August 24, 2004

Higher female effective population size in recent human evolution

The common maternal ancestor of all living humans appears to be much older than the common paternal ancestor of all living men. It has been suggested that selection on the Y-chromosome is responsible for this discrepancy, that is, that a recent male line has wiped out older lines because of some unspecified advantage. This new study indicates that this is not the case, and rather that the female effective population size was higher than the male one, which is likely due to the practice of successful males to have many children with different wives (polygyny).

Mol Biol Evol. 2004 Aug 18 [Epub ahead of print]

Genetic Evidence for Unequal Effective Population Sizes of Human Females and Males.

Wilder JA et al.

The time to the most recent common ancestor (TMRCA) of the human mitochondria (mtDNA) is estimated to be older than that of the non-recombining portion of the Y chromosome (NRY). Surveys of variation in globally distributed humans typically result in mtDNA TMRCA values just under 200 thousand years (kya) while those for the NRY range between 46 and 110 kya. A favored hypothesis for this finding is that natural selection has acted on the NRY leading to a recent selective sweep. An alternate hypothesis is that sexbiased demographic processes are responsible. Here we re-examine the disparity between NRY and mtDNA TMRCAs using data collected from individual human populations--a sampling strategy that minimizes the confounding influence of population subdivision in global datasets. We survey variation at 782 bp of the mitochondrial cytochrome c oxidase subunit 3 gene as well as at 26.5 kb of non-coding DNA from the NRY in a sample of 25 Khoisan, 24 Mongolians, and 24 Papua New Guineans. Data from both loci in all populations are best described by a model of constant population size, with the exception of Mongolian mtDNA which appears to be experiencing rapid population growth. Taking these demographic models into account, we estimate the TMRCAs for each locus in each population. A pattern that is remarkably consistent across all three populations is an approximately two-fold deeper coalescence for mtDNA than for the NRY. The oldest TMRCAs are observed for the Khoisan (73.6 kya for the NRY and 176.5 kya for mtDNA) while those in the non-African populations are consistently lower (averaging 47.7 kya for the NRY and 92.8 kya for mtDNA). Our data do not suggest that differential natural selection is the cause of this difference in TMRCAs. Rather, these results are most consistent with a higher female effective population size.

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