Genetic Architecture, Spatial Heterogeneity, and the Arms Race between Newts and Snakes: Exploring Coevolution with Simulations

AbstractCoevolution between two species can lead to exaggerated phenotypes that vary in a correlated manner across space. However, the conditions under which we expect such spatially varying coevolutionary patterns in polygenic traits are not well understood. We investigate the coevolutionary dynamics between two species undergoing reciprocal adaptation across space and time using simulations inspired by the <i>Taricha</i> newt/<i>Thamnophis</i> garter snake system. One striking observation from this system is that newts in some areas carry much more tetrodotoxin than in other areas, and garter snakes that live near more toxic newts tend to be more resistant to this toxin, a correlation seen across several broad geographic areas. Furthermore, snakes seem to be "winning" the coevolutionary arms race, that is, having a high level of resistance compared with local newt toxicity, despite substantial variation in both toxicity and resistance across the range. We explore how possible genetic architectures of the toxin and resistance traits would affect the coevolutionary dynamics by manipulating both mutation rate and effect size of mutations across many simulations. We find that coevolutionary dynamics alone were not sufficient in our simulations to produce the striking mosaic of levels of toxicity and resistance observed in nature, but simulations with ecological heterogeneity (in trait costliness or interaction rate) did produce such patterns. We also find that differences in polygenicity do not seem sufficient to explain the observation that snakes seem to be winning.