Complex patterns of genetic and phenotypic divergence in populations of the Lake Malawi cichlid Maylandia zebra
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Ecological speciation, which relies heavily on selection driving the emergence of new species, has become the primary paradigm through which rapid species radiations are understood. In this way, selection, particularly ecological selection, is assumed to be the driver of most species radiations. However, in many radiations, such as the radiation of Lake Malawi’s cichlids, the assumption of selection as the driver of speciation has rarely been explicitly tested, and drift, often, has completely been ignored as potential factor. In order to understand the forces driving the divergence of Lake Malawi cichlids at the microevolutionary level, we studied the genetic and phenotypic divergence of ten allopatric populations of Maylandia zebra. We estimated effective population sizes as proxy for drift. Further, we compared neutral genetic differentiation to divergence in three phenotypic traits: body size, body shape, and melanophore counts. We found small, yet significant, population differentiation in all the studied traits across most populations. Population sizes were small rendering the potential for drift to be high. However, phenotypic differentiation exceeded neutral expectations for all traits suggesting divergent local selection. Our data suggest that natural, and potentially also sexual, selection may be the dominant force driving population differentiation in Lake Malawi’s rock-dwelling cichlids, despite the potential for drift in small populations.
KeywordsPopulation genetics Ecological selection Sexual selection Radiation Geometric morphometrics Body shape Body size Melanophores
We wish to thank Richard Zatha, Aimee Howe, and Jason Curole for field assistance. We are grateful to Ming Chen and Aimee Howe, as well as the reviewers for comments on previous versions of the manuscript. Jan C. Habel provided some useful suggestions for the analyses. We are grateful to the officers of Lake Malawi National Park, members of Malawi Fisheries Department, and faculty at the University of Malawi for assistance in conducting the necessary field work. The study was funded by a Jordan Grant from the American Cichlid Association to MH; two Folmar Grants from the Biology Department of Baylor University to MH & BD; and funding from the C. Gus Glasscock, Jr. Endowed Fund for Excellence in Environmental Sciences in the College of Arts and Sciences to MH. This study was further supported by funds to PDD from the Faculty Research Investment Program and the Vice Provost for Research at Baylor University.
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