Coalescent Simulations and Field Experiments Support Natural Selection as the Driving Force Maintaining Color Differences Between Adjacent Populations of Ceroglossus chilensis (Coleoptera: Carabidae)

TL;DR

This study shows that natural selection, particularly from predators, drives color differences in adjacent populations of a ground beetle species.

Contribution

The study provides direct evidence that selection, not genetic drift, maintains color divergence in Ceroglossus beetles.

Findings

Color fixation in beetles deviates from expectations under genetic drift, indicating selection.

Beetles that do not mimic local color patterns face higher predation rates.

Predators are the main selective force maintaining color differences in the species.

Abstract

Observable characteristics of organisms can evolve by different mechanisms such as genetic drift and selection. In this study, we aimed to assess which of these forces are responsible for the differences in color between various adjacent populations of a ground beetle species, Ceroglossus chilensis, using genetics and field experiments. Our approach supported selection as the main force driving color differences and showed that predators are the main selective force. This study contributes to the understanding of the processes driving diversity in Ceroglossus chilensis and provides insights regarding which processes may be involved in the pattern of mimicry observed within the genus.

Determining the drivers of phenotypic evolution and their role on shaping regional phenotypic diversity is a fundamental aspect of mimicry research. Beetles of the genus Ceroglossus are well known for their rampant color diversity, and species have been shown to exhibit convergence in elytral coloration when in sympatry. Despite solid phylogenetic evidence showing that color convergence is not the result of shared ancestry, more direct evidence of the role of selection on color evolution is lacking. In this study, we used two different, but complementary approaches to test whether selection plays a role in color divergence and whether predators are responsible for its maintenance. We first used molecular data to assess gene flow. Subsequently, we used the sequence data to conduct coalescent simulations and generate expectations for color fixation under the hypothesis of genetic drift. Second, we conducted field experiments to test whether predators exhibited different preferences on mimetic versus non-mimetic phenotypes. Results from the coalescent simulations showed that color fixation exhibited strong deviations from the genetic drift expectation, implying that selection must be responsible for color divergence. Field experiments showed that translocated beetles (into areas where they were not mimics) suffered higher rates of attacks than those beetles that mimicked the locally abundant phenotypes. Thus, our study supports the hypothesis of selection as a driving force of color evolution in Ceroglossus beetles and provides the context that explains and supports previous claims of Müllerian mimicry.

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