The remarkable increase in species richness as one travels from the poles to the tropics (the latitudinal diversity gradient, LDG) is the Earth’s predominant biodiversity pattern, recognized since the time of Wallace and Darwin. Although a consensus explanation for the LDG remains elusive, recent advances have narrowed the possibilities to geographical variation in: 1) productivity, 2) time and area available for diversification, and 3) differences in speciation and/or extinction rates. We develop and analyze a unique data set on the global distribution of endemic freshwater fishes in the 2,746 largest lakes that allows us to test how speciation probability and net diversification are independently attributable to lake age, area, productivity, and geographical location.
We find that 1,933 endemic fish from 47 families are restricted to a single lake in the 2,746 largest lakes worldwide. The probability of a lake containing an endemic species and the total number of endemics per lake are both higher in lakes that are older, larger, and at lower latitudes. Further, a family-controlled contrast of the relative distributions of endemic and non-endemic fish shows that endemic fish are significantly more likely to be found at lower latitudes in 34 of 41 families analyzed. Our results demonstrate an association between latitude and the probability of speciation (and species richness) that is independent of age, area, and energy, suggesting that there are unique features of low-latitude (tropical) environments that promote speciation. These results, along with evidence for greater and more divergent niche utilization in tropical fish, suggest that low latitude fish may diversify more rapidly through increased ecological opportunities in tropical climates and the coevolution of strong species interactions.