Data from: Weak disruptive selection and incomplete phenotypic divergence in two classic examples of sympatric speciation: Cameroon crater lake cichlids

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.

近期，若干具有说服力的同域物种形成（sympatric speciation）实例被报道，推动了相关理论模型的大量涌现。遗憾的是，自然界中具备合理性的实例极少被用于检验模型预测——例如强歧化选择（disruptive selection）的初始存在性。本研究针对两例经典的同域物种形成实例，估算了选择的形式与强度：分别为局限于巴伦比湖（Lake Barombi Mbo）与埃贾哈姆湖（Lake Ejagham）的喀麦隆丽鱼（cichlid）适应辐射类群。本研究对两个湖泊中处于物种形成初期的物种复合体的500余尾个体，测定了5项功能性状与相对生长速率。两类类群的单性状轴与多性状轴上均普遍存在歧化选择，但相较于其他性状所受的稳定选择（stabilizing selection），以及绝大多数已发表的歧化选择强度估算值，本研究观测到的歧化选择强度较弱。此外，尽管两个物种复合体均存在遗传结构、选型交配（assortative mating）以及双峰的物种诊断性体色，但性状分布均为单峰，表明其正处于物种形成的最早阶段。当初始歧化选择强度较弱时，物种形成过程可能耗时过长，或是同域物种形成无法完全完成。此外，本研究还为两个物种复合体存在的额外限制因素提供了证据：包括体色与形态间的关联较弱、与非线性选择曲面（nonlinear selection surfaces）相符的形态变异度降低，以及极有限的生态分化（ecological divergence）。尽管该适应辐射类群中的其他物种已呈现出完全的表型分化，但与性征体色的快速分化形成鲜明对比的是，这些物种复合体的形态与生态分化，在最优参数范围之外可能进展缓慢，或是无法完全完成。