Biology Department, Baylor University, Waco, TX, USA.
J Evol Biol. 2014 May;27(5):826-39. doi: 10.1111/jeb.12353. Epub 2014 Mar 12.
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.
马拉维湖的慈鲷鱼类代表了已知脊椎动物中最多样化的适应性辐射之一。在岩栖慈鲷（mbuna）中，密切相关的同域近缘种具有相似的营养形态（即颅骨和颌骨结构），保卫重叠或相邻的领地，但可以根据雄性婚色轻易区分。然而，同域近缘种的明显形态相似性导致了一个生态难题：理论预测，生态竞争应该导致竞争排斥。因此，我们假设，在性信号的分歧中伴随着微小但显著的生态差异，并且生态和性特征的分歧是相关的。为了评估这个假设，我们量化了身体形状，这是一个已知具有生态重要性的特征，在马莱迪亚斑马（一种有条纹的、广泛分布的 mbuna）和几个同域的非条纹近缘种的种群中。我们发现，有条纹的种群在身体形状上与它们的非条纹同域近缘种不同，并且形状差异的方向在所有有条纹与非条纹的比较中都是一致的。有条纹的种群通常体型更深，这可能是对它们更喜欢的结构复杂的栖息地的适应，而无条纹的物种则具有更梭形的身体形状，这可能在它们更开放的小生境中是适应性的。此外，与非条纹近缘种同域的 M. zebra 种群与非条纹表型不存在且占据较少形态空间的种群不同，这表明可能存在生态特征替代。线粒体 DNA 以及已发表的 AFLP 数据表明，无条纹种群不是单系的，因此可能已经独立进化了多次。总的来说，我们的数据表明，颜色和身体形状的进化可能是由于相关选择而耦合的。我们假设，性选择和生态特征的相关进化可能促成了快速的物种形成，以及已知最具多样性的适应性辐射之一的多样性的维持。
