Significance Heterostyly is an adaptation to promote outbreeding in plants. In heterostylous primroses, plants form flowers either with long styles and low anthers or with short styles and high anthers. This difference is due to a chromosomal segment containing five predicted genes, yet their roles and the evolution of this segment remain unclear. Here we identify the gene responsible for raising the anthers in short-styled flowers. This gene arose by duplication from a classical floral-organ identity gene and gained a novel function. Surprisingly, the responsible chromosomal segment appears to have evolved by stepwise gene duplications rather than duplication of an entire chromosomal block. These findings thus provide detailed insight into the evolution of complex polymorphisms involving different individual traits.