Balancing Selection Shared Between Humans and Chimpanzees

The new issue of Science will appear tomorrow, but I still wanted to spend some more words on the current issue, which has clearly been a treat for the ecologically minded reader – not only did it solve the mystery of fairy circles, but there are also two papers on pollination (services) and a quite interesting study that demonstrates the importance of mycorrhiza and other fungi for the accumulation of soil-organic matter in boreal forests (see also a blog post here).

And there is the study I want to highlight here, namely “Multiple Instances of Ancient Balancing Selection Shared Between Humans and Chimpanzees” by Ellen M. Leffler and colleagues, which I found highly interesting. They test for instances of balancing selection, i.e.

adaptation that leads to the persistence of variation in a population or species in the face of stochastic loss by genetic drift.

As the authors explain, it is difficult to search for balancing selection in the genome of one species alone, as

patterns of diversity can occur by chance because of the tremendous variance in coalescence times due to genetic drift alone (14). As an illustration,
under a simple demographic model with no selection, the probability that two human lineages do not coalesce before the split with chimpanzee is on the order of 10−4 (15, 16). Although this probability is small, the human genome is large, and so many such regions could occur by chance.

To overcome this problem, the study used

complete genome sequences from 59 humans from sub-Saharan Africa (Yoruba) (20) and 10 Western chimpanzees (Pan troglodytes verus) (21) in order to identify shared polymorphisms— namely, high-quality orthologous SNPs with identical alleles in the two species

The methods for identification of shared polymorphisms and also the interpretation of the identified genes in the conclusions seemed a bit voodoo to me, but given that I know very little on this field I was happy to give the authors the benefit of the doubt and simply enjoy this very nicely written and clearly motivated paper.

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