A paradigm of evolutionary biology is that adaptation and reproductive isolation are caused by a nearly infinite number of mutations of
individually small effect. Here, we test this hypothesis by
investigating the genetic basis of pollinator discrimination in two
closely related species of monkeyflowers that differ in their major
pollinators. This system provides a unique opportunity to investigate
the genetic architecture of adaptation and speciation because floral
traits that confer pollinator specificity also contribute to premating
reproductive isolation. We asked: (i) What floral traits cause
pollinator discrimination among plant species? and (ii) What is the
genetic basis of these traits? We examined these questions by using
data obtained from a large-scale field experiment where genetic markers
were employed to determine the genetic basis of pollinator visitation.
Observations of F-2 hybrids produced by crossing bee-pollinated Mimulus
lewisii with hummingbird-pollinated Mimulus cardinalis revealed that
bees preferred large flowers low in anthocyanin and carotenoid
pigments, whereas hummingbirds favored nectar-rich flowers high in
anthocyanins. An allele that increases petal carotenoid concentration
reduced bee visitation by 80\%, whereas an allele that increases nectar
production doubled hummingbird visitation. These results suggest that
genes of large effect on pollinator preference have contributed to
floral evolution and premating reproductive isolation in these
monkeyflowers. This work contributes to growing evidence that
adaptation and reproductive isolation may often involve major genes.