Abstract:
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A standard approach to model how selection shapes phenotypic traits is the
analysis of capture–recapture data relating trait variation to survival. Divergent
selection, however, has never been analyzed by the capture–recapture approach.
Most reported examples of differences between urban and nonurban animals
reflect behavioral plasticity rather than divergent selection. The aim of this
paper was to use a capture–recapture approach to test the hypothesis that
divergent selection can also drive local adaptation in urban habitats. We
focused on the size of the black breast stripe (i.e., tie width) of the great tit
(Parus major), a sexual ornament used in mate choice. Urban great tits display
smaller tie sizes than forest birds. Because tie size is mostly genetically determined,
it could potentially respond to selection. We analyzed capture/recapture
data of male great tits in Barcelona city (N = 171) and in a nearby (7 km) forest
(N = 324) from 1992 to 2008 using MARK. When modelling recapture rate,
we found it to be strongly influenced by tie width, so that both for urban and
forest habitats, birds with smaller ties were more trap-shy and more cautious
than their larger tied counterparts. When modelling survival, we found that
survival prospects in forest great tits increased the larger their tie width (i.e.,
directional positive selection), but the reverse was found for urban birds, with
individuals displaying smaller ties showing higher survival (i.e., directional negative
selection). As melanin-based tie size seems to be related to personality,
and both are heritable, results may be explained by cautious personalities being
favored in urban environments. More importantly, our results show that divergent
selection can be an important mechanism in local adaptation to urban
habitats and that capture–recapture is a powerful tool to test it. |