Why are some groups of animals made of up many species, while others only a few? One explanation is that older clades are more diverse simply because they have been around longer and had more time to accumulate species. However, different evolutionary processes may also change the rate at which species arise or disappear, like key innovations, extinction events, movement into new geographic regions and other factors.
As part of a project focused on the evolution of some of the largest radiations of mammals, I examined phylogenetic imbalance (the extent to which species diversity is unevenly distributed across clades) across Primates. While old groups of Primates do tend to be more species rich, one clade (Old World Monkeys, Cercopithicidae) was much more diverse than expected based on its age alone.
I investigated whether rates of lineage diversification (the difference between speciation and extinction rates) varied across the evolution of Primates. A “blind” analysis showed that Old World Monkeys have experienced a considerable increase in diversification rate. This shift was sufficient to explain the difference in species diversity from that expected based on the age of primate clades. We could not link any traditional biogeographic hypothesis (latitude, geographic range size, sympatry vs. allopatry) to this shift in diversification rates, and future research will focus on ecological adaptations that may have permitted the rapid radiation of Old World Monkeys.
Arbour, J.H. & Santana, S.E.. 2017. A major shift in diversification rate helps explain macroevolutionary patterns in primate species diversity. Evolution 71(6):1600-1613. (link)