Conspicuous differences in floral morphology are partly responsible for reproductive isolation between two sympatric species of monkeyflower
because of their effect on visitation of the flowers by different
pollinators. Mimulus lewisii flowers are visited primarily by
bumblebees, whereas M. cardinalis flowers are visited mostly by
hummingbirds. The genetic control of 12 morphological differences
between the flowers of M. lewisii and M. cardinalis was explored in a
large linkage mapping population of F-2 plants (n = 465) to provide an
accurate estimate of the number and magnitude of effect of quantitative
trait loci (QTLs) governing each character. Between one and six QTLs
were identified for each trait. Most (9/12) traits appear to be
controlled in part by at least one major QTL explaining greater than or
equal to 25\% of the total phenotypic variance. This implies that
either single genes of individually large effect or linked clusters of
genes with a large cumulative effect can play a role in the evolution
of reproductive isolation and speciation.