>>As noted in the earlier lecture on the distribution
of human genetic variation and the genetic race hypothesis being rejected in humans,
in part this is because on average alleles that are common in one population are typically
common globally. And alleles that are restricted to one geographic region, such as Africa,
are also typically rare in that population. For example, if we find an allele that is
only found in Africa and you happen to be African, you likely don’t have that allele.
If there is an allele that is only found in Europe and you happen to be European, you
likely don’t have that allele. Ex cetera. That said, we do find rare deviations from
this general pattern of human genetic variation, and these deviations are often very relevant
for understanding human biology. The most obvious deviation is skin color.
Cultural perceptions of race often include skin color. Skin color does have a strong
genetic factor. On average, Africans will tend to have similar skin color genetic traits
with each other than they do to most other areas of the world. But this is a very rare
exception to the general rule about human genetic traits. But it’s also misleading.
For example, if we look at skin color globally, we see that using African as a proxy for darker
skin would neglect a large number of Native people both within and outside of Africa.
Moreover, the genetic aspect of skin color reflects a very small part of the functional
human genome. It does not reflect the pattern of functional genetic variation more generally
and more globally. Nevertheless, finding these exceptions to
the rule often provide the basis for the practical importance of human evolution research. These
exceptions occasionally happen from major genetic drift events where an allele that
was previously rare won the chance lottery and happened to increase in frequency in the
population. But such extraordinary genetic drift events are also extraordinarily rare.
As noted in the population genetics section of this class, the other possibility is that
this particular genetic type is under natural selection. This is certainly the case for
skin color. For example, when controlling for sunlight exposure, light skinned people
have a tenfold risk for skin cancer. In fact, skin cancer rate among native African albinos
is estimated to be a startling 23 percent. Clearly this will have an enormous selective
pressure on a population. Darker skin in these high UV areas will be of an incredible reproductive
advantage. Thus, it should not be surprising that 200,000 years ago, when all humanity
lived in Africa, they also all had dark skin. Light skin evolved much later. Now very dark
skin can be too protective of ultraviolet (UV) light in areas that receive less intense
sunlight. Exposure to UV radiation permits your skin to produce Vitamin D, which helps
maintain a strong skeleton. During the peopling of more northern latitudes where the sun is
less intense, there appears to have been at least two separate events in which natural
selection favored lighter skin tones. One event contributed to the European skin tones,
and one event contributed to the Asian skin tones. Today technology such as protective
clothing, and sunscreens, and fortified foods have largely removed these selective pressures
in wealthy countries, but they remain an important consideration in developing countries with
less access to these interventions.