Speciation, ecological opportunity, and latitude

TL;DR

This paper evaluates hypotheses explaining higher tropical biodiversity, finding recent speciation rates are higher in temperate zones, but historical data support greater tropical diversification due to age and area, with ecological opportunity playing a role.

Contribution

It provides a comprehensive analysis of speciation and diversification rates across latitudes, emphasizing the importance of historical factors and ecological opportunity in shaping biodiversity.

Findings

Per capita speciation rates are currently higher in temperate zones.

Tropical clades show higher speciation and diversification rates historically.

Ecological opportunity can lead to rapid speciation in depauperate environments.

Abstract

Evolutionary hypotheses to explain the greater numbers of species in the tropics than the temperate zone include greater age and area, higher temperature and metabolic rates, and greater ecological opportunity. These ideas make contrasting predictions about the relationship between speciation processes and latitude, which I elaborate and evaluate. Available data suggest that per capita speciation rates are currently highest in the temperate zone, and that diversification rates (speciation minus extinction) are similar between latitudes. In contrast, clades whose oldest analyzed dates precede the Eocene thermal maximum, when the extent of the tropics was much greater than today, tend to show highest speciation and diversification rates in the tropics. These findings are consistent with age and area, which is alone among hypotheses in predicting a time trend. Higher recent speciation rates in the temperate zone than the tropics suggest an additional response to high ecological opportunity associated with low species diversity. These broad patterns are compelling but provide limited insights into underlying mechanisms, arguing that studies of speciation processes along the latitudinal gradient will be vital. Using threespine stickleback in depauperate northern lakes as an example, I show how high ecological opportunity can lead to rapid speciation. The results support a role for ecological opportunity in speciation, but its importance in the evolution of the latitudinal gradient remains uncertain. I conclude that per-capita evolutionary rates are no longer higher in the tropics than the temperate zone. Nevertheless, the vast numbers of species that have already accumulated in the tropics ensure that total rate of species production remains highest there. Thus, tropical evolutionary momentum helps to perpetuate the steep latitudinal biodiversity gradient.