In today’s world skin, hair and eye colour are often the basis of “racial” differentiation and then of much of the racism that exists. The population of Europe is a genetic mix of migrations and genetic evolution and while migratory effects may have been dominant in the ancient past and again in modern times, the genetic evolution by selection during the last 5,000 years – when large scale migrations have been less prevalent – is quite considerable.
Ancient DNA analysis is now providing a treasure trove of direct evidence for genetic changes over thousands of years. 5,000 years ago is about 250 generations ago and that is apparently a sufficient number of generations for the genetic effects of “natural” selection to show up.
But over this time scale the selection was probably more due to sexual selection consequent to the attractiveness and selection of partners and kin-groups rather than for a selection to better meet environmental surroundings. A new paper addresses the changes in Europe over the last 5,000 years.
The suggestion is that while skin colour was clearly darker in Europe 5,000 years ago and the changes could be explained by sunlight deficiencies at northern latitudes, it does not explain changes in skin and eye colour. It could be that these have changed genetically due to the selection of partners based on “attractiveness”.
“Instead, it may be that lighter hair and eye color functioned as a signal indicating group affiliation, which in turn played a role in the selection of a partner.” Sexual selection of this kind is common in animals and may also have been one of the driving forces behind human evolution over the past few millennia.
Wilde, Sandra et al. Direct evidence for positive selection of skin, hair, and eye pigmentation in Europeans during the last 5,000 years. Proceedings of the National Academy of Sciences, 10 March 2014 DOI: 10.1073/pnas.1316513111
Johannes Gutenberg University Mainz Press Release:
There has been much research into the factors that have influenced the human genome since the end of the last Ice Age. Anthropologists … have analyzed ancient DNA from skeletons and found that selection has had a significant effect on the human genome even in the past 5,000 years, resulting in sustained changes to the appearance of people. …… This involved analyzing DNA from archaeological skeletons and then comparing the prehistoric data with that of contemporary Europeans using computer simulations. Where the genetic changes could not be explained by the randomness of inheritance, the researchers were able to infer that positive selection played a role, i.e., that frequency of a certain mutation increased significantly in a given population. …
Sandra Wilde of the Palaeogenetics Group at the JGU Institute of Anthropology noticed striking differences in genes associated with hair, skin, and eye pigmentation. “Prehistoric Europeans in the region we studied would have been consistently darker than their descendants today,” says Wilde, first author of the PNAS article. “This is particularly interesting as the darker phenotype seems to have been preferred by evolution over hundreds of thousands of years. All our early ancestors were more darkly pigmented.” However, things must have changed in the last 50,000 years as humans began to migrate to northern latitudes.
“In Europe we find a particularly wide range of genetic variation in terms of pigmentation,” adds co-author Dr. Karola Kirsanow, who is also a member of the Palaeogenetics Group at Mainz University. “However, we did not expect to find that natural selection had been favoring lighter pigmentation over the past few thousand years.” The signals of selection that the Mainz palaeogeneticists and their colleagues at University College London have identified are comparable to those for malaria resistance and lactase persistence, meaning that they are among the most pronounced that have been discovered to date in the human genome. The authors see several possible explanations. “Perhaps the most obvious is that this is the result of adaptation to the reduced level of sunlight in northern latitudes,” says Professor Mark Thomas of UCL, corresponding author of the study. “Most people of the world make most of their vitamin D in their skin as a result UV exposure. But at northern latitudes and with dark skin, this would have been less efficient. If people weren’t getting much vitamin D in their diet, then having lighter skin may have been the best option.”
“But this vitamin D explanation seems less convincing when it comes to hair and eye color,” Wilde continues. “Instead, it may be that lighter hair and eye color functioned as a signal indicating group affiliation, which in turn played a role in the selection of a partner.” Sexual selection of this kind is common in animals and may also have been one of the driving forces behind human evolution over the past few millennia.
“We were expecting to find that changes in the human genome were the result of population dynamics, such as migration. In general we expect genetic changes due to natural selection to be the exception rather than the rule. At the same time, it cannot be denied that lactase persistence, i.e., the ability to digest the main sugar in milk as an adult, and pigmentation genes have been favored by natural selection to a surprising degree over the last 10,000 years or so,” adds Professor Joachim Burger, senior author of the study. “But it should be kept in mind that our findings do not necessarily mean that everything selected for in the past is still beneficial today. The characteristics handed down as a result of sexual selection can be more often explained as the result of preference on the part of individuals or groups rather than adaptation to the environment.”
Abstract: Pigmentation is a polygenic trait encompassing some of the most visible phenotypic variation observed in humans. Here we present direct estimates of selection acting on functional alleles in three key genes known to be involved in human pigmentation pathways—HERC2, SLC45A2, and TYR—using allele frequency estimates from Eneolithic, Bronze Age, and modern Eastern European samples and forward simulations. Neutrality was overwhelmingly rejected for all alleles studied, with point estimates of selection ranging from around 2–10% per generation. Our results provide direct evidence that strong selection favoring lighter skin, hair, and eye pigmentation has been operating in European populations over the last 5,000 y.