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Abstract

Novel selection pressures in new environments arise through two distinct processes. First, environmental conditions directly affect the fitness of different phenotypes. Second, phenotypic plasticity alters the distribution of phenotypes, thereby placing populations in new selective regimes. A small isolated population of dark-eyed juncos Junco hyemalis became established in San Diego, probably in the early 1980s and probably from the nearby mountains. The relatively mild coastal climate has resulted in an increase in both the mean and the variance of the length of time females breed each year, and this is assumed to be a result of phenotypic plasticity. The population has evolved reduced white in the tail. We studied contemporary patterns of selection on tail white, in the context of the altered breeding season length. Late-hatched nestlings had higher survival and were in better condition than early-hatched nestlings, but among survivors, late-hatched birds had less tail white. We suggest this reflects juvenile mortality favoring individuals with less tail white. In adults, we found weak sexual selection and no viability selection but positive selection on female tail white in association with fecundity. We argue that altered breeding season length had a major impact on patterns of selection and evolution in this population.