Island biology and morphological divergence of the Skyros wall lizard Podarcis gaigeae: a combined role for local selection and genetic drift on color morph frequency divergence?

 

















We have recently published a paper on evolutionary processes in isolated islet populations of the Skyros wall lizard, Podarcis gaigeae in BMC Evolutionary Biology. The paper investigates the relationship between neutral genetic divergence and morphological divergence in islet- and mainland populations. The morphological trait we use in the comparisons is throat colour morph, and islet populations show pronounced frequency differences with different morphs being common on different islets. Our data suggests that stochastic forces such as genetic drift and/or founder effects can interact with selection and have an effect even at a morphological level in islet populations with low effective sample sizes. BMC Evolutionary Biology is an open access journal and a link to the paper is found here. The abstract is as follows:

Island biology and morphological divergence of the Skyros wall lizard Podarcis gaigeae: a combined role for local selection and genetic drift on color morph frequency divergence?

Anna Runemark, Bengt Hansson, Panagiotis Pafilis, Efstratios D. Valakos & Erik I. Svensson

Background

Patterns of spatial variation in discrete phenotypic traits can be used to draw inferences about the adaptive significance of traits and evolutionary processes, especially when compared to patterns of neutral genetic variation. Population divergence in adaptive traits such as color morphs can be influenced by both local ecology and stochastic factors such as genetic drift or founder events. Here, we use quantitative color measurements of males and females of Skyros wall lizard, Podarcis gaigeae, to demonstrate that this species is polymorphic with respect to throat color, and the morphs form discrete phenotypic clusters with limited overlap between categories. We use divergence in throat color morph frequencies and compare that to neutral genetic variation to infer the evolutionary processes acting on islet- and mainland populations.

Results

Geographically close islet- and mainland populations of the Skyros wall lizard exhibit strong divergence in throat color morph frequencies. Population variation in throat color morph frequencies between islets was higher than that between mainland populations, and the effective population sizes on the islets were small (N_e:s <>ST) for throat color morph frequencies fell within the neutral F_ST-distribution estimated from microsatellite markers, and genetic drift could thus not be rejected as an explanation for the pattern. Moreover, for both comparisons among mainland-mainland population pairs and between mainland-islet population pairs, morph frequency divergence was significantly correlated with neutral divergence, further pointing to some role for genetic drift in divergence also at the phenotypic level of throat color morphs.

Conclusions

Genetic drift could not be rejected as an explanation for the pattern of population divergence in morph frequencies. In spite of an expected stabilising selection, throat color frequencies diverged in the islet populations. These results suggest that there is an interaction between selection and genetic drift causing divergence even at a phenotypic level in these small, subdivided populations.