Balancing selection and drift in a polymorphic salamander metapopulation

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 demonstrates the importance of metapopulation-scale studies for capturing a broad range of evolutionary dynamics.

Methods
Raw egg mass morph and water chemistry data for 31 ponds included in this study. We relocated 31 previously surveyed spotted salamander populations from annotated maps in Sadinski (1991) and resurveyed them in April 2020. To survey each population one observer slowly waded each pond and counted each color morph. The attached file includes count data; of the number of white morphs (n white morph), and the total number of egg masses seen (n total egg masses). Prior to each survey, the observer measured pH with an Oakton PCTS 35 probe and collected water for dissolved organic carbon (DOC) analysis with a Turner handheld fluorometer. Historical methods are detailed in Ruth (1992). Dissolved organic carbon (DOC), Ca, Na, K and Mg concentrations are all in mg/L. 

Sadinski WJ. 1991 Direct and indirect effects of low pH on the communities of temporary ponds. PhD Thesis, The Pennsylvania State University, Pennsylvania USA.

Ruth BC. 1992 Possible Adaptive Significance of the Visible Polymorphism of Spotted Salamander Egg Masses. Master’s Thesis, The Pennsylvania State University, Pennsylvania USA.