astronaut, connecting all events simultaneous with the arrival of the rocket at Canopus (simultaneous in that frame). Among these events is event \(A\) , the Earth clock reading of \(3.96\) years, which occurs at Earth position \(x=0\) . In brief, at the moment the rocket arrives at Canopus, the Earth clock reads \(3.96\) years as observed in the outgoing rocket frame.
To go back over the astronaut trip while looking at the spacetime map is (finally!) to solve the Twin Paradox. As the astronaut travels outward toward Canopus, many colleagues follow her at the same speed, with clocks synchronized in her frame. As they whiz past Earth, each records the reading on the Earth clock. Later analysis leads them to agree that the time between ticks of Earth's clock is longer than the time between ticks of their own outward-moving clocks. (They say, "The Earth clock runs slow.") At any event point on her outward worldline, the astronaut's line of simultaneity slopes upward to the right in the Earth spacetime diagram, as shown in the figure. Simultaneous with astronaut arrival at Canopus (event \(T\) , when all outward-moving clocks read 20 years), one of her colleagues reads a time \(3.96\) years on the Earth clock (event A).
Now the astronaut jumps from the outward-moving rocket to a returning rocket. She inherits a completely new set of colleagues, with a new set of synchronized clocks. The astronaut's new line of simultaneity slopes upward to the left in the Earth spacetime diagram. Simultaneous with her departure from Canopus (event \(T\) , when all inward-moving clocks read 20 years), one of her new colleagues reads a time \(202-\) \(3.96=198.04\) years on the passing Earth clock (event \(B\) ). Thereafter new colleague after new colleague streaks past Earth, recording the fact that Earth clock ticks are farther apart in time than the ticks on their own clocks. (They say, "The Earth clock runs slow.")
The analysis so far accounts for the short time segments \(O A\) and \(B C\) recorded by the Earth clock on its vertical worldline \(A C\) . What about the omitted time lapse \(A B\) ? This is recorded, sure enough, by the Earth clock plowing forward along worldline \(O C\) in its comfortable single free-float frame. However, the story of time \(A B\) is quite different for the turn-around astronaut. Before she reaches turnaround at \(T\) , events on line \(A B\) are in her future. All those Earth clock ticks are yet to be recorded by her outgoing colleagues. These events lie above her line of simultaneity \(A T\) as she arrives at Canopus at \(T\) . However, as she turns around, her line of simultaneity also slews forward, swinging from line \(A T\) to line \(B T\) . Suddenly the events on line \(A B\) - all those intermediate ticks of the Earth clock - are in the astronaut's past. These events lie below the line of simultaneity \(B T\) as she starts back at \(T\) . Her outward-moving colleague reads \(3.96\) years on the Earth clock as she reaches Canopus; an instant later on her clock, her new inward-moving colleague reads \(198.04\) on the Earth clock.
Shall we say that the Earth clock "jumps ahead" as the astronaut turns around? No! Utterly ridiculous! For what single observer does it jump ahead? Not for the Earth observer. Not for the outgoing set of clock readers. Not for the returning set of clock readers. For whom then? Nobody! At the same time as she reaches Canopus-old meaning of simultaneous! - the astronaut's outgoing colleague records \(3.96\) years for the Earth clock. At the same time as she leaves Canopus - new meaning of simultaneous! - her new ingoing colleague records \(198.04\) years on the Earth clock. The astronaut has nobody but herself to blame for her misperception of a "jump" in the Earth clock reading.
The "lost Earth time" \(A B\) in the figure makes consistent the story each observer tells about the clocks. Simple is the story told by the Earth observer: "My clock ticked along steadily at the 'proper' rate from astronaut departure to astronaut return. In contrast, ticks on the astronaut clock were far apart in time on both the outgoing and incoming legs of her trip. We agree that her total ticks are less than my total ticks: she is younger than I when we meet again." More complicated is the astronaut account of clock behavior: "Ticks on the Earth clock were far apart in time as I traveled to Canopus; ticks on the Earth clock were also far apart as I traveled home again. But as I turned around, a whole bunch of Earth clock ticks went from my future to my past. This accounts for the larger number of total ticks on the Earth clock than on my clock during the trip. We agree that I am younger when we meet again."
So saying, the astronaut renounces her profession and becomes a stand-up comedian.
Reference: E. Lowry, American Journal of Physics , Volume 31, page 59 (1963) .


The Domains of Life
As of the 1970's, life has been classified into three domains: Bacteria, Archaea, and Eukarya (shown in Figure \(\PageIndex{1}\) as Eukaryota). All known life on Earth is thought to have descended from a single common ancestor: LUCA (last universal common ancestor). The relationship between the three domains of life is depicted in the phylogenetic tree below. This diagram represents Archaea and Eukarya as sister taxa, more closely related than either group is to Domain Bacteria.
Figure \(\PageIndex{1}\): "A
phylogenetic tree
of living things, based on RNA data and proposed by Carl Woese, showing the separation of bacteria, archaea, and eukaryotes. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, thanks to long branch attraction. The exact relationships of the three domains are still being debated, as is the position of the root of the tree. It has also been suggested that due to lateral gene transfer, a tree may not be the best representation of the genetic relationships of all organisms. For instance some genetic evidence suggests that eukaryotes evolved from the union of some bacteria and archaea (one becoming an organelle and the other the main cell)." This vector version produced in 2006: Eric Gaba (Sting, fr:Sting), Cherkash, Public domain, via Wikimedia Commons.
Relationships Within Eukarya
Depicting Relationships
Branching tree diagrams (like the phylogenies shown above) are used to communicate a hypothesis for how organisms are related. In Figure \(\PageIndex{3}\) and Figure \(\PageIndex{4}\), some example trees are drawn to show how this works.
Figure \(\PageIndex{3}\): These diagrams (cladograms) show hypotheses for four different
taxa
: A, B, C, and D. These taxa could represent individuals, different species, or any other level of classification. Trees 1a and 1b are drawn differently but represent the same hypothesis: C and D are most closely related (sister taxa) and A is the most ancestral lineage. Trees 2a and 2b represent a slightly different hypothesis: C and D are sister taxa, but A and B are also sister taxa; neither lineage is depicted as "older" than the other. This last relationship is also represented in the "Unrooted" tree: there is no hypothesis in this tree for which group is most ancestral (no "root"). Images drawn by Maria Morrow,
CC-BY
.
Figure \(\PageIndex{4}\): In these four trees, we are again looking at hypotheses for how A, B, C, and D are related. However, in these trees we have some information about why these determinations were made: traits have been included. In trees 1 and 2, vascular tissue and megaphylls have been added to the two different hypotheses presented in Figure \(\PageIndex{3}\). Vascular tissue appears on the root of the trees as an ancestral trait. This means that A, B, C, and D all have vascular tissue. Megaphylls appears in both trees as a shared derived characteristic (synapomorphy) of C and D. This means C and D have megaphylls, but A and B do not. Both tree 1 and tree 2 are equally parsimonious. In trees 3 and 4, a third trait has been added: heterospory. In addition to the information from trees 1 & 2, we now see that A and B are heterosporous, while C and D are not. In tree 3, heterospory appears as an ancestral trait which is lost in C and D. In tree 4, heterospory is a synapomorphy of A and B. Tree 3 has four changes, while tree 4 only has three. This means that tree 4 is more parsimonious and therefore more likely (though not necessarily correct!). Images drawn by Maria Morrow,
CC-BY
.