Assortative mating, where individuals non‐randomly mate with respect to phenotype or genotype, can occur when preferences between potential mates have evolved. When such mate preferences occur in a population it can drive evolutionary and phenotypic divergence. But the extent to which assortative mating, mate preference, and development are evolutionarily linked remains unclear. Here we use Streblospio benedicti, a marine annelid with a rare developmental dimorphism, to investigate if mate‐choice could contribute to developmental evolution. For S. benedicti two types of ecologically and phenotypically similar adults persist in natural populations, but they give rise to distinctly different offspring with alternative life‐histories. This dimorphism persists despite the absence of post‐zygotic reproductive barriers, where crosses between the developmental types can produce phenotypically intermediate offspring. How this life‐history strategy evolved remains unknown, but assortative mating is a typical first step in evolutionary divergence. Here we investigate if female mate‐choice is occurring in this species. We find that mate preferences could be contributing to the maintenance of alternative developmental and life‐history strategies.

Assortative mating, where individuals non-randomly mate with respect to phenotype or genotype, can occur when preferences between potential mates have evolved. When such mate preferences occur in a population it can drive evolutionary and phenotypic divergence. But the extent to which assortative mating, mate preference, and development are evolutionarily linked remains unclear. Here we useStreblospio benedicti, a marine annelid with a rare developmental dimorphism, to investigate if mate-choice could contribute to developmental evolution. ForS. benedictitwo types of ecologically and phenotypically similar adults persist in natural populations, but they give rise to distinctly different offspring with alternative life-histories. This dimorphism persists despite the absence of post-zygotic reproductive barriers, where crosses between the developmental types can produce phenotypically intermediate offspring. How this life-history strategy evolved remains unknown, but assortative mating is a typical first step in evolutionary divergence. Here we investigate if female mate-choice is occurring in this species. We find that mate preferences could be contributing to the maintenance of alternative developmental and life-history strategies.