Taxonomies attempt to portray evolutionary relationships. Thus, the current taxonomic approach of placing western and eastern gorilla forms into two separate species reflects new mitochondrial-DNA (mtDNA) analyses that suggest these two gorilla forms appear to have been separated from each other for about as long an evolutionary time as that for bonobos and chimpanzees, perhaps as old as 2.5 million years ago. As interesting, the split representing the sub-speciation of Grauer's gorillas and Virunga mountain gorillas may date back as far as 500,000 years ago.
These studies also reveal that Grauer's gorilla may have one of the more interesting evolutionary histories of all the gorilla forms. Studies of mtDNA and cranial morphology of Grauer's gorilla indicate that this population shows less variation in these characters than is the case for western lowland gorillas and the much smaller Virunga mountain gorilla population. This suggests that Grauer's gorillas may not have had much evolutionary time to develop genetic or morphological variation. The best explanation for this is that their current distribution reflects a relatively recent expansion from a small, genetically bottlenecked population that may have survived in a forest refuge during the last glacial maximum 18,000 years ago. At that time, global conditions were much drier and the majority of the Congo Basin was not rainforest as it is today, but was covered in savanna and woodland habitats. It is thought that there were pockets of wetter mountain forests, perhaps in the Tshiaberimu range in the north or around Mount Kahuzi in the south. This may have served as a “refuge” for a small population of Grauer's gorillas, and as conditions became warmer and wetter in the last 18,000 years, rainforests recovered and re-expanded into what is now eastern DRC. As these forests reached their current configuration, the Grauer's gorilla population expanded into the distribution we see today.
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