Island-wide population genomic analysis of the coastal shrub, beach naupaka ( Scaevola taccada L.), on Oʻahu

Abstract Plant species face myriad threats to their survival, including climate change and habitat destruction. On Oʻahu in Hawaiʻi, Scaevola taccada L., a coastal dune shrub, is highly impacted by human activities and environmental threats. Despite its ubiquity across Pacific islands, the extent and distribution of genetic diversity within individual islands is unclear. We surveyed genomic diversity using double digest restriction-associated DNA sequencing (ddRAD-seq) in eight populations of S. taccada on Oʻahu. Using principal components (PCA) and STRUCTURE analyses, we assessed whether genetic diversity was structured geographically and the extent to which selection and migration structured genetic diversity. We analysed climate and leaf thickness data to understand how patterns of genetic diversity were correlated with environmental and trait variation. Our analyses pointed to a trend in which populations in protected areas were genetically differentiated from, and harboured less genetic diversity than, human-dominated areas. Analyses of genetic structure indicated that western beach populations were differentiated from eastern beach populations. FST outlier analyses suggest that selection is not primarily responsible for the differentiation seen between the protected populations and the human-dominated areas. Our results demonstrate that S. taccada populations in protected natural areas harboured unique genetic variation compared to human-dominated beach parks, but that protected populations also had significantly reduced genetic variation compared to most other populations. Relatively few FST outliers, paired with evidence of continued gene flow, suggest that current levels of genetic diversity are more a function of isolation and drift as opposed to selection.