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?

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.

The influence of stochastic and selective forces in the population divergence of female colour polymorphism in damselflies of the genus Ischnura

“…Runemark et al, 2010). The degree of differentiation between pairs of populations at the colour locus in I. elegans was in several cases considerably larger than at the neutral loci, suggesting that divergent selection can have important effects in shaping morph frequencies in I. elegans.…”

“…The degree of correlation can inform us about the importance of genetic drift and selection in maintaining colour-morph frequencies (Runemark et al, 2010). A strong correlation would suggest that the colour-morph frequency in local populations is largely affected by genetic drift, whereas a weak correlation would be indicative of selection.…”

“…A strong correlation would suggest that the colour-morph frequency in local populations is largely affected by genetic drift, whereas a weak correlation would be indicative of selection. A moderate correlation would indicate a combination between drift genetic and selection (Runemark et al, 2010).…”

Antagonistic selection factors induce a continuous population divergence in a polymorphism

“…The degree of correlation can inform us about the importance of genetic drift and selection in maintaining color morph frequencies (Runemark et al, 2010). A strong correlation would suggest that the color morph frequency in local populations is largely affected by genetic drift, whereas a weak correlation would be indicative of selection (Sánchez-Guillén et al, 2011).…”

“…Correlation between F ST values for neutral loci with those for the loci in question indicates the significance of the effect of random drift on population divergence (Runemark et al, 2010). In addition, insights into the presence of divergent selection, balancing selection and random drift can be obtained by contrasting the degree of genetic differentiation (e.g., F ST ) at a focused locus to the degree of differentiation at neutral markers (Gillespie and Oxford, 1998); divergent selection, balancing selection and no selection is acting if ratio F ST (focused)/F ST (neutral) is greater than, smaller than and equal to 1, respectively, as in the case of phenotypic traits (Lynch and Walsh, 1998).…”