Leaf shape variation and taxonomic boundaries in two sympatric rupicolous species of Helichrysum (Asteraceae: Gnaphalieae), assessed by linear measurements and geometric morphometry

Quantifying and understanding the causes of population differentiation in plants are critical for assessing the taxonomic structure of species, as the level of differentiation among populations may correspond to incipient speciation. Using linear and geometric morphometric approaches, phenotypic variation and associations between leaf traits and geographic and environmental characteristics were investigated in Helichrysum crassifolium and the widespread and sympatric Helichrysum pendulum: two species exhibiting conspicuous leaf variation in size and shape. The morphometric results show that both species have distinct patterns of leaf shape variation. However, overlapping values link the ranges of variation: a significant variance is explained by withinindividual and population differentiation, and some environmental variables are moderately correlated with leaf shape parameters in both species. The fact that reproductive barriers between both species are weak, and that the morphological plasticity detected in both species appears to be non-adaptive, and to a high extent nonenvironmentally induced, allow us to postulate that the variations observed could be mediated by hybridization processes. Our results suggest that niche differentiation, which involves the colonization of a specialized rupicolous habitat, does not necessarily lend itself as an effective strong reproductive barrier between closely related species.