Random and non-random variation in flower colour along an urban-rural gradient in the introduced mustard Hesperis...

Urbanization can alter the interplay of evolutionary processes such as natural selection and genetic drift, but these effects will depend on the biology and history of a species. To explore the influences of drift and selection in an urban context, we investigated patterns of flower colour variation among stands of the introduced ornamental mustard Hesperis matronalis along an urban-rural gradient in eastern Ontario, Canada.

We surveyed 136 naturalized stands of H. matronalis over three generations, and for each stand estimated the diversity of the three colour morphs (white, pink, purple), the number of reproductive plants and the degree of urbanization based on night sky brightness.

Flower colour morph diversity increased with both stand size and urbanization, which is consistent with effects of genetic drift during colonization combined with multiple introductions of this horticultural plant in urban areas. However, the frequency and fixation of the purple morph systematically increased towards the rural end of the gradient. Although lifetime seed production did not vary among morphs, pre-dispersal seed predation by a recent adventive weevil was higher in the purple morph, particularly in rural areas. Estimated seed production in the absence of predation possibly suggests a previous fitness advantage for the purple and pink morphs in rural areas and for the white morph in urban areas.

Random variation in flower colour diversity may be influenced by stochastic processes and colonization history, while systematic variation in colour morph frequencies may reflect past fitness differences among morphs that have been recently erased by seed predation.