Fluctuating selection and the maintenance of individual and sex-specific diet specialization in free-living oystercatchers.

Fluctuating and disruptive selection are important mechanisms for maintaining intrapopulation trait variation. Nonetheless, few field studies quantify selection pressures over long periods and identify what causes them to fluctuate. Diet specialists in oystercatchers differ in short-term payoffs (intake), but their long-term payoffs are hypothesized to be condition dependent. We test whether phenotypic selection on diet specialization fluctuates between years due to the frequency of specialists, competitor density, prey abundance, and environmental conditions. Short-term payoffs proved to be poor predictors of long-term fitness payoffs of specialization. Sex-differences in diet specialization were maintained by opposing directional fecundity and viability selection between the sexes. Contrasting other studies, selection on individual diet specialization was neither negative frequency- or density-dependent nor dependent on prey abundance. Notwithstanding, viability selection fluctuated strongly (stabilizing<-->disruptive) over the 26-year study period: slightly favoring generalists in most years, but strongly disfavoring generalists in rare harsh winters, suggesting generalists cannot cope with extreme conditions. Although selection fluctuated, mean selection on specialists was weak, which can explain how individual specialization can persist over long periods. Because rare events can dramatically affect long-term selective landscapes, more care should be taken to match the timescale of evolutionary studies to the temporal variability of critical environmental conditions.