Figure 1.

Urban populations of white clover, Trifolium repens, have evolved reduced cyanogenesis relative to non-urban populations in three cities. We observed parallel clines in cyanogenesis in: (a) Toronto (n populations = 121), (b) New York City (n = 48) and (c) Boston (n = 44), but not in (d) Montreal (n = 49). Each point represents the proportion of plants within a population that tested positive for cyanogenesis (mean n per population = 19.7 plants). Statistics were calculated using linear regression.