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Phenotypic variation may be genetically determined or reflect phenotypic plasticity. More common plants are expected to be less differentiated between and within regions and more adapted than less common ones. However, such differences might not develop in hybridizing species which cannot evolve completely independently. We collected 311 genets of Carex flava, 215 of C. viridula and 46 of their hybrid C. × subviridula from 42 natural populations in cold temperate Estonia, mild temperate Lowland Switzerland and alpine Highland Switzerland. Three plantlets from each genet were planted to three experimental gardens, one in each region. We measured survival, growth, reproduction and morphological traits. The experimental transplants showed strong plasticity and grew smallest in the alpine garden. The less common C. viridula was slightly more differentiated between regions of origin than the more common C. flava and the hybrid. However, this depended on the experimental garden. Significant origin-by-garden-by-taxon and taxon-by-garden interactions suggest differential adaptation among populations and taxa. Regional differed from non-regional plants in several traits indicating both adaptations and, especially for C. viridula, maladaptations to the home regions. For C. flava, plant seed production was higher when mean annual temperature and precipitation were more similar between population of origin and garden, suggesting local adaptation to climate. Hybrids were intermediate between parental taxa or more similar to one of them, which was retained across gardens. We conclude that plasticity, genetic variation and genotype–environment interactions all contributed to regional differentiation of the closely related species. Hybridization did not completely align evolutionary patterns, and the less common species showed slightly more genetic differentiation between populations and more maladapted traits than the more common one.

Rarer species are expected to show stronger geographic differentiation than more common species. However, if rare species hybridize with common species, differentiation may be quite similar between the two due to genetic admixing via backcrossing. We studied morphological differentiation of plants of 21 natural populations of the more common Carex flava, 16 of the less common Carex viridula and 6 of their hybrids from 27 sites in three climatically different regions, Estonia, Lowland Switzerland and Highland Switzerland. Univariate ANOVA and multivariate principal component analysis of 14 morphological characters, describing both vegetative and reproductive characters, allowed to clearly distinguish C. flava from C. viridula. Carex viridula populations showed stronger regional variation than C. flava. Hybrids had both intermediate and transgressive characters in Switzerland and Estonia. On average, hybrids from Lowland Switzerland were more similar to Swiss C. flava than to C. viridula, while hybrids from Estonia were morphologically intermediate between plants of Estonian populations of the parental species. The results suggest that within-region genetic admixing between species has limited potential to lead to region-specific similarity between species, at least in our model system of the C. flava complex. We conclude that C. flava and C. viridula are clearly distinct species and that, despite hybridization, geographic differentiation is more pronounced in the less common C. viridula than in C. flava.