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.

近年来，学界对若干极具说服力的同域物种形成（sympatric speciation）实例的记录，推动了相关理论模型的大量涌现。遗憾的是，自然界中看似合理的实例极少被用于验证模型预测——例如强歧化选择（disruptive selection）的初始存在性。本研究针对两例经典同域物种形成实例，估算了选择作用的形式与强度：分别为仅分布于巴伦比湖（Lake Barombi Mbo）与埃贾哈姆湖（Lake Ejagham）的喀麦隆慈鲷辐射演化类群。研究人员对两个湖泊中处于物种形成初期的物种复合群内的500余尾个体，测定了5项功能性状与相对生长速率。结果显示，两类群在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），以及多数已发表的歧化选择评估结果，本次测得的歧化选择强度相对较弱。此外，尽管两个物种复合群均存在遗传结构、选型交配（assortative mating）以及可区分物种的双峰色彩特征，但二者的性状分布均为单峰，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，可能会导致物种形成所需的等待时间过长，或是同域物种形成过程无法完成。除此之外，本研究还为两类物种复合群存在额外限制因素提供了证据：包括色彩与形态间的关联较弱、与非线性选择曲面（nonlinear selection surfaces）相符的形态方差降低，以及极小的生态分化水平。尽管该辐射演化类群中的其他物种已表现出完全的表型分化，但相较于其性色彩的快速分化，本研究涉及的物种复合群的形态与生态分化，在最优参数范围之外可能进展缓慢或无法完成。