How does ecological opportunity influence rates of speciation, extinction, and morphological diversification in New World ratsnakes (tribe Lampropeltini)?

Ecological adaptive radiation theory predicts an increase in both morphological and specific diversification when organisms colonize new environments. Accordingly, bursts of morphological diversification, characterized by low within-subclade morphological disparity, may be associated with these increases in speciation rates. Conversely, increasing species density, reduction in available habitat, or increasing extinction rates are expected to cause rates of diversification to decline. We test these hypotheses by examining the tempo and mode of speciation in the lampropeltinine snakes, a morphologically variable group that colonized the New World approximately 24 million years ago and radiated throughout the Miocene. We show that specific diversification increased early in the history of the group, and that most morphological variation is partitioned among, rather than within subclades. These patterns provide further evidence for the hypothesis that morphological variation tends to be strongly partitioned among lineages when clades undergo early bursts of species diversification. A reduction in speciation rates may be indicative of density dependent effects due to a saturation of available ecological opportunity, rather than increases in extinction rates at the onset of the Pleistocene/Pliocene glacial cycles. This evidence runs counter to the general Pleistocene species pump model.