Balancing selection and drift in a polymorphic salamander metapopulation

Abstract

Understanding how genetic variation is maintained in a metapopulation is a longstanding problem in evolutionary biology. Historical resurveys of polymorphisms have offered efficient insights about evolutionary mechanisms, but are often conducted on single, large populations, neglecting the more comprehensive view afforded by considering all populations in a metapopulation. Here, we resurveyed a metapopulation of spotted salamanders ( Ambystoma maculatum ) to understand the evolutionary drivers of frequency variation in an egg mass colour polymorphism. We found that this metapopulation was demographically, phenotypically and environmentally stable over the last three decades. However, further analysis revealed evidence for two modes of evolution in this metapopulation—genetic drift and balancing selection. Although we cannot identify the balancing mechanism from these data, our findings present a clear view of contemporary evolution in colour morph frequency and demonstrate the importance of metapopulation-scale studies for capturing a broad range of evolutionary dynamics.

Ecosystem effects of intraspecific variation in a colour polymorphic amphibian

“…However, the colour morph difference in embryo survival we observed here (figure 2) matches the results of Pintar & Resetarits [27], who also reared egg masses in shallow outdoor mesocosms (electronic supplementary material, S1). Given the sensitivity of developing Ambystoma embryos and larvae to ultraviolet radiation [53,[66][67][68][69][70], we suspect that the lower survival of clear morphs under these conditions is explained by the low transmission of harmful ultraviolet light through white jelly [21]. To date, a photoprotective function for white jelly has only been evaluated twice [53,54].…”

“…Although egg masses with intermediate coloration are occasionally reported, evidence for an intermediate morph is limited [22,23]. Indeed, this polymorphism is most commonly treated as a discrete dimorphism comprising clear and white morphs [21,[24][25][26][27], an approach we follow here. Polymorphic populations are common and can be found across the large geographical range of the spotted salamander [21][22][23]25].…”

“…Indeed, this polymorphism is most commonly treated as a discrete dimorphism comprising clear and white morphs [21,[24][25][26][27], an approach we follow here. Polymorphic populations are common and can be found across the large geographical range of the spotted salamander [21][22][23]25]. Within this extensive geographical range, morph frequencies exhibit striking among-population variability-even reaching down to microgeographic spatial scales [21,23,27,28].…”

“…Polymorphic populations are common and can be found across the large geographical range of the spotted salamander [21][22][23]25]. Within this extensive geographical range, morph frequencies exhibit striking among-population variability-even reaching down to microgeographic spatial scales [21,23,27,28]. As with many polymorphisms, there has been little work to explore whether this extensive variation has any ecological relevance.…”

Microgeographic evolution of metabolic physiology in a salamander metapopulation

“…In this study, we explored evidence for evolution of metabolic physiology in a spotted salamander (Ambystoma maculatum) metapopulation in northeastern Connecticut, USA. Previous work in spotted salamanders shows that despite gene flow and genetic drift, spatial heterogeneity in predation risk, population density, and water chemistry can generate clear signatures of adaptation in foraging behavior, morphology, and physiology (Urban 2007a, 2013, Brady 2012, Urban and Richardson 2015Giery et al 2021b). Notably, this trait variation emerges across distances less than the average dispersal distance of adult spotted salamanders, demonstrating the microgeographic scale at which evolutionary differentiation can occur in this system (Richardson and Urban 2013).…”