FLUCTUATING SELECTION AND RESPONSE IN A POPULATION OF FRESHWATER COPEPODS

The timing of springtime production of diapausing eggs by a population of the freshwater copepod, Diaptomus sanguineus, has been shown previously to be consistent with avoidance of seasonally intense fish predation. Natural selection acting on the timing of diapause fluctuates between years depending upon the population density of fish. Here we show that, in the field, the mean timing of diapause shifts between years in response to fluctuations in selection. Diapause is earlier in years following high predator density, and is later in years following low predator density. Although selection intensity in individual years may be large, the mean intensity over the decade of fluctuating selection investigated here is close to zero. Photoperiod sensitivity of the diapausetiming trait is heritable in the laboratory. The combination of fluctuating selection and multi-generation storage of genotypes as diapausing eggs in lake sediments may contribute to the maintenance of the genetic variation that permits the rapid selection response seen in the field.