H. sapiens is one variable species that is held together through large-scale gene flow. Between populations with contrasting allele frequencies there exist populations with intermediate frequencies. This phenomenon is known as a cline, or an allele frequency gradient in space held together by gene flow. Human variation represented by a cline is continuous in spectrum and there are no discrete groups. Therefore, the concept of "race" as is used today has been deemed horribly flawed and should be replaced with a notion of continuous geographic diversity in biology and culture.

Although, the modern scientific mantra that "there is no biological basis for race" is true on many levels, it is also misleading. Obviously, there are regional phenotypic differences among populations of people and these are evident in, for instance, skin color, hair color and texture, eye color and shape, nose shape, height, limb lengths and body proportions, skull and face shape, ear wax type, blood type, disease susceptibility and resistance, body fat distribution, high altitude adaptations, lactose tolerance, and so on. These differences are caused by random genetic drift, sexual selection, differing levels of gene flow, and of course, regional environmental adaptations. Regional variation also records ancestral history. The eye shape and hair texture of Native American Indians reflects their ancestral Asian origins. The traits we use to identify "races" are nearly always continuous traits that exist to varying degrees in all human populations.

Nuclear DNA studies of geographic variation have shown that the traits that reflect regional variation are determined by a tiny fraction of our genes.

Figure 6.2 Arrows on the map of the Old World show approximate dates for the first arrival of modern humans to geographic regions, including the first dispersal to the New World (the Americas) by about 30 Kya. Illustration by Jeff Dixon.

Plus, an underwhelming 15 percent of human genetic variation separates major human groups. Compared to various species of large-bodied mammals with excellent dispersal abilities (e.g., the gray wolf and white-tailed deer), humans have very little variation. Most human diversity exists as differences among individuals within populations. That is, there is more variation within populations than between them.

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