Temperate extinction in squamate reptiles and the roots of latitudinal diversity gradients
- Editor: Miguel Olalla-Tárraga
Many ecological and evolutionary hypotheses have been proposed to explain latitudinal diversity gradients. However, any mechanistic explanation must include factors that change at least one of the three processes that can directly affect species richness: speciation, extinction and dispersal. I hypothesize that higher extinction in temperate areas, higher speciation in tropical areas and reduced dispersal into temperate regions drives latitudinal diversity gradients in squamates, and that these processes may be common in other groups.
I test for these processes using phylogenetic methods that can untangle speciation and extinction with respect to latitudinal position (GeoSSE), using a dated phylogeny containing c. 45% of all squamates (4161 species) with data on their geographic occurrence.
I find that lineages in the tropics have high speciation and low extinction, but that temperate lineages have even higher speciation and extinction, leading to lower net diversification and higher turnover in temperate areas, with much higher rates of dispersal into the tropics from temperate areas than the reverse.
Recent empirical studies using dated molecular phylogenies appear to be consistent in supporting a similar set of simple and intuitive results for processes driving latitudinal gradients in species richness: higher net diversification rates in the tropics, higher relative extinction fractions in temperate regions and reduced dispersal out of the tropics. I suggest that higher temperate extinction represents a dominant force for the origin and maintenance of latitudinal gradients, particularly in groups with ancient temperate clades.