Credit: Image courtesy of D. Schemske and T. Bradshaw.

Turning down flowers on the basis of their colour is unlikely to raise your popularity stakes. Yet, some animal pollinators are adapted for this particular choosiness, which enables them to single out the specfic plant species that they help to propagate by their colour. For example, the red-flowered species of the monkeyflower genus Mimulus is visited exclusively by hummingbirds, whereas its pink sister species is selectively pollinated by bumblebees. Such fastidiousness ensures the reproductive isolation of the two flower species in overlapping habitat ranges. By experimentally switching the colour of the two types of Mimulus, Toby Bradshaw and Doug Schemske have now shown that the adaptation of each plant species to a specific pollinator might be due, in large measure, to a mutation at a single locus that affects flower colour.

The YUP (yellow upper) gene controls whether yellow pigment is deposited in flowers: the pigment is present in the red, hummingbird-pollinated M. cardinalis, but absent in the pink, bumblebee-pollinated M. lewisii (shown in the figure). The authors substituted the YUP allele of one species by crossing it into the near-isogenic line of the other, to create pale, yellow-orange M. lewisii and dark-pink M. cardinalis. The effect of this colour change on pollinator presence was striking: bumblebees preferred the 'mutant' pink M. cardinalis — the species normally selected by birds — over the red, wild-type variety by 74-fold, and hummingbirds visited the 'mutant' yellow-orange M. lewisii — the species normally favoured by bees — 68 times more than the pink, wild-type one. That the effect was so large and symmetrical reinforces the idea that mutations in YUP have a marked effect on pollinator preference and that this locus alone — under the right ecological conditions — could initiate an adaptive shift in pollinator choice.

Could Ronald A. Fisher — the innovative twentieth-century statistician and evolutionary biologist — possibly have got it wrong? His theory that adaptation proceeds by the gradual accumulation of an infinite number of infinitesimally small steps might now have to give way to the increasingly popular view that the same result can be achieved by taking just a few, perhaps even one, big step. How close this study of pollinator shifts has come to demonstrating such a paradigm shift will depend on identifying additional mutations in the 'adaptive walk' from bumblebee to hummingbird pollination.