Evolutionary biologists continue to disagree about the relative importance of natural selection, drift and phylogenetic constraint in determining characteristics of an organism1. Because of the difficulty of identifying examples of selection in nature there are few rigorous field studies of selection2−6. We have been studying selection on flower colour in the small perennial larkspur Delphinium nelsonii, a native to mountains of the western USA. Previously we showed that white-flowered forms, which are very rare in natural populations, produce fewer seeds than their common blue-flowered conspecifics, and that this selective disadvantage results from partial discrimination against white flowers by bumblebee and hummingbird pollinators7. Here we present evidence that discrimination occurs because white flowers have inferior 'nectar guides' and therefore require longer handling times than blue flowers. Pollinators may thus experience lower net rates of energy intake on white flowers, a sufficient reason for undervisitation by optimally-foraging animals.
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