Wednesday, 16 November 2016

Rapid Human Evolution in America!

Partners from Canada’s northwest coast (from left, Barbara Petzelt, Harold Leighton, Bill Pahl, Wendy Pahl, Yvonne Ryan and Joycelynn Mitchell) collaborated with an international team of researchers on a genetic study of First Nations peoples – both present day and ancient.
Photo courtesy Metlakatla First Nation
A time transect of exomes from a Native American population before and after European contact | Nature Communications.

Right on the heels of the paper showing how a change in the domestic dog's environment drove their evolution, pushing them closer to humans, we have this example of how changes in the environment by one human group can have a major impact on the evolution of another human group.

In this case, the environmental change was in the microbial and viral environment of Native Americans brought about by contact with European colonists.

It has long been recognised that the huge range of viruses carried by Europeans, probably as a result of prolonged close association with domestic animals, was a major factor in softening and debilitating other human populations when Europeans began to colonise the rest of the world. Isolated groups, especially in the New World and the Pacific, had not been exposed to viruses such as mumps, measles and rubella. In a population where these had been endemic for generations, herd resistance was high and, although they weren't harmless, they were mild in comparison to their effect on populations that had never been exposed before. To these people the effects were devastating, leaving a weak and shocked population that could easily be dominated by resistant newcomers.

The viruses had acted as the advance guard of the newcomers, showing how even pathogens can, in the right circumstances, as as symbionts with their carriers. This was explored in some detail in Jared Diamond's, Guns, Germs and Steel.

This paper, published in Nature Communications by a team led by Professor Michael DeGiorgio, and anthropologist with the University of Illinois, shows how contact with Europeans led to a rapid change in allele frequency of a variant of the HLA-DQ associated with resistance to colonization-era diseases.

A major factor for the population decline of Native Americans after European contact has been attributed to infectious disease susceptibility. To investigate whether a pre-existing genetic component contributed to this phenomenon, here we analyse 50 exomes of a continuous population from the Northwest Coast of North America, dating from before and after European contact. We model the population collapse after European contact, inferring a 57% reduction in effective population size. We also identify signatures of positive selection on immune-related genes in the ancient but not the modern group, with the strongest signal deriving from the human leucocyte antigen (HLA) gene HLA-DQA1. The modern individuals show a marked frequency decrease in the same alleles, likely due to the environmental change associated with European colonization, whereby negative selection may have acted on the same gene after contact. The evident shift in selection pressures correlates to the regional European-borne epidemics of the 1800s.

First Nations history mainly consists of oral stories passed from generation to generation. Our oral history tells of the deaths of a large percentage of our population by diseases from the European settlers. Smallpox, for our area, was particularly catastrophic. We are pleased to have scientific evidence that corroborates our oral history. As technology continues to advance, we expect that science will continue to agree with the stories of our ancestors.

Joycelynn Mitchell, a Metlakatla co-author on the study.
Briefly, following the initial contact with disease-carrying Europeans about 175 years ago, the population of the ancestors of modern Coast Tsimshian people declined by some 57%. In these people the variant of HLA-DQ gene which had been beneficial in the prevailing, pre-colonial conditions became disadvantageous, placing another allele under positive selection pressure because its carriers better survived the colonial-era diseases. One of these alleles is now 64% less common than in pre-colonial times.

So here we have a very clear example of rapid human evolution brought about by an environmental change under the extreme selection pressure of one allele facilitating survival of often-lethal disease and the other making the carrier more susceptible. It would take a creationist genius at reinterpretation of everyday words to be able to explain this away as 'not evolution' or not the result of natural selection.

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