Abstract
Increasing urbanization offers a unique opportunity to study adaptive responses to rapid environmental change. Numerous studies have demonstrated phenotypic divergence between urban and rural organisms. However, comparing the direction and magnitude of natural selection between these environments has rarely been attempted. Using seven years of monitoring of great tits (Parus major) breeding in nest-boxes across the city of Montpellier and in a nearby oak forest, we find phenotypic divergence in four morphological and two life-history traits between urban and forest birds. We then measure reproductive selection on these traits, and compare selection between the habitats. Urban birds had significantly smaller morphological features than their rural counterparts, with a shorter tarsus, lower body mass, and smaller wing and tail lengths relative to their overall body size. While urban female tarsus length was under stabilizing selection, and forest males show positive selection for tarsus length and negative selection for body mass, selection gradients were significantly divergent between habitats only for body mass. Urban great tits also had earlier laying dates and smaller clutches. Surprisingly, we found selection for earlier laying date in the forest but not in the city. Conversely, we detected no linear selection on clutch size in the forest, but positive selection on clutch size in the urban habitat. Overall, these results do not support the hypothesis that contemporary reproductive selection explains differences in morphology and life history between urban- and forest-breeding great tits. We discuss how further experimental approaches will help confirm whether the observed divergence is maladaptive while identifying the environmental drivers behind it.
Highlights
Human activities drive tremendous environmental changes in a wide range of ecosystems around our planet
Research in urban ecology during the past decade has revealed many cases of phenotypic divergence between urban and native habitats. This is especially true for birds, which have been studied for over a century [3], and for whom studies have shown divergence in morphology, physiology, behaviour, song [4 – 6] and life history [7], highlighting the existence of urban versus rural ecotypes & 2018 The Authors
As all four morphological traits were correlated with body size, estimations of selection differentials were performed on measures corrected for size by using residuals of a linear regression between the trait and tarsus length run on the pooled dataset with both urban and forest birds
Summary
Human activities drive tremendous environmental changes in a wide range of ecosystems around our planet. As our goal was mean + s.d. females males fixed effects habitat year sex age tarsus length habitatÂyear sexÂage hour to understand the adaptive mechanisms explaining urban/forest divergence in a set of traits with little access to complete life histories, we opted for an annual measure of fitness rather than lifetime reproductive success [31]. As all four morphological traits were correlated with body size (electronic supplementary material, table S1), estimations of selection differentials were performed on measures corrected for size by using residuals of a linear regression between the trait (body mass, wing or tail length) and tarsus length run on the pooled dataset with both urban and forest birds. Significance (i.e. p-values) of these selection differentials and gradients were estimated from linear mixed models including year and individual ring number (only female identity for life-history traits) as random effects to control for variation across years and repeated measurements on the same individuals. Made between habitats for each trait with a Fisher variance ratio test (var.test(), R package stats)
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