Developmental mode consists of suites of phenotypic and behavioral traits that can influence micro- and macro-evolutionary patterns and processes including gene-flow, local adaptation, and speciation and extinction. In marine invertebrates there are typically two discrete types of developmental mode: large non-feeding, non-dispersive lecithotrophic larvae and small feeding and highly dispersive planktotrophic larvae. A few species exhibit intraspecific variation for developmental mode (termed poecilogony) and provide powerful systems to identify the minimum number of genetic changes and environmental influences that underlie developmental mode evolution. I used an evolve and resequence approach to identify the genomic response to selection for increased proportions of lecithotrophy in the poecilogonous sea slug Alderia willowi. Lecithotrophy increased from 36% to 60-70% after 5 generations of selection in low (16 ppt) and high salinity (32 ppt) across 6 replicate populations. In a genome-wide test of association I found 10 loci on 8 scaffolds to be associated with developmental mode, showing the polygenic nature of developmental mode in A. willowi. Sequencing the replicate lines after selection I found a significant change in allele frequency for these loci that was shared across replicate lines. These results show that there is standing genetic variation for developmental mode in A. willowi that may be maintained via phenotypic plasticity.