Genetic population structure and neighbourhood population size estimates of the caddisfly Plectrocnemia conspersa

1. We used both genetic and ecological methods to evaluate the role of history and the scale of colonisation in structuring populations of the caddisfly Plectrocnemia conspersa. There was no genetic differentiation between sites up to 20 km apart, despite population sizes suggesting that genetic drift could create substantial variation at this scale. 2. Genetic differentiation between populations separated by more than 20 km was greater than expected given the contrasting short‐range trend, and implied a neighbourhood population size that is implausibly small. Therefore, the evolutionary processes that affect the short‐range trend do not explain differentiation over greater distances. 3. At small scales (<20 km), relatively short flights by winged adults spread over a number of generations could account for the spread of genes. For instance, dispersing individuals could found small (often temporary) populations, which may then grow and exchange genes with larger and more permanent local populations, amplifying the effects of the initial gene flow. 4. Over larger scales (20–500 km), substantial gaps between regions containing suitable habitat patches could reduce the number of colonisation events. Genetic patterns at this scale may date from the time they were last colonised. Previous ecological studies have rarely examined the dynamics of aquatic insect populations over these larger geographical scales, yet these processes may be central to their persistence and spread.