The Central African Rainforest (as distinct from the West African Rain Forest) spans an area from the Atlantic coast to nearly Lake Victoria in Uganda and Tanzania. In fairly recent times (the mid Holocene) this forest was probably continuous all the way to Victoria, and probably extended farther north and south than one might imagine from looking at its current distribution.
Within the forest are major rivers, including the Congo. The Congo River is the only major river in the world that crosses the Equator twice. This trans-equatorial configuration guarantees that the rivers picks up rain from both of the equatorial rainy seasons, making it a huge and virtually uncrossable barrier for terrestrial mammals. During glaical periods, the forest is believed to have shrunk to either small refugia, or to have virtually disappeared entirely with only riverine forest remaining. Between the shrinkage of the forest and the major riverine barriers, terrestrial (non-flying, non-swimming) forest-dwelling animals that might have had a more continuous distribution would have been broken into many smaller units. Likely, many of these small populations would have gone extinct, but others may have changed over time such that when the forest was re-established, they may have constituted different subspecies or species. This breaking up and rejoining of the rain forest, over and over again, during the Pleistocene is thought to have caused much of the modern day variation we see among closely related forest species of primates, small carnivores, and forest ungulates such as duikers.
Anthony, et. al, in a paper just coming out in PNAS, explore this question with respect to gorilla genetics in particular.
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. [and] 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. … tests reveal a significant isolation-by-distance 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.
People always want to know, “How does this study change our thinking of evolution/whatever.” People want to know this because we have become accustom to the press reporting every scientific finding that they report as changing our thinking. This paper does not change our thinking at all, but rather, offers empirical proof of something that was previously modeled, suspected, and here and there, demonstrated with one taxon or another. Science marches on.
Anthony, Nicola M., Mireille Johnson-Bawe, Kathryn Jeffery, Stephen L. Clifford, Kate A. Abernethy, Caroline E. Tutin, Sally A. Lahm, Lee J. T. White, John F. Utley, E. Jean Wickings, and Michael W. Bruford. (2007). The role of Pleistocene refugia and rivers in shaping gorilla genetic diversity in central Africa. PNAS published December 12, 2007, 10.1073/pnas.0704816105