Experimental studies of selection and adaptation in Clarkia xantiana (Onagraceae). III. Phenotypic selection across a subspecies

Sister taxa often differ phenotypically, yet we generally do not know whether trait divergence reflects adaptation to spatially-varying selection. In a reciprocal transplant experiment, we ask whether selection favors the native phenotype in two parapatric subspecies of Clarkia xantiana (Onagraceae), an annual plant endemic to California. We quantified phenotypic selection in each of the exclusive ranges and the contact zone in two years with contrasting climates. We predicted that in the arid, pollinator-scarce eastern region of the distribution, phenotypes of the native subspecies (parviflora) have an advantage: small leaves, slow leaf growth, accelerated flowering, and diminutive flowers. In the wetter, pollinator-rich, western range of subspecies xantiana, we expected selection for the opposite phenotypes. We predicted that temporal variation in selection would favor xantiana¬-like traits during the first wet El Niño year and parviflora-like traits in the second dry La Niña year. For subspecies xantiana, selection generally favored native phenotypes in the native garden. However, for parviflora, selection sometimes favored non-native trait values. We found little evidence for pollinator-mediated selection, subspecies differences in pollen limitation to reproduction, or temporal variation in selection. Phenotypic differentiation appears to reflect divergent selection in xantiana, but not parviflora.
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