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 populat ions in the wild are rare. Those that exist have resulted from major change s in environment during an ongoing study. Selection response should be more common and more readily observable in short-lived organisms where the dire ction of selection changes from year to year. We examined how the interacti on of fluctuating selection, and emergence from long-term diapause, caused ongoing microevolutionary change over eight years in an important life-hist ory trait (diapause timing) in the freshwater calanoid copepod Diaptomus sa nguineus. Emergence from long-term diapause releases into the population li neages that did not experience the most recent bout of selection, thereby p romoting the maintenance of the heritable trait variation that allows conti nual selection response. A mechanistic selection model was created on the b asis of field and laboratory studies to predict how interannual variations in predation intensity generate year-to-year changes in mean diapause timin g and in net reproductive success for alternate trait values. The predicted selection response and the estimated effect of emergence from diapause wer e both significantly correlated with observed changes in trait mean. A line ar model combining selection response and emergence from diapause explained 59% of the variance in year-to-year changes in trait mean. According to th is model, strong selection occurred in about half of the years studied, and the average annual contributions to changes in trait mean from selection a nd emergence were roughly equal. Thus, both fluctuating natural selection a nd emergence from prolonged diapause affect the expression of diapause timi ng by D. sanguineus. Fluctuating selection is ubiquitous in nature and may provide opportunities in other populations to witness ongoing natural selec tion without directional trends in mean phenotype.