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.