The role of major mutations in adaptive evolution has been debated for more than a century(1,2). The classical view is that adaptive mutations
are nearly infinite in number with infinitesimally small phenotypic
effect(3), but recent theory suggests otherwise(4). To provide
empirical estimates of the magnitude of adaptive mutations in wild
plants, we conducted field studies to determine the adaptive value of
alternative alleles at a single locus, YELLOW UPPER5-7 (YUP). YUP
controls the presence or absence of yellow carotenoid pigments in the
petals of pink-flowered Mimulus lewisii, which is pollinated by
bumblebees(5,8-10), and its red-flowered sister species(11) M.
cardinalis, which is pollinated by hummingbirds(5,8-10). We bred
near-isogenic lines (NILs) in which the YUP allele from each species
was substituted into the other. M. cardinalis NILs with the M. lewisii
YUP allele had dark pink flowers and received 74-fold more bee visits
than the wild type, whereas M. lewisii NILs with the M. cardinalis yup
allele had yellow-orange flowers and received 68-fold more hummingbird
visits than the wild type. These results indicate that an adaptive
shift in pollinator preference may be initiated by a single major
mutation.