Measuring the strength of selection and pollen limitation on flower colour along an urban rural cline in Hesperis matronalis

Urban and rural Hesperis matronalis show no detectable pollen limitation or flower-colour selection differences, leaving the urban–rural flower-colour cline unexplained by these mechanisms.

• Why it matters: Urban and rural habitats can impose different ecological pressures, so the cline in flower colour could reflect altered pollinator limitation or selection.
• What they did: Plants from urban and rural settings were tested with pollen supplementation to assess seed production, and flower-colour selection was quantified by measuring linear and non-linear selection across populations.
• The result: Pollen supplementation had no effect on seed production in either setting, and in all but one population significant linear or non-linear selection on flower colour was not detected, implying stochastic forces may underlie the cline.

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