Maintenance of soil ecotypes of Solidago virgaurea in close parapatry via divergent flowering time and selection against immigrants

The often patchy distribution of serpentine geology can lead to abrupt changes in soil and microclimates. Thus, serpentine areas provide an ideal natural setting to understand how divergent selection drives the process of local adaptation in edaphically specialized plants. When the serpentine ecotype is surrounded by a related nonserpentine ecotype, a balance of natural selection and potential gene flow should maintain the different ecotypes over very short distances. We aimed to reveal the mechanisms allowing soil ecotypes of a goldenrod species to co‐occur sympatrically in Japan. We performed field surveys to characterize microenvironments and flowering phenology of each ecotype, common garden and reciprocal transplant experiments and artificial crossing, and population genetic analysis to investigate the levels of genetic differentiation between ecotypes. Growth chamber experiments show that serpentine plants showed lower specific leaf area (SLA) and greater resource allocation to their root systems than did their nonserpentine counterparts, a potential adaptation to drier and less fertile soil condition in serpentine habitats. Reciprocal transplant studies demonstrated a clear pattern of local adaptation in the plant growth rate. Importantly, serpentine populations completed flowering by midsummer versus late summer in nonserpentine plants. This pattern is consistent with the hypothesis that early flowering ensures reproductive success, before the microclimatic conditions becomes severe in open habitats. Although prezygotic isolation was a strong barrier to gene flow, genetic differentiation was very low, indicating a recent origin for the serpentine ecotypes and/or gene flow at low frequencies. Synthesis. The findings indicate that the early flowering times of serpentine ecotypes, which would have been selected for by microclimates in serpentine areas, can play roles in local adaptation, but also population isolation via a by‐product of diverged reproductive timings. This study contributes to general understanding of the initial stages of plant ecological speciation under potential gene flow in very small geographic scales. Without any effective geographic barriers, the Solidago soil ecotypes were maintained by divergent selection imposed by microclimatic conditions in alternative habitats. Importantly, the divergent flowering time played dual roles of a local adaptation to each habitat and assortative mating to further restrict gene flow between popu