Department of Evolution & Ecology and Center for Population Biology, University of California, Davis, California 95616, USA.
Am Nat. 2012 Oct;180(4):E90-E109. doi: 10.1086/667586. Epub 2012 Aug 24.
Recent documentation of a few compelling examples of sympatric speciation led to a proliferation of theoretical models. Unfortunately, plausible examples from nature have rarely been used to test model predictions, such as the initial presence of strong disruptive selection. Here I estimated the form and strength of selection in two classic examples of sympatric speciation: radiations of Cameroon cichlids restricted to Lakes Barombi Mbo and Ejagham. I measured five functional traits and relative growth rates in over 500 individuals within incipient species complexes from each lake. Disruptive selection was prevalent in both groups on single and multivariate trait axes but weak relative to stabilizing selection on other traits and most published estimates of disruptive selection. Furthermore, despite genetic structure, assortative mating, and bimodal species-diagnostic coloration, trait distributions were unimodal in both species complexes, indicating the earliest stages of speciation. Long waiting times or incomplete sympatric speciation may result when disruptive selection is initially weak. Alternatively, I present evidence of additional constraints in both species complexes, including weak linkage between coloration and morphology, reduced morphological variance aligned with nonlinear selection surfaces, and minimal ecological divergence. While other species within these radiations show complete phenotypic separation, morphological and ecological divergence in these species complexes may be slow or incomplete outside optimal parameter ranges, in contrast to rapid divergence of their sexual coloration.
最近有一些关于同域物种形成的引人注目的例子的记录，这导致了理论模型的大量涌现。不幸的是，自然界中可信的例子很少被用来检验模型的预测，例如最初存在强烈的破坏性选择。在这里，我估计了两个同域物种形成经典例子中的选择形式和强度：喀麦隆慈鲷在巴龙比姆博湖和埃贾加姆湖的辐射。我在每个湖中测量了 500 多个来自初始物种复合体的个体的五个功能性状和相对生长率。在两个群体中，在单个性状轴和多变量性状轴上都存在着破坏性选择，但相对于其他性状和大多数已发表的破坏性选择估计值来说，这种选择是较弱的。此外，尽管存在遗传结构、交配选择和双峰物种诊断性颜色，但在两个物种复合体中，性状分布都是单峰的，表明这是物种形成的最早阶段。当破坏性选择最初较弱时，可能会导致等待时间长或不完全的同域物种形成。或者，我在这两个物种复合体中都提出了额外限制的证据，包括颜色和形态之间的弱联系、与非线性选择面一致的形态方差减小，以及最小的生态分歧。虽然这些辐射中的其他物种表现出完全的表型分离，但在这些物种复合体中，形态和生态的分歧可能在最佳参数范围之外是缓慢的或不完全的，这与它们的性色迅速分化形成对比。
