Fluctuating selection among years in a wild insect (Gryllus campestris)

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.