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.