MBE Advance Access originally published online on July 4, 2008
Molecular Biology and Evolution 2008 25(9):1979-1994; doi:10.1093/molbev/msn148
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Research Articles |
Estimation of Hominoid Ancestral Population Sizes under Bayesian Coalescent Models Incorporating Mutation Rate Variation and Sequencing Errors
* Galton Laboratory, Department of Biology, University College London, London, United Kingdom
Watson School of Biological Sciences, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY
Laboratory of Biometrics, Graduate School of Agriculture and Life Sciences, University of Tokyo, Tokyo, Japan
E-mail: z.yang{at}ucl.ac.uk.
Accepted for publication June 29, 2008.
Estimation of population parameters for the common ancestors of humans and the great apes is important in understanding our evolutionary history. In particular, inference of population size for the human–chimpanzee common ancestor may shed light on the process by which the 2 species separated and on whether the human population experienced a severe size reduction in its early evolutionary history. In this study, the Bayesian method of ancestral inference of Rannala and Yang (2003. Bayes estimation of species divergence times and ancestral population sizes using DNA sequences from multiple loci. Genetics. 164:1645–1656) was extended to accommodate variable mutation rates among loci and random species-specific sequencing errors. The model was applied to analyze a genome-wide data set of 
15,000 neutral loci (7.4 Mb) aligned for human, chimpanzee, gorilla, orangutan, and macaque. We obtained robust and precise estimates for effective population sizes along the hominoid lineage extending back 30 Myr to the cercopithecoid divergence. The results showed that ancestral populations were 5–10 times larger than modern humans along the entire hominoid lineage. The estimates were robust to the priors used and to model assumptions about recombination. The unusually low X chromosome divergence between human and chimpanzee could not be explained by variation in the male mutation bias or by current models of hybridization and introgression. Instead, our parameter estimates were consistent with a simple instantaneous process for human–chimpanzee speciation but showed a major reduction in X chromosome effective population size peculiar to the human–chimpanzee common ancestor, possibly due to selective sweeps on the X prior to separation of the 2 species.
Key Words: hominoid • ancestral population size • Bayesian inference • MCMC • coalescent model • sequencing errors
Rasmus Nielsen, Associate Editor