Abstract
Islands are bounded areas where high endemism is explained either by allopatric speciation through the fragmentation of the limited amount of space available, or by sympatric speciation and accumulation of daughter species. Most empirical evidence point out the dominant action of allopatric speciation. We evaluate this general view by looking at a case study where sympatric speciation is suspected. We analyse the mode, tempo and geography of speciation in Agnotecous, a cricket genus endemic to New Caledonia showing a generalized pattern of sympatry between species making sympatric speciation plausible. We obtained five mitochondrial and five nuclear markers (6.8 kb) from 37 taxa corresponding to 17 of the 21 known extant species of Agnotecous, and including several localities per species, and we conducted phylogenetic and dating analyses. Our results suggest that the diversification of Agnotecous occurred mostly through allopatric speciation in the last 10 Myr. Highly microendemic species are the most recent ones (<2 Myr) and current sympatry is due to secondary range expansion after allopatric speciation. Species distribution should then be viewed as a highly dynamic process and extreme microendemism only as a temporary situation. We discuss these results considering the influence of climatic changes combined with intricate soil diversity and mountain topography. A complex interplay between these factors could have permitted repeated speciation events and range expansion.
Highlights
Diversity hotspots in islands are characterized by very high specific richness and endemism resulting from local diversification of groups which are ‘‘trapped’’ in very limited spatial areas [1,2,3]
The theory behind this diversity rests upon the simple idea that local diversification occurred either by allopatric speciation, through the fragmentation of the limited space available into even more restricted daughter areas, or by accumulation of daughter species without vicariance, through sympatric speciation based on non geographic isolation, for example involving ecological specializations [4,5,6]
Empirical case studies showed or suggested that diversifications in islands most often result from repeated events of local allopatric speciation and subsequent establishment of microendemism [7] due to Plio-Pleistocene climatic fluctuations resulting in rainforest fragmentation [8,9,10], or by the barriers made by large rivers [11,12]
Summary
Diversity hotspots in islands are characterized by very high specific richness and endemism resulting from local diversification of groups which are ‘‘trapped’’ in very limited spatial areas [1,2,3]. Empirical case studies showed or suggested that diversifications in islands most often result from repeated events of local allopatric speciation and subsequent establishment of microendemism [7] due to Plio-Pleistocene climatic fluctuations resulting in rainforest fragmentation [8,9,10], or by the barriers made by large rivers [11,12] Contrasting with this general pattern, sympatric speciation has been documented in islands [4,5,13,14] and a few cases have been suspected in a context where distribution of species is restricted to small areas, i.e. microendemism [8,15,16,17,18,19]. The relative importance of allopatric and sympatric speciation has not been addressed in many cases because of the low number of opportunities and the difficulty to distinguish between primary and secondary sympatry [7,14,28]
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