Latitude and rates of diversification in birds and butterflies

Central to many explanations of latitudinal diversity gradients is the idea that rates of species diversification increase towards the equator. However, there have been few explicit tests of whether or not this pattern exists. Using sister-group analyses to compare 48 clades of passerine birds and swallowtail butterflies from different latitudes, I found evidence that relative rates of diversification per unit time are indeed higher towards the equator. This pattern is explicable in terms of abiotic factors which vary continuously with latitude, and may be further enhanced by diversity-dependent speciation and extinction processes.