Résumé
Amounts of genetic variability, genetic differentiation among taxa and populations, and population sizes were studied in five populations of Centaurea maculosa ssp. maculosa (a widespread taxon), all six populations of C. corymbosa (a narrowly endemic species), and the single population of C. maculosa ssp. albida. Seventeen isozyme loci were studied, of which nine were polymorphic. Results suggest that C. corymbosa and C. maculosa ssp. albida are likely derived from C. maculosa ssp. maculosa because the former represent a sample of the diversity of the latter. The percentage of polymorphic loci and Nei's genetic diversity were positively and significantly correlated with population size over all populations, but not within each taxon. Populations of both the widespread C. maculosa ssp. maculosa and the rare C. corymbosa were strongly differentiated: overall, FST values were 0.26 and 0.34, respectively. Differentiation among populations of different taxa was of the same order of magnitude as that observed among populations within taxa. Nevertheless, significant differentiation among the three taxa was found by a hierarchical analysis of variance on allele frequencies. We suggest that bottlenecks or founder effects associated with colonization events and ecological specialization in some populations of C. maculosa ssp. maculosa have led to new taxa such as C. corymbosa and C. maculosa ssp. albida. This may be a direct consequence of the particularly strong differentiation among populations of the widespread C. maculosa ssp. maculosa. Our study highlights the utility of considering closely related widespread taxa in order to understand the population biology and evolution of rare species, as well as to design proper management programs.