Data from: Contrasting the effects of natural selection, genetic drift and gene flow on urban evolution in white clover (Trifolium repens)

Urbanization is a global phenomenon with profound effects on the ecology
and evolution of organisms. We examined the relative roles of natural
selection, genetic drift and gene flow in influencing the evolution of
white clover (Trifolium repens), which thrives in urban and rural areas.
Trifolium repens exhibits a Mendelian polymorphism for the production of
hydrogen cyanide (HCN), a potent antiherbivore defence. We quantified the
relative frequency of HCN in 490 populations sampled along urban-rural
transects in 20 cities. We also characterized genetic variation within 120
populations in eight cities using 16 microsatellite loci. HCN frequency
increased by 0.6% for every km from an urban centre, and the strength of
this relationship did not significantly vary between cities. Populations
did not exhibit changes in genetic diversity with increasing urbanization,
indicating that genetic drift is unlikely to explain urban-rural clines in
HCN frequency. Populations frequently exhibited isolation-by-distance and
extensive gene flow along most urban-rural transects, with the exception
of a single city that exhibited genetic differentiation between urban and
rural populations. Our results show that urbanization repeatedly drives
parallel evolution of an ecologically important trait across many cities
that vary in size, and this evolution is best explained by urban-rural
gradients in natural selection.