Phenotypic differences may have genetic and plastic components. Here, we investigated the contributions of both for differences in body shape in two species of Lake Malawi cichlids using wild‐caught specimens and a common garden experiment. We further hybridized the two species to investigate the mode of gene action influencing body shape differences and to examine the potential for transgressive segregation. We found that body shape differences between the two species observed in the field are maintained after more than 10 generations in a standardized environment. Nonetheless, both species experienced similar changes in the laboratory environment. Our hybrid cross experiment confirmed that substantial variation in body shape appears to be genetically determined. The data further suggest that the underlying mode of gene action is complex and cannot be explained by simple additive or additive‐dominance models. Transgressive phenotypes were found in the hybrid generations, as hybrids occupied significantly more morphospace than both parentals combined. Further, the body shapes of transgressive individuals resemble the body shapes observed in other Lake Malawi rock‐dwelling genera. Our findings indicate that body shape can respond to selection immediately, through plasticity, and over longer timescales through adaptation. In addition, our results suggest that hybridization may have played an important role in the diversification of Lake Malawi cichlids through creating new phenotypic variation.

The cichlid fishes of Lake Malawi represent one of the most diverse adaptive radiations of vertebrates known. Among the rock-dwelling cichlids (mbuna), closely related sympatric congeners possess similar trophic morphologies (i.e. cranial and jaw structures), defend overlapping or adjacent territories, but can be easily distinguished based on male nuptial coloration. The apparent morphological similarity of congeners, however, leads to an ecological conundrum: theory predicts that ecological competition should lead to competitive exclusion. Hence, we hypothesized that slight, yet significant, ecological differences accompanied the divergence in sexual signals and that the divergence of ecological and sexual traits is correlated. To evaluate this hypothesis, we quantified body shape, a trait of known ecological importance, in populations of Maylandia zebra, a barred, widespread mbuna, and several sympatric nonbarred congeners. We found that the barred populations differ in body shape from their nonbarred sympatric congeners and that the direction of shape differences was consistent across all barred vs. nonbarred comparisons. Barred populations are generally deeper bodied which may be an adaptation to the structurally complex habitat they prefer, whereas the nonbarred species have a more fusiform body shape, which may be adaptive in their more open microhabitat. Furthermore, M. zebra populations sympatric with nonbarred congeners differ from populations where the nonbarred phenotype is absent and occupy less morphospace, indicating potential ecological character displacement. Mitochondrial DNA as well as published AFLP data indicated that the nonbarred populations are not monophyletic and therefore may have evolved multiple times independently. Overall our data suggest that the evolution of coloration and body shape may be coupled as a result of correlational selection. We hypothesize that correlated evolution of sexually selected and ecological traits may have contributed to rapid speciation as well as the maintenance of diversity in one of the most diverse adaptive radiations known.

No abstract

We examined survival, competition, and recruitment among cyprinids that were subjected to interspecific and intraspecific competition and chronic heat stress in large, outdoor experimental streams. The study was conducted in 2011 during the hottest summer (also one of the driest summers) recorded in Texas. We measured survival of Red Shiners Cyprinella lutrensis, Blacktail Shiners Cyprinella venusta, and Central Stonerollers Campostoma anomalum stocked at varying densities to examine competitive interactions. Trophic interactions among Blacktail Shiner density treatments were assessed with stable isotope analysis (SIA), and mitochondrial markers were used to examine the lineage of young-of-the-year shiners spawned during the study. Stocking survival was significantly greater for Red Shiners, and both shiner species demonstrated higher overall survival rates than Central Stonerollers. Although SIA results were variable among replicate streams, more generalist foraging patterns were observed for Blacktail Shiners in the high-density treatment (intraspecific competition; no Red Shiners present) relative to the low-density Blacktail Shiner treatment. Less trophic redundancy was observed in the interspecific competition treatment (Blacktail Shiners and Red Shiners co-occurring) compared with the same density of Blacktail Shiners only (high-density treatment). Despite the potentially lethal water temperatures, including 4 d on which water temperatures exceeded 37 C, mitochondrial sequences showed that both Red Shiners and Blacktail Shiners were able to spawn during the study. These results collectively highlight potential mechanisms for explaining fish assemblage responses at local and landscape scales, such as the Red Shiner's recent range expansion and increased abundance in some areas of Texas. Our findings are relevant for forecasting regional changes in fish species distributions in response to more frequent droughts and warmer summers due to climate change.