Genetics and biogeography of east African chimpanzees (Pan troglodytes schweinfurthii)

This study is a geographic survey of genetic variability in the easternmost subspecies of chimpanzee, Pan troglodytes schweinfurthii. The study tests predictive hypotheses about the historical biogeography of the subspecies. Hypotheses are derived from biogeographic theory. Forests in eastern Africa have expanded and contracted in response to oscillations in Pleistocene global climate. Biogeographic theory predicts that (1) chimpanzees were restricted to refugia during periods of minimal forest cover, and (2) chimpanzees dispersed outward from refugia when forests re-expanded. Population genetic theory is used to predict the genetic consequences of such demographic events. The genetic hypotheses tested are that (1) eastern chimpanzees show evidence of a recent population-size bottleneck and (2) the geographic apportionment of eastern chimpanzee genetic diversity is consistent with expansion from Pleistocene forest refugia.; Chimpanzees were sampled from 19 populations in Uganda, Rwanda, Tanzania and Zaire. Sampling locations were chosen to allow investigation of the potential roles of conspicuous topographic features (e.g. major rivers) as barriers to dispersal. DNA was collected in the form of shed hair. Two hundred eighty-one mitochondrial control region DNA sequences were obtained.; Eastern chimpanzees display levels of genetic variation which are low, and which are similar to levels observed in humans. Mismatch distribution analyses reveal direct evidence of recent population expansion in the subspecies. These lines of evidence support the bottleneck hypothesis. However, insular biogeographic analyses fail to support the hypothesis that chimpanzees were restricted to refugia prior to expansion. Genetic diversity was unrelated to distance from refugia, forest area, or isolation. In addition, gene flow has been relatively uninterrupted by phylogeographic barriers.; These lines of evidence suggest that the eastern chimpanzee population responded to Pleistocene climatic fluctuations by growing and shrinking in numbers. The geographic range of the subspecies was unaffected by the contraction and expansion of forest into and out of refugia. Eastern chimpanzees have lived in dry, mosaic environments for much of their evolutionary history. These results are discussed in light of their implications for biogeography, evolution, and conservation. Additional analyses are presented which address the correlation between genetics and chimpanzee social behavior.