Abstract
Human evolution exhibits repeated speciations and conspicuous morphological change: fromAustralopithecustoHomo habilis, H. erectus,andH. sapiens;and from their hominoid ancestor to orangutans, gorillas, chimpanzees, and humans. Theories of founder-event speciation propose that speciation often occurs as a consequence of population bottlenecks, down to one or very few individual pairs. Proponents of punctuated equilibrium claim in addition that founder-event speciation results in rapid morphological change. The major histocompatibility complex (MHC) consists of several very polymorphic gene loci. The genealogy of 19 human alleles of theDQB1locus coalesces more than 30 million years ago, before the divergence of apes and Old World monkeys. Many human alleles are more closely related to pongid and cercopithecoid alleles than to other human alleles. Using the theory of gene coalescence, we estimate that these polymorphisms require human populations of the order ofN= 100,000 individuals for the last several million years. This conclusion is confirmed by computer simulations showing the rate of decay of the polymorphisms over time. Computer simulations indicate, in addition, that in human evolution no bottlenecks have occurred with fewer than several thousand individuals. We evaluate studies of mtDNA, Y-chromosome, and microsatellite autosomal polymorphisms and conclude that they are consistent with the MHC result that no narrow population bottlenecks have occurred in human evolution. The available molecular information favors a recent African origin of modern humans, who spread out of Africa approximately 100,000 to 200,000 years ago.
Highlights
The question arises whether the notable morphological changes that have occurred in hominoid evolution may have been triggered by population bottlenecks, perhaps even extreme bottlenecks down to only one or a few pairs of individuals, as postulated by theories of founder-event speciation
The theory of punctuated equilibrium makes the additional claim that bursts of morphological change occur in association with speciation, whereas stasis prevails over the long time intervals between speciation events
In the present paper we investigate another class II locus, the DQB1 locus, at which many alleles are known in humans as well as in other primates
Summary
According to this theory, extreme population bottlenecks would facilitate speciation as well as rapid morphological change. Earlier species-splitting events include at least the successive divergence of the orangutan, gorilla, and chimpanzee lineages; and earlier still, the divergence of the hylobatids. The question arises whether the notable morphological changes that have occurred in hominoid evolution may have been triggered by population bottlenecks, perhaps even extreme bottlenecks down to only one or a few pairs of individuals, as postulated by theories of founder-event speciation. The theory of punctuated equilibrium makes the additional claim that bursts of morphological change occur in association with speciation, whereas stasis prevails over the long time intervals between speciation events
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