Biogeographic and Evolutionary Mechanisms Driving Diversification in Caribbean Ground Crickets (genus Amphiacusta).

Abstract

Amphiacusta is a species-rich genus of flightless ground crickets distributed throughout the Caribbean which exhibits substantial variation habitat use and male reproductive traits. This ecological and sexual diversity displayed by Amphiacusta furnishes a natural experiment to investigate driving population and species divergence because geographic isolation on islands can lead to the reduction or elimination of gene flow This research addresses three questions. First, what biogeographic mechanisms have influenced the pattern of species diversification of Amphiacusta? Estimates of the phylogenetic relationships among Amphiacusta are used to test the predictions of two biogeographic models for the Caribbean: island-island vicariance and overwater dispersal. Both topological evidence and estimates of the timing of divergence events provide little evidence for a strict interpretation of the island-island vicariance model, suggesting rather that both vicariance and overwater dispersal, followed by intra-island diversification, have played a role in the history of this genus. Second, is morphological divergence in the Virgin Islands species Amphiacusta sanctaecrucis the result of neutral or selective divergence? A comparison of phenotypic differentiation in three morphological traits (body size and shape, mandible shape, and genitalia shape) with neutral gene differentiation, indicates that all three exhibit significant variation between populations and have diverged by natural or sexual selection. Finally, are these traits evolving solely through local selection in isolation, or do extrinsic factors, such as the geographic configuration of islands and the ocean currents between them, influence morphological divergence between populations? An analysis of microsatellite loci finds that gene flow is very limited between islands, and furthermore, that there is no association between distance or gene flow and morphological divergence. This confirms that morphological divergence is the result of local adaptation and suggests that even weak selection over the course of their recent isolation could result in significant morphological divergence between populations. Overall, these results demonstrates that both the geological history of the Caribbean, as well as the evolutionary mechanisms operating within isolated populations are involved in the diversification of Amphiacusta.