17 Population Biology and Population Genetics of Pleistocene Hominins
The population genetics of Pleistocene hominins is deduced from three types of data: coalescent processes and haplotype trees estimated from surveys of genetic variation in present-day human populations, haplotypes inferred from ancient DNA extracted from fossils, and overlays of current quantitative genetic variance/covariance matrices upon hominin fossils. The haplotype trees are subjected to nested clade phylogeographic analyses, including many new analyses never before published. These analyses show that there were three major expansion events of hominins out of Africa during the last 2 million years. The first expansion event marked the original dispersal of Homo erectus out of Africa into Eurasia. The quantitative genetic analysis of hominin fossils indicates that there was relaxed selection upon at least some morphological features at this time, perhaps due to an increased use of cultural inheritance in dealing with the environment. Coalescent analyses indicate that the colonization of Eurasia was marked by strong selection at many loci, so although morphological selection may have been relaxed, adaptive processes were still proceeding as humans colonized this new geographical area. Eurasian and African populations also established recurrent gene flow restricted by isolation by distance by 1.46 million years ago. A second expansion out of Africa was marked by the spread of the Acheulean culture, implying that the spread of this culture was due to a spread of peoples and not just ideas. The expanding Acheulean populations interbred with existing Eurasian populations, and recurrent gene flow continued after the Acheulean expansion. A third expansion out of Africa marked the spread of many anatomically modern traits that had earlier appeared in Africa. This expansion was also marked by interbreeding, so regional continuity persisted for some traits. Although total replacement of Eurasian populations is rejected with a p < 10–17, it is still possible that some local populations were replaced. Ancient DNA studies are inconclusive about the status of Neanderthals in this regard. Coalescent studies are also inconclusive and contradictory about the size of hominin populations before this last out-of-Africa expansion and the degree of population growth during the expansion phase. Because of interbreeding and gene flow, humanity evolved into its modern form as a single evolutionary lineage but with some geographical differentiation at any given time.
KeywordsExpansion Event Census Size Eurasian Population Coalescent Analysis Infinite Site Model
I am grateful for support by NIH grant GM065509. I also want to thank Rebecca Ackermann and James Cheverud for kindly making their manuscript on hominin evolution available to me prior to its publication.
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