Investigating developmental evolution usually requires comparing differences across related species to infer how phenotypic change results from embryological modifications. However, when comparing organisms from different environments, ecologies, and evolutionary histories there can be many confounding factors to finding a genetic basis for developmental differences. In the marine annelid Streblospio benedicti, there are two distinct types of offspring with independent developmental pathways that converge on the same adult phenotype. To my knowledge, S. benedicti is the only known species that has heritable (additive) genetic variation in developmental traits that results in alternative life-history strategies. Females produce either hundreds of small, swimming and feeding larvae, or dozens of large, nonfeeding larvae. The larvae differ in their morphology, ecology, and dispersal potential. This developmental dimorphism makes S. benedicti a unique and useful model for understanding how genetic changes result in developmental modifications that ultimately lead to overall life-history differences. Because the offspring phenotypes of S. benedicti are heritable, we can use forward genetics within a single evolutionary lineage to disentangle how development evolves, and which genes and regulatory mechanisms are involved.