section_path: Biology/Biological variation
section_name: Biological variation
paragraph_text: Biological variation
  
  There is biological variation in the human species – with traits such as blood type, genetic diseases, cranial features, facial features, organ systems, eye color, hair color and texture, height and build, and skin color varying across the globe. The typical height of an adult human is between 1.4 and 1.9 m (4 ft 7 in and 6 ft 3 in), although this varies significantly depending on sex, ethnic origin, and family bloodlines. Body size is partly determined by genes and is also significantly influenced by environmental factors such as diet, exercise, and sleep patterns.
  
  There is evidence that populations have adapted genetically to various external factors. The genes that allow adult humans to digest lactose are present in high frequencies in populations that have long histories of cattle domestication and are more dependent on cow milk. Sickle cell anemia, which may provide increased resistance to malaria, is frequent in populations where malaria is endemic. Populations that have for a very long time inhabited specific climates tend to have developed specific phenotypes that are beneficial for those environments – short stature and stocky build in cold regions, tall and lanky in hot regions, and with high lung capacities or other adaptations at high altitudes. Some populations have evolved highly unique adaptations to very specific environmental conditions, such as those advantageous to ocean-dwelling lifestyles and freediving in the Bajau.
  
  Human hair ranges in color from red to blond to brown to black, which is the most frequent. Hair color depends on the amount of melanin, with concentrations fading with increased age, leading to grey or even white hair. Skin color can range from darkest brown to lightest peach, or even nearly white or colorless in cases of albinism. It tends to vary clinally and generally correlates with the level of ultraviolet radiation in a particular geographic area, with darker skin mostly around the equator. Skin darkening may have evolved as protection against ultraviolet solar radiation. Light skin pigmentation protects against depletion of vitamin D, which requires sunlight to make. Human skin also has a capacity to darken (tan) in response to exposure to ultraviolet radiation.
  
  There is relatively little variation between human geographical populations, and most of the variation that occurs is at the individual level. Much of human variation is continuous, often with no clear points of demarcation. Genetic data shows that no matter how population groups are defined, two people from the same population group are almost as different from each other as two people from any two different population groups. Dark-skinned populations that are found in Africa, Australia, and South Asia are not closely related to each other.
  
  Genetic research has demonstrated that human populations native to the African continent are the most genetically diverse and genetic diversity decreases with migratory distance from Africa, possibly the result of bottlenecks during human migration. These non-African populations acquired new genetic inputs from local admixture with archaic populations and have much greater variation from Neanderthals and Denisovans than is found in Africa, though Neanderthal admixture into African populations may be underestimated. Furthermore, recent studies have found that populations in sub-Saharan Africa, and particularly West Africa, have ancestral genetic variation which predates modern humans and has been lost in most non-African populations. Some of this ancestry is thought to originate from admixture with an unknown archaic hominin that diverged before the split of Neanderthals and modern humans.
  
  Humans are a gonochoric species, meaning they are divided into male and female sexes. The greatest degree of genetic variation exists between males and females. While the nucleotide genetic variation of individuals of the same sex across global populations is no greater than 0.1%–0.5%, the genetic difference between males and females is between 1% and 2%. Males on average are 15% heavier and 15 cm (6 in) taller than females. On average, men have about 40–50% more upper-body strength and 20–30% more lower-body strength than women at the same weight, due to higher amounts of muscle and larger muscle fibers. Women generally have a higher body fat percentage than men. Women have lighter skin than men of the same population; this has been explained by a higher need for vitamin D in females during pregnancy and lactation. As there are chromosomal differences between females and males, some X and Y chromosome-related conditions and disorders only affect either men or women. After allowing for body weight and volume, the male voice is usually an octave deeper than the female voice. Women have a longer life span in almost every population around the world. There are intersex conditions in the human population, however these are rare.