The Temporal and Spatial Scale of Microevolution: Fine-scale Color Pattern Variation in the Lake Erie Watersnake

Abstract

Question: What is the temporal and spatial scale of microevolution? Hypotheses: The combined effects of natural selection and gene flow result in variation in heritable traits on fine spatial and geographic scales. Organism: The Lake Erie watersnake, Nerodia sipedon insularum. Field site: US and Canadian islands in western Lake Erie. Methods: We tested for variation in colour pattern frequency within islands, among islands, and over time using data from nearly annual censuses conducted since 1980, museum specimens, and published sources. We compared FST for a presumptive major colour pattern locus to FST for allozyme loci to determine whether spatial variation exceeded that expected by chance. We computed effective population size (Ne) based on temporal frequency changes in presumptive colour pattern alleles to determine whether temporal variation exceeded that expected by chance (Ne significantly less than ∞). Conclusions: Morph frequencies did not differ significantly within islands or between islands separated by short distances. Morph frequencies did sometimes differ significantly among distant islands and among sampling periods from 1980 to the present, but no more than expected by chance. In contrast, a marked change in morph frequency occurred between historic (prior to 1961) and recent (1980–2003) samples. Possible mechanisms include changes in the strength of selection (due to changes in predator assemblages and visual environments) and rates of gene flow (due to changes in island watersnake population size)