| Summary: | Lay Summary: The potential for populations to adapt rapidly to new environments will depend on the amount of genetic variation in multiple traits that make up a multidimensional phenotype. If traits are tightly correlated at the genetic level, then selection will change all traits together and adaptation will be forced into directions determined by the genetic architecture of the traits. However, genotypes can produce different phenotypes in different environments, known as plasticity. If genotypes vary in their responses to the environment, then plasticity in a novel environment could determine how much genetic variation lies in the direction of selection, and therefore the potential for rapid adaptation. We focus on two closely related sister species of Sicilian daisy (Senecio) that are native to low and high elevations on Mt. Etna. We generated and then reciprocally planted seeds of both species at four elevations on Mt. Etna, including their native habitats and two intermediate elevations. We tested how genetic variation in ecologically important leaf traits changed across elevations, and whether such changes should help or hinder rapid adaptation at the edge of species’ native ranges, and in novel environments beyond their existing ranges. We found that genetic variance in leaf traits changed less between species than across elevations. Genetic variance in the high-elevation species changed most across elevations, which occurred in ways that would be likely to prevent adaptation to low elevations and, by extension, the warmer conditions being created by climate change. Genetic variance in the low-elevation species changed least across elevations and showed more potential to aid adaptation to high-elevation habitats. Together, our results show that two sister species vary in their phenotypic and genotypic responses to the environment, which suggests that closely related species can differ greatly in their potential to persist and then adapt to novel environments.
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