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Balancing selection can maintain polymorphisms in natural populations for extended periods of evolutionary time. However, in this paper, Dannemann et al. 2016 identify three archaic haplotypes in the modern human genome that are due to adaptive introgression. Why can we exclude that haplotypes III, IV and VII (the introgressed haplotypes) have remained in the human gene pool since the split between modern humans and Neanderthals because of balancing selection?

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    $\begingroup$ I'm closing this question because based on a discussion in the comments it seems that the premise of the question is a homework assignment that isn't reproduced here. $\endgroup$
    – Bryan Krause ♦
    May 2, 2022 at 2:28

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Balancing selection and positive selection are not mutually exclusive. Neither are balancing selection and negative selection. Sickle cell is a classic example where there is negative selection against the homozygote for the sickling mutation, and positive selection for the heterozygote. The result is that there both alleles are maintained in the population under balancing selection.

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  • $\begingroup$ Thank you. I understand that both balancing and positive selection can occur together (along with introgression). In the paper, the authors present evidence for positive selection of the archaic haplotypes. However, I'm wondering if there is conclusive evidence to exclude balancing selection in this case? (The question comes from a homework assignment, so balancing selection is already presumed to be excluded...) $\endgroup$
    – sq99
    Dec 17, 2018 at 22:37
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    $\begingroup$ There is nothing in the paper about balancing selection being excluded. Are you saying that the only reason you think it's excluded is because a homework assignment is telling you this? Also, you keep mentioning introgression along with selection--those two terms need not be related at all. Introgression in the absence of selective signatures is common. $\endgroup$
    – DavidR
    Dec 18, 2018 at 1:00
  • $\begingroup$ Yes, the question assumes it's excluded which is why I'm wondering what evidence can be drawn from the paper for that? From what I understand, introgression and balancing selection can leave similar genomic signatures so it would be difficult to distinguish between the two of them. $\endgroup$
    – sq99
    Dec 18, 2018 at 16:02

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