Abstract
Evidence is growing that not only allopatric but also sympatric speciation can be important in the evolution of species. Sympatric speciation has most convincingly been demonstrated in laboratory experiments with bacteria, but field-based evidence is limited to a few cases. The recently discovered plethora of subterranean diving beetle species in isolated aquifers in the arid interior of Australia offers a unique opportunity to evaluate alternative modes of speciation. This naturally replicated evolutionary experiment started 10-5 million years ago, when climate change forced the surface species to occupy geographically isolated subterranean aquifers. Using phylogenetic analysis, we determine the frequency of aquifers containing closely related sister species. By comparing observed frequencies with predictions from different statistical models, we show that it is very unlikely that the high number of sympatrically occurring sister species can be explained by a combination of allopatric evolution and repeated colonisations alone. Thus, diversification has occurred within the aquifers and likely involved sympatric, parapatric and/or microallopatric speciation.
Highlights
Strong evidence for sympatric speciation has recently been provided in vitro [1], and the concept is well supported by theoretical analyses [2,3,4,5,6,7]
We show that it is unlikely that the observed high frequency of co-occurring sister species pairs and triplets is the result of repeated colonisations, and that the pattern is better explained by diversification within the aquifers
Repeated colonisation model We used a model to test the repeated colonisation hypothesis in order to predict the fraction of aquifers with pairs and triplets of sympatric sister species
Summary
Strong evidence for sympatric speciation has recently been provided in vitro [1], and the concept is well supported by theoretical analyses [2,3,4,5,6,7]. Even after the presence of sympatric, closely related sister species has been established, it remains uncertain whether the co-occurring species pairs have evolved in sympatry or whether the divergence of the species occurred in isolation and involved multiple invasions [13,14,15,16]. To distinguish between these modes of speciation a statistical approach is needed, which requires the presence of multiple sympatrically occurring species pairs. The recent discovery of communities of invertebrates in Australian subterranean aquifers that have evolved in isolation for millions of years provides a unique opportunity to evaluate the occurrence of sympatric versus allopatric modes of speciation in a natural environment
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