Evolution
The P. kirkii population on Zanzibar, represents a population of red colobus that is believed to have been isolated on the island after sea levels rose toward the end of the Pleistocene epoch. Furthermore, it is suggested through mitochondrial analysis, that phylogenetic groups within the red colobus have been genetically isolated from another since the Pliocene.
Examining cranial morphology has shown that Procolobus kirkii has diverged from mainland Procolobus to its own species. It has experienced an acceleration in morphological evolution of size which is suggested to be the result of insularity on the island and environmental pressures such as competition, habitat, predation and/or resource availability. There has been no evidence for population bottlenecks in the species.
The smaller cranium of Procolobus kirkii in contrast to the mainland colobus monkey, is consistent with Foster's rule (also known as the island rule) in which the original (larger) animal becomes smaller over time when there is limited resources. Males tend to have pedomorphic traits which include a shorter face, large orbits and an enlarged neurocranium. It is not certain how long ago and where this evolutionary change occurred.
Through molecular analyses, it is indicated that P. kirkii is more closely related to P. gordonorum compared to other red colobus species. This analysis has also placed the divergence of P. kirkii from its sister species P. godronorum at about 600,000 years ago, which actually allows for an older evolutionary age compared to the previous assumption that it had become its own species around the last glacial period.
Read more about this topic: Zanzibar Red Colobus