Evidence for ecological processes driving speciation among endemic lizards of Madagascar

Although genetic patterns produced by population isolation during speciation are well documented, the biogeographic and ecological processes that trigger speciation remain poorly understood. Alternative hypotheses for the biogeography and ecology of speciation include geographic isolation combined with niche conservation (soft allopatry) or parapatric distribution on an environmental gradient with niche divergence (ecological speciation). Here, we use species' distributions, environmental data, and two null models (the Random Translation and Rotation and the Background Similarity Test) to test these alternative hypotheses among 28 sister pairs of microendemic lizards in Madagascar. Our results demonstrate strong bimodal peaks along a niche divergence-conservation spectrum, with at least 25 out of 28 sister pairs exhibiting either niche conservation or divergence, and the remaining pairs showing weak ecological signals. Yet despite these significant results, we do not find strong associations of niche conservation with allopatric distributions or niche divergence with parapatric distributions. Our findings thus provide strong evidence of a role for ecological processes driving speciation, rather than the classic expectation of speciation through geographic isolation, but demonstrate that the link between ecological speciation and parapatry is complex and requires further analysis of a broader taxonomic sample to fully resolve.