1. Department of Biology, University of Washington, Seattle, Washington 98195, USA
2. Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824, and W. K. Kellogg Biological Station, Hickory Corners, Michigan 49060, USA

Correspondence to: H. D. Bradshaw, Jr¹ Email: toby@u.washington.edu

Abstract

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 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 UPPER^{5, 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 bumblebees^{5, 8, 9, 10}, and its red-flowered sister species¹¹ M. cardinalis, which is pollinated by hummingbirds^{5, 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.