Fluctuating selection among years in a wild insect.

Abstract

Temporal or spatial variation in selection has the potential to explain long standing evolutionary problems such as evolutionary stasis and the maintenance of genetic variation. Long-term field studies of plants and wild vertebrates have provided some insights, but multigenerational measures of selection in wild invertebrates remain scarce. Short-lived ectothermic animals are likely to experience more pronounced environmental variation across generations than longer-lived and endothermic species. As a result, variation in selection may be particularly significant in these groups. Over ten years, we have monitored an individually tagged population of wild crickets (Gryllus campestris) using a network of up to 133 day-night video cameras. The over a million hours of video that we watched allowed us to capture detailed information about naturally and sexually selected traits and life-history parameters. Over ten discrete generations, population size ranged from 51 to 546 adults. There were also substantial differences among years in the average values of traits including adult emergence date, body size, lifespan, and several behavioral traits. We combined measurements of these traits with individual fitness, measured as the number of adult offspring inferred from genetic-marker-based parentage assignments. This revealed substantial variation in selection gradients across years in several traits, with evidence that in one trait, adult emergence date, selection switched from positive to negative among years. Our findings suggest that fluctuations in selection gradients are common but complete reversals in the direction of selection may not be very frequent.