The role of Pleistocene forest refugia and rivers in the evolutionary diversification of tropical biota has been the subject of considerable debate. A range-wide analysis of gorilla mitochondrial and nuclear variation was used to test the potential role of both refugia and rivers in shaping genetic diversity in current populations. Results reveal strong patterns of regional differentiation that are consistent with refugial hypotheses for central Africa. Four major mitochondrial haplogroups are evident with the greatest divergence between eastern (A, B) and western (C, D) gorillas. Coalescent simulations reject a model of recent east-west separation during the last glacial maximum but are consistent with a divergence time within the Pleistocene. Microsatellite data also support a similar regional pattern of population genetic structure. Signatures of demographic expansion were detected in eastern lowland (B) and Gabon/Congo (D3) mitochondrial haplogroups and are consistent with a history of postglacial expansion from formerly isolated refugia. Although most mitochondrial haplogroups are regionally defined, limited admixture is evident between neighboring haplogroups. Mantel tests reveal a significant isolation-bydistance effect among western lowland gorilla populations. However, mitochondrial genetic distances also correlate with the distance required to circumnavigate intervening rivers, indicating a possible role for rivers in partitioning gorilla genetic diversity. Comparative data are needed to evaluate the importance of both mechanisms of vicariance in other African rainforest taxa.control region ͉ mitochondrial ͉ phylogeography ͉ refugium M echanisms underlying evolutionary diversification in tropical forests have intrigued biologists for more than a century. Numerous hypotheses have been proposed (1-2), of which the Pleistocene forest refugium and riverine barrier hypotheses have provoked considerable interest and controversy (3-7). Whereas most studies have focused on the Amazon and Australian wet tropics (5,(8)(9)(10), data on central African rainforest taxa remain relatively sparse.According to Pleistocene refuge theory, forest fragmentation during glacial maxima led to the isolation and subsequent diversification of forest-associated taxa (3). During periods of climate amelioration and population expansion, zones of secondary contact may have also formed between neighboring refugial populations. Although palynological and biogeographical data have been used to infer forest refugia (11-16), their precise location and role in Pleistocene diversification is controversial. Several molecular studies suggest that refugia may have played an important role in structuring montane birds (17-19), primates (20-22), and trees (23). However, other forest species such as chimpanzees (24-26) and elephants (27,28) show relatively weak regional genetic structure, suggesting that wideranging and/or savannah-tolerant species may be poor indicators of range changes in tropical forest cover. One criticism of the Pleistocene...

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