Allele substitution at a flower colour locus produces a pollinator shift in monkeyflowers

The role of major mutations in adaptive evolution has been debated for more than a century1,2. The classical view is that adaptive mutations are nearly infinite in number with infinitesimally small phenotypic effect3, but recent theory suggests otherwise4. 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 UPPER5,6,7 (YUP). YUP controls the presence or absence of yellow carotenoid pigments in the petals of pink-flowered Mimulus lewisii, which is pollinated by bumblebees5,8,9,10, and its red-flowered sister species11 M. cardinalis, which is pollinated by hummingbirds5,8,9,10. 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.