Key innovations and island colonization as engines of evolutionary diversification: a comparative test with the australasian

Abstract

A major challenge in evolutionary biology is understanding the main drivers that un- derlie morphological and species diversity. Ecological opportunity—access to new or previously inaccessible niches—has been classically identified as one of the most im- portant drivers of both phenotypic and species diversification. This can arise from three main sources: (i) the extinction of ecological competitors that open up previously “filled” niches; (ii) exposure to new environments through dispersal (e. g. island colonization) or changes to existing environments through extrinsic forces that modify the environment (e.g., climate change); and (iii) the evolution of key innovations that allow taxa to use environments or resources in novel ways. Our study examined the latter two sources of ecological opportunity—specifically, the colonization of islands (in this case, New Caledonia and New Zealand) and the evolution of two putative key innovations (adhe- sive toepads and a snake-like phenotype)—and explored the extent these have driven diversification in a morphologically diverse and species rich vertebrate group: the Aus- tralasian diplodactyloid geckos.

To study this question, we developed a robust time-calibrated phylogeny of the whole ra- diation, and compiled body size data (our proxy to phenotype) for most of the species in it. We then applied a variety of recently developed comparative methods to test whether key innovations and island colonization were associated with accelerated rates of body size evolution and species diversification.

Our results show that island colonization has played the most prominent role in the evolutionary diversification of Australasian geckos, producing accelerated rates of phe- notypic and species diversification. This is consistent with the expected scenario of a rapid niche-filling process coupled with high rates of speciation during the early stages of island colonization, when groups experience high levels of ecological opportunity. Regarding the two key innovations studied, only one of them, the snake-like phenotype, was associated to a pattern similar of that found for island colonization. Adhesive toe- pads, however, failed to show any direct impact on the total evolutionary diversification experienced by the group. This shows how key innovations, despite of allowing new inte- ractions with the environment, not necessarily open the door to high rates of phenotypic and species diversification. Studies wishing to confirm the putative link between a key innovation and subsequent evolutionary diversification must therefore show that it has been the acquisition of an innovation specifically, not the colonization of new areas more generally, that has prompted diversification.