Climatic Change as an Engine for Speciation in Orthoptera Species: : Flightless Species on African Mountains as a Model System

Abstract

Climatic fluctuations characterized the climate in East Africa during the Plio-Pleistocene. These variable conditions established opportunities for ecological fragmentation with subsequent genetic isolation of species dependent on both savannah and forest habitats. This study addresses the hypothesis that historical climatic fluctuations in the region have had a major evolutionary impact on the fauna in East Africa. To do this, mitochondrial DNA sequences from a group of bush crickets consisting of both forest and savannah inhabiting taxa were analyzed in relation to Plio-Pleistocene range fragmentations indicated by palaeoclimatic studies. Parts of the COI and 12S mitochondrial genes were sequenced to infer phylogeographic patterns. Coalescent modeling and mismatch distributions were used to distinguish between alternative biogeographic scenarios. The same methods, together with a traditional molecular clock approach, were also used to date the divergence times between species. The results indicate that two radiations, one between 6 and 3.5 million years ago and the other about 0.8 million years ago, gave rise to most of the species analyzed. The earliest radiation overlaps in time with the global spread of C4 grasslands and speciation through adaptation to this new type of plants may therefore explain the contemporary sudden burst of savannah inhabiting lineages. Climate seems to have been the driving force behind the more recent of the two radiations. An intensified drying and cooling of the climate right after the last warm and wet maximum in the region about 1 million years ago resulted in the retraction of forest to higher altitudes. Forest dependent species were consequentially trapped in forest refuges and simultaneous vicariant speciation events in these lineages followed. Accordingly, this study shows that the East African continental archipelago is a suitable model system for phylogenetic research and for illuminating how climatic fluctuations may influence speciation and other evolutionary processes. Further, my results suggest that savannah inhabiting species have experienced intense selective constraints on the mtDNA COI gene compared to forest dependent species. One possible explanation could be severe competition on the savannah for the right microclimate and food resources. Finally, paraphyly at both the genus and the species level was detected and some taxonomic revisions are suggested.