Meeting Abstract
One of the most celebrated examples of sympatric speciation in nature were believed to be four sympatric radiations of cichlid fishes endemic to three crater lakes in Cameroon. Here I critically examine the evidence for sympatric speciation in these cichlids, combining measurements of natural selection and assortative mating in the field with population genomics using restriction site associated DNA sequencing to evaluate the evidence for secondary gene flow. First, I found considerable evidence of gene flow between all four radiations and neighboring riverine populations after initial colonization. In a few cases, some sympatric species were more closely related to outgroups than others, consistent with secondary gene flow facilitating their speciation. Second, many sympatric species complexes exhibited a unimodal distribution of phenotypes, suggesting that sympatric divergence is slow or incomplete. I found evidence of significant disruptive selection on trophic morphology in two lakes, but models suggest it is not strong enough to drive the speciation process to completion. In contrast, assortative mating was extremely strong along two independent axes and was primarily driven by breeding coloration. Overall, this suggests that sympatric divergence in these craters is driven by sexual selection, not natural selection, and may have benefited from secondary gene flow. These results do not rule out sympatric speciation in Cameroon cichlids, but instead reveal a complex history of speciation with gene flow, including allopatric and sympatric phases, resulting in both reproductively isolated species and incipient species complexes. Future work will aim to pinpoint where sympatric divergence may have occurred and continue to measure the relevant ecological, behavioral, and genetic variables to test existing models for this process.
