Narrowing the gap between rich and poor

The species richness of a given clade is explained in simple terms by rate of diversification and clade age, but what other factors might play a role? 

Data resources used via GBIF : 4,400,000 species occurrences
Isolepis prolifera

Isolepis prolifera by memopob via NatureWatch NZ. Photo licensed under CC BY-NC 4.0.

Why are some taxonomic groups rich in species while others are poor? Evolutionarily speaking, richness is limited by diversification rate and age, but reality is more complex. This study focused on sedges (Cyperaceae family) and the observed correlation between occupied area and its richness. Using molecular data from four genes sequenced in 384 taxa combined with GBIF-mediated occurrences, researchers modelled ancestral ranges and measured diversification rates, and tested for correlation of richness with range size, overlap, niche clade age and rate of diversification. Their results point to South America as the origin of Cyperaceae in the late Cretaceuos. Increases in diversification took place three times, two of which occurred in the temperate Northern Hemisphere. Species-rich clades occupy more space and exhibit patterns consistent with niche differentiation correlating with diversification.

Spalink D, Drew BT, Pace MC, Zaborsky JG, Starr JR, Cameron KM, Givnish TJ and Sytsma KJ (2016) Biogeography of the cosmopolitan sedges (Cyperaceae) and the area-richness correlation in plants. Journal of Biogeography. Wiley-Blackwell 43(10): 1893–1904. Available at doi:10.1111/jbi.12802.