Genetic variation in small, isolated fern populations

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Abstract
Differences in genetic variability of several small, isolated populations of four fern species in a restricted area in the Swiss lowlands reflect differences in breeding system, population size, the degree of population fragmentation, and ecological requirements. The investigated populations ofAsplenium septentrionaleshow only little genetic variability (isozyme variation) without gene flow among populations (based on the banding pattern of multi‐locus phenotypes), and they persist for long periods despite the small population sizes. InAsplenium ruta‐muraria, genetic variability is correlated with age. Young populations show no genetic variation, while old populations show some. All individuals ofPolypodium vulgareinvestigated, either epiphytic or epilithic, share exactly the same enzyme phenotype. The results for these three species can be related to predominance of inbreeding, lack of inbreeding depression, polyploidy, long‐distance dispersal, production of large amounts of diaspores, single‐spore colonization, and perennial life cycles. Genetic variability in these three species does not seem to be absolutely necessary for the maintenance of their populations. Ecological and demographic factors are considered to be more important. An isolated, glacial relict population of diploidAsplenium virideshows high variability in two out of eight enzyme systems, which may be due to prevailing outbreeding. We discuss aspects of the importance of genetics and life history for conservation biology.