Rapid, Adaptive Human Evolution Facilitated by Admixture in the Americas

Abstract

Humans have migrated from their ancestral homelands in Africa to nearly every part of the world. Human migration is characterized by a recurrent process of physical isolation and genetic diversification followed by admixture, whereby previously isolated populations come together and exchange genes. Admixture results in the introgression of alleles from ancestral source populations into hybrid admixed populations, and introgression can facilitate rapid, adaptive evolution by introducing beneficial alleles at intermediate frequencies. We provide examples of adaptive introgression between archaic and modern human populations and for admixed populations in the Americas, which were formed relatively recently via admixture among African, European, and Indigenous American ancestral populations. Adaptive introgression has had an outsized effect on the human immune system. In light of the ubiquity of admixture in human evolution, we propose that adaptive introgression is a fundamentally important mechanism for driving rapid, adaptive evolution in human populations.