Abstract
Understanding how genetic variation is generated and maintained in natural populations, and how this process unfolds in a changing environment, remains a central issue in biological research. In this work, we analysed patterns of genetic diversity from several populations of three cichlid species from Lake Tanganyika in parallel, using the mitochondrial DNA control region. We sampled populations inhabiting the littoral rocky habitats in both very deep and very shallow areas of the lake. We hypothesized that the former would constitute relatively older, more stable and genetically more diverse populations, because they should have been less severely affected by the well-documented episodes of dramatic water-level fluctuations. In agreement with our predictions, populations of all three species sampled in very shallow shorelines showed traces of stronger population growth than populations of the same species inhabiting deep shorelines. However, contrary to our working hypothesis, we found a significant trend towards increased genetic diversity in the younger, demographically less stable populations inhabiting shallow areas, in comparison with the older and more stable populations inhabiting the deep shorelines. We interpret this finding as the result of the establishment of metapopulation dynamics in the former shorelines, by the frequent perturbation and reshuffling of individuals between populations due to the lake-level fluctuations. The repeated succession of periods of allopatric separation and secondary contact is likely to have further increased the rapid pace of speciation in lacustrine cichlids.
Highlights
The role of small- vs. large-scale environmental changes in the generation and maintenance of genetic variation in natural populations remains a central but neglected issue in biological research (Leffler et al 2012)
We sampled populations inhabiting the littoral rocky habitats in both very deep and very shallow areas of the lake. We hypothesized that the former would constitute relatively older, more stable and genetically more diverse populations, because they should have been less severely affected by the welldocumented episodes of dramatic water-level fluctuations
Species with larger and more stable population sizes are expected to maintain higher levels of neutral genetic diversity due to the reduced effect of genetic drift, while more complex interactions are expected for genetic variation under direct or indirect selection (Smith & Haigh 1974; Charlesworth et al 1993)
Summary
The role of small- vs. large-scale environmental changes in the generation and maintenance of genetic variation in natural populations remains a central but neglected issue in biological research (Leffler et al 2012). Levels of genetic diversity within populations will depend on the net balance between gain and loss of genetic variants, but while mechanisms behind the generation of new genetic variants are relatively uncontroversial, those involved in their maintenance or disappearance remain the subject of debate. While a central and relatively simple prediction, this correlation between population sizes and stability on one hand, and genetic diversity levels on the other, remains divisive
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