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“There’s no map
To human behaviour
They’re terribly moody
Then all of a sudden turn happy…”

Bjork - “Human Behavior”

Evolution of species is driven by selective forces, usually changes in the environment that require a species to adapt or to die out. For example, a plant may be living happily in a marshy environment, perfectly adapted to a wet climate. Then a drought hits their environment. Year after year more of these plants dies out because they can not cope with a dry climate. However, if one member of this population develops a genetic mutation that confers some sort of drought tolerance and is able to breed then the offspring of this plant are given a selective advantage and will outcompete the non-mutated members of the population. Ultimately, if the drought continues, only mutated drought resistant plants will remain.

Since the advent of agriculture, humans have been applying their own selective pressure on species that come into contact with them. A rudimentary understanding of genetics is not necessary to select for higher yielding wheat or meatier cattle or woollier sheep. There is evidence that since humans began farming this type of artificial selection has been a major factor in our species’ own evolutionary success. Along with intentional selective pressure on agriculture, we apply selective pressure indirectly as well on species that we hunt or whose environment we destroy or modify. So, I guess it’s really no surprise that we have been applying artificial selection to ourselves as well.

This is what a large consortium of researches along with scientists from MIT, Harvard and Massachusetts General Hospital, describe in the most recent issue of Nature. It is not the first time that scientists have discovered evidence of selection in humans, there are two cases that have been published in the last couple of years looking at selection in single genes. What is novel about this research is the sheer scale.

First, we need to define some terms. Whenever we’re looking at selection, we’re looking at genetic mutations, in other words a change to the DNA sequence. Not all mutation is bad, a change that results in a gene having a new function can be beneficial. Positive selection refers to a beneficial mutation being an evolutionarily advantage and organisms with that mutation outcompeting organisms without it. After several generations that mutation may become fixed - in other words, all of the organisms in the population now bear the beneficial mutation. Most mutations are thought to be harmful, when a harmful change occurs, the negative (or purifying) selection eliminates the mutation through what most of us thing of as survival of the fittest. Organisms bearing the harmful mutation are not as fit as those without it and if they do breed their progeny are not as fit as others. After only a few generations, harmful mutations are eliminated from a population by negative selection.

The goal of the research described in this week’s Nature was to explore the recent evolutionary history of humans. To this end, the researchers used DNA sequencing to look for single nucleotide polymorphisms (SNP) in different human populations. A SNP is a type of mutation, one where one letter of the genetic information is changed to a different letter. This may or may not effect a gene, most SNPs are silent mutations - ones that do not change a gene’s function. What these researchers are interested in is SNPs that do effect a gene’s function, are recent changes and are present in one population but not in another. In this work DNA of over 400 people from three populations - European, central African and Japanese or Chinese - were analyzed. The researchers sequenced over 3,000,000 SNPs and filtered through them until finding 22 regions that fit the criteria in which they were interested.

Of these 22, two regions represent SNPs in genes that have already been reported to be different in different populations which demonstrates that their analysis is working. Of the remaining 20, three were particularly interesting to the researchers as showing evidence of selection in one of the three populations. The first of these was a gene known as SLC45A2. The ortholog of this gene is zebrafish, mouse and horses is important in pigmentation. A particular SNP showed strong evidence of positive selection in the European populations, but was absent in the Asian and African populations. This permutation of the SLC45A2 gene is associated with fair skin and non-black hair. These data can be interpreted as positive selection in European populations for fair skin and hair.

The second case of positive selection in a single population involved two genes, LARGE and DMD, which were associated with one SNP in African populations and a different SNP in Asian and European populations. LARGE and DMD encode proteins that are required for the Lassa fever virus (endemic to Nigeria) to infect and spread. The African permutations of these genes presumably render some resistance to the Lassa virus. Since Asian and European populations are not generally exposed to the virus there is no selective pressure.

The final regions of interest surround two genes, EDAR and EDA2R, whic are involved in the ectodysplasin pathway - important for development of hair, teeth and exocrine (sweat, tear, and mammary) glands. Mutations in these genes are present in Asian populations and absent in European and Africa. Interestingly, the Asian mutation is also present in American aboriginal populations which indicates that it was fixed prior to the migration from Asia into the Americas which occurred 15 - 20,000 years ago. Defects in these genes result in a condition known as hypohidrotic ectodermal dysplasia, which results in hair, teeth and skin defects. The mutation present in the Asian populations does not result in this disorder, but it is not clear what it does do. However, because the mutation is fixed and has been so for a relatively long time, it is safe to assume that it confered some sort of evolutionary advantage.

This work is revolutionary largely because of the techniques employed. This type of analysis would have been virtually impossible even five years ago just due to the huge amount of data generated and the analysis of that data. What I think is more interesting is the clues that the results give us into our ancestral behavior and indicates that we’ve been applying selection - intentionally or inadvertantly, on ourselves for thousands of years.

Image Credits:

Evolution cartoon

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