Beschreibung:
Studies comparing rare and widespread congeners are particularly valuable when examining hypotheses regarding the condition of rarity. Comparison of close relatives can minimize confounding effects of phylogenetic history and, optimally, eliminate ecological factors not directly relevant. Among various hypotheses regarding rarity, this paper focuses on genetic correlates and potential ecophysiological restrictions, using a comparison from the coneflower genus Echinacea as an example. Echinacea tennesseensis is an endangered species endemic to limestone "cedar glades" of Tennessee. Echinacea angustifolia is a widespread prairie species. Genetic comparison of these close relatives supports the hypothesis that rare species have lower levels of genetic variability within populations than do widespread species. However, an ecophysiological study measuring photosynthesis following different light and soil moisture preconditioning regimes provides no evidence that narrow tolerances for these factors are responsible for the endemic species' narrow geographical range. These results for the Echinacea congeners are compared with other relevant studies.