It's no surprise that natural selection does not always take an evolutionary time scale. 

When thousands of knights died during the Crusades at Acre, natural selection was being channeled. Yes, there were still other mechanisms of evolution but when the available pool of people is changed, positive selection is changed also. Pinpointing that on a time scale can be tough but there has been much research into the factors that have influenced the human genome since the end of the last Ice Age and a team of anthropologists, geneticists and archaeologists 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.



A grave with an about 5,000-year-old skeleton from a kurgan of the Yamnaya culture near the town Kirovograd in Ukraine. The Kurgan was also one of the best villains of all time, in "The Highlander" film. Credit: Alla V. Nikolova

While investigating numerous genetic markers in archaeological and living individuals, first author Sandra Wilde of the Palaeogenetics Group at the 
Johannes Gutenberg Universitaet Mainz
 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. 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."


"In Europe we find a particularly wide range of genetic variation in terms of pigmentation," adds co-author Dr. Karola Kirsanow, 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 they 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 corresponding author professor Mark Thomas of University College London. "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."



 Article: "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 (PNAS).
Source: Johannes Gutenberg Universitaet Mainz