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Section Summary
Darwin’s Theory
Guide for Reading
■ What important observations did Darwin make on his voyage?
What hypothesis did Darwin make to explain the differences
between similar species?
How does natural selection lead to evolution?
In 1831, Charles Darwin left England on board the HMS Beagle. Darwin’s
important observations included the diversity of living things, the
remains of ancient organisms, and the characteristics of organisms on the
Galápagos Islands. Darwin saw many different species. A species is a group
of similar organisms that can mate with each other and produce fertile
offspring. Darwin saw the fossil bones of animals that had died long ago. A
fossil is the preserved remains or traces of an organism that lived in the past.
In 1835, the Beagle reached the Galápagos Islands in the Pacific Ocean.
Darwin was surprised that many of the plants and animals on the Galápagos
Islands were similar to organisms on mainland South America. However,
there were also important differences. Darwin inferred that a small number of
different species had come to the islands from the mainland. Eventually, their
offspring became different from the mainland relatives. The finches on the
Galápagos Islands were noticeably different from one island to another. The
most obvious differences were the varied sizes and shapes of the birds’ beaks.
Beak shape is an example of an adaptation, a trait that helps an organism
survive and reproduce. Darwin reasoned that plants or animals that arrived
on the Galápagos Islands faced conditions that were different from those on
the mainland. Perhaps, Darwin hypothesized, the species gradually
changed over many generations and became better adapted to the new
conditions. The gradual change in a species over time is called evolution.
In his book The Origin of Species, Darwin proposed that evolution occurs by
means of natural selection. Natural selection is the process by which
individuals that are better adapted to their environment are more likely to
survive and reproduce than other members of the same species. A number of
factors affect the process of natural selection: overproduction, competition,
and variations. Any difference between individuals of the same species is
called a variation. Some variations make certain individuals better adapted to
their environment because of helpful traits they possess. Darwin proposed
that, over a long period of time, natural selection can lead to change.
Helpful variations may gradually accumulate in a species, while
unfavorable ones may disappear. Without variations, all members of a
species would have the same traits. Only traits that are inherited, or controlled
by genes, can be acted upon by natural selection.
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