Data from: Speciation, ecological opportunity, and latitude

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. 28 pages,
4 figures; Supplement EndDryadContent