Humans vary more than most similar-sized mammals in size and body shape but less in genetic patterns.
Human variation in skin color is due to differential density of melanin in the epidermal layers. All humans have the same number of melanocytes, but the activity of these cells is related to adaptations to UV light stress. More melanin production leads to darker skin color.
Human body size and shape are determined in large part by natural selection, gene flow, and nutrition. Allen's and Bergmann's rules demonstrate the mammalian-wide patterns of heat/cold stress on body form. Combined with gene flow and cultural patterns, these stresses impact the distribution of body size and shape in human populations.
Human males and females have about a 15% difference in body size, as well as specific morphological differences in skeletal and muscle morphology.
Human cranial form is quite flexible, but nearly 80% of cranial variation is found within populations, and only about 20% is found between populations. Forensic scientists can identify clusters of cranial variation within populations and between populations; however, these clusters generally reflect gene flow and other evolutionary processes.
Disease may play an important role in human evolution. Some populations have higher allele frequencies for certain genetic disorders and so may be more susceptible to those disorders. Human cultural modification of the environment can alter the patterns and contexts of natural selection through disease patterns.
Blood group allele frequencies vary across human populations due to both natural selection and gene flow.
Human genetic variation is surprisingly distributed. Nearly 85% of the genetic variation in the species Homo sapiens sapiens is found in each H.s.s population. The overall genetic Fst for our species is approximately 0.17, well below the level for subspecies distinction.
There is no evidence from any morphological or genetic measures that supports the division of modern humans into biological races or subspecies. No morphologically or genetically unique or even substantially divergent populations of humans can currently be divided into specific geographic clusters. Despite this, there is a continued use of Linnaean like taxonomies for humanity that place humans into the "big few" categories of Asian, Black, and White.
In the United States we use socially constructed race categories rather than biological ones. Comparing differences in socially constructed race categories cannot involve biological comparisons or causes, only sociocultural ones. The history of colonial expansion, Linnaean classification, the eugenics movement, and specific aspects of U.S. history all contribute to the use of these categories.
