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,Erik I Svensson,Panayiotis Pafilis,Bengt Hansson,Efstratios D Valakos

doi:10.1186/1471-2148-10-269

Abstract

BackgroundPatterns 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.ResultsGeographically 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 (Ne:s < 100). Population divergence (FST) for throat color morph frequencies fell within the neutral FST-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.ConclusionsGenetic 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.

Highlights

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
Color morphs: visual classification and spectral analysis A discriminant function analysis based on the seven first principle components extracted from the throat images revealed that the six visually classified throat color morph groups were highly statistically different from each other (F = 31.81, df = 7, 318, P < 0.001) (Fig. 3). 14 out of 15 post-hoc tests between the different groups were highly significant (P < 0.001), with the only exception for the yellow-orange morph that did not differ from the orange (P = 0.11)
All main colors (O, Y, W) - not all six throat color morphs - were found in all mainland populations and in two of the three islet populations of P. g. gaigeae (Fig. 2B)

Summary

Introduction

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. 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. Theoretical models of speciation, so-called “peak-shift models”, suggested that genetic drift must be a strong force if a population was to move from one adaptive peak to another, and cross a valley with lower fitness [5,6]. These early models during a short and transient period if selection is temporally relaxed [7,8,9,10]. Various forms of stochasticity have been ignored in most evolutionary studies, and it has been pointed out that genetic drift can take place in large populations [12]

Objectives

Methods

Results

Discussion

Conclusion