THE ROLES OF FLUCTUATING SELECTION AND LONG-TERM DIAPAUSE IN MICROEVOLUTION OF DIAPAUSE TIMING IN A FRESHWATER COPEPOD

Direct observations of selection response in natural, unmanipulated populations in the wild are rare. Those that exist have resulted from major changes in environment during an ongoing study. Selection response should be more common and more readily observable in short-lived organisms where the direction of selection changes from year to year. We examined how the interaction of fluctuating selection, and emergence from long-term diapause, caused ongoing microevolutionary change over eight years in an important life-history trait (diapause timing) in the freshwater calanoid copepod Diaptomus sanguineus. Emergence from long-term diapause releases into the population lineages that did not experience the most recent bout of selection, thereby promoting the maintenance of the heritable trait variation that allows continual selection response. A mechanistic selection model was created on the basis of field and laboratory studies to predict how interannual variations in predation intensity generate year-to-year changes in mean diapause timing and in net reproductive success for alternate trait values. The predicted selection response and the estimated effect of emergence from diapause were both significantly correlated with observed changes in trait mean. A linear model combining selection response and emergence from diapause explained 59% of the variance in year-to-year changes in trait mean. According to this model, strong selection occurred in about half of the years studied, and the average annual contributions to changes in trait mean from selection and emergence were roughly equal. Thus, both fluctuating natural selection and emergence from prolonged diapause affect the expression of diapause timing by D. sanguineus. Fluctuating selection is ubiquitous in nature and may provide opportunities in other populations to witness ongoing natural selection without directional trends in mean phenotype.