Résumé
Individual variation among and within natural populations can have
eco-evolutionary implications by, for example, affecting species
interactions or evolutionary potential. Urban systems present a unique
opportunity to evaluate how environmental change shapes variation since
urban phenotypic differentiation is widely documented on contemporary
timescales. We introduce and test three hypotheses to determine how
urbanization affects phenotypic variation at different population levels.
Combining 21 long-term datasets in a mega-analysis approach, we synthesize
how urbanization impacts variation in tarsus length and lay date among and
within subpopulations of great and blue tits (Parus major, Cyanistes
caeruleus) at a continental scale. Our synthesis reveals that urbanization
is associated with increased phenotypic variation within subpopulations by
11% on average, and by as much as 25% across the species and traits
examined. We also find some evidence (for tarsus length in great tits)
that urbanization increases differentiation between subpopulations. We did
not, however, find that urbanization increases differences between
subpopulations in their within-subpopulation variation. Our synthesis
provides novel insights into how urban contexts impact individual
diversity at different spatial scales and we highlight future directions
that could establish the genetic and environmental effects that underlie
these continental patterns of urban phenotypic variation.