Evolutionary potential is dependent on the additive genetic variance displayed by important adaptive behavioral and life-history traits, such as phenology. However, the genetic variance of such traits may vary over the life span. Using a long-term study (1994â€“2008) of arrival date from spring migration in the long-lived common tern (Sterna hirundo), we examined changes in variances of this key phenological trait across reproductive stages (before, during, and after the first breeding event). Based on 5315 records from 1232 individuals, arrival date at the breeding grounds exhibited significant declines in phenotypic and additive genetic variances across reproductive stages. Canalization of this phenological trait and selective disappearances across reproductive stages are hypotheses to explain these declines. Canalization was revealed by significant reductions in phenotypic and additive genetic variations at progressive reproductive stages. Viability analyses rejected the selective disappearance hypothesis. Heritability of arrival date also declined with reproductive stage (from 0.23 to 0.11). Finally, arrival date was under fecundity selection for experienced breeders, suggesting a current influence on canalization via fecundity selection. These results highlight how fine-tuned quantitative genetic investigations can reveal canalization in behavioral traits, reflecting past and present selective forces, and refining predictive evolutionary potential.
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