Résumé
Urban areas are increasing globally, providing opportunities for
biodiversity researchers to study the process in which species become
established in novel, highly disturbed habitats. This ecological process
can be understood through analyses of morphological and genetic variation,
which can shed light on patterns of neutral and adaptive evolution.
Previous studies have shown that urban populations often diverge
genetically from non-urban source populations. This could occur due to
neutral genetic drift, but an alternative is that selection could lead to
allele frequency changes in urban populations. The development of genome
scan methods provides an opportunity to investigate these outcomes from
samples of genetic variation taken along an urbanization gradient. Here we
examine morphological variation in wing size and diversity at neutral
amplified fragment length polymorphisms in the butterfly Pieris rapae L.
(Lepidoptera, Pieridae) sampled from the center to the periphery of
Marseille. We utilize established and novel environmental correlation
approaches to scan genetic variation for evidence of selection. We find
significant morphological differences in urban populations, as well as
weak genetic structure and decreased genetic diversity in urban versus
non-urban sites. However, environmental correlation tests provide little
support for selection in our dataset. Our comparison of different methods
and allele frequency clines suggests that loci identified as significant
are false positives. Although there is some indication that selection may
be acting on wing size in urban butterflies, genetic analyses suggest P.
rapae are undergoing neutral drift.