Bio geography and Systematics of Gorillas

The genus Gorilla occurs in two geographically distinct regions, Western Central Africa and Eastern Central Africa. Separated by 1000 km, gorillas in these two areas were historically viewed as two subspecies, the western lowland (Gorilla gorilla gorilla), and the eastern or mountain gorilla (G. g. beringei) gorilla. This view persisted until 1970 when an examination of skeletal and dental morphology of different gorillas led to the proposal of three subspecies, with the eastern subspecies further divided into two distinct subspecies, the eastern lowland (G. g. graueri) and the mountain gorilla (G. g. beringei) (1,2). This view gained wide acceptance and was, until recently, used to classify gorillas.

Over the last several years studies of DNA sequences from different gorilla populations are changing the way we think about gorilla taxonomy. Results from several independent mitochondrial DNA sequence studies (3-11) indicate that gorillas show extreme variability in their genetic makeup and that there is a clear distinction between the eastern and western populations with divergence being dated at about 2-3 million years (3). The differences are greater than those observed between the common chimpanzee (Pan troglodytes) and the pygmy chimpanzee (Pan panicus) (5) two different species with clear morphological, ecological and behavioral differences. Genetic studies also revealed that the eastern lowland and mountain gorilla sequences are distinct (5,8) and these gorillas diverged from a common ancestor about
100 000 years ago3. These results raise the possibility that eastern and western gorillas are two distinct species, Gorilla beringei and Gorilla gorilla, a view that has been proposed (12,13) and is gaining wide acceptance.

There is greater debate about classification within each of the two proposed species. Within the western
gorillas, it has been proposed, based on morphological and genetic evidence (14,15), that in addition to G.
g. gorilla (in Gabon, Central African Republic, Congo, Cameroon and Equatorial Guinea) a second subspecies (G. g. diehli) should be recognized. These Cross River Gorillas number less than 300 individuals, are found on the Nigeria-Cameroon border and are separated from other gorillas by more than 200 km. Western gorillas have been shown to contain much more variation (7,10,11) than eastern gorillas and future studies may reveal further sub-structuring. Within the eastern gorillas, genetic results indicate that both Bwindi and Virunga mountain gorillas belong to the mountain gorilla subspecies (G. b. beringei) (5,8), whereas other scientists suggest that Bwindi gorillas could comprise a third eastern subspecies (in addition to G. b. beringei and G. b. graueri) on the basis of limited morphological and behavioral evidence (16).