Introgressive hybridization facilitates adaptive divergence in a recent radiation of monkeyflowers

<jats:p>A primary goal in evolutionary biology is to identify the historical events that have facilitated the origin and spread of adaptations. When these adaptations also lead to reproductive isolation, we can learn about the evolutionary mechanisms contributing to speciation. We reveal the complex history of the gene<jats:italic>MaMyb2</jats:italic>in shaping flower colour divergence within a recent radiation of monkeyflowers. In the<jats:italic>Mimulus aurantiacus</jats:italic>species complex, red-flowered<jats:italic>M. a.</jats:italic>ssp<jats:italic>. puniceus</jats:italic>and yellow-flowered<jats:italic>M. a.</jats:italic>ssp.<jats:italic>australis</jats:italic>are partially isolated because of differences in pollinator preferences. Phylogenetic analyses based on genome-wide variation across the complex suggest two origins of red flowers from a yellow-flowered ancestor: one in<jats:italic>M. a.</jats:italic>ssp<jats:italic>. puniceus</jats:italic>and one in<jats:italic>M. a.</jats:italic>ssp.<jats:italic>flemingii</jats:italic>. However, in both cases, red flowers are caused by<jats:italic>cis</jats:italic>-regulatory mutations in the gene<jats:italic>MaMyb2</jats:italic>. Although this could be due to distinct mutations in each lineage, we show that the red allele in<jats:italic>M. a.</jats:italic>ssp.<jats:italic>puniceus</jats:italic>did not evolve<jats:italic>de novo</jats:italic>or exist as standing variation in its yellow-flowered ancestor. Rather, our results suggest that a single red<jats:italic>MaMyb2</jats:italic>allele evolved during the radiation of<jats:italic>M. aurantiacus</jats:italic>that was subsequently transferred to the yellow-flowered ancestor of<jats:italic>M. a.</jats:italic>ssp.<jats:italic>puniceus</jats:italic>via introgressive hybridization. Because gene flow is still possible among taxa, we conclude that introgressive hybridization can be a potent driver of adaptation at the early stages of divergence that can contribute to the origins of biodiversity.</jats:p>