Genetic Variation within and among Populations of Ferns

Abstract

The evolutionary potential of a species depends in part on the levels of genetic variation maintained within the species and also on the distribution of that variation both within and among its populations. Analyses of electrophoretic data for seed plants have resulted in several generalizations regarding levels and distribution of genetic variation: (1) High levels of genetic variation are maintained in populations of species with outcrossing mating systems, wide (regional) geographic distributions, habitats of late successional stages, and wind-pollination (Hamrick et al., 1979), (2) Inbreeding populations often exhibit genetic structure (i.e. a nonrandom distribution of genotypes in space), whereas genotypes are typically randomly distributed in highly outcrossing populations (Loveless & Hamrick, 1984), and (3) Outcrossing species maintain genetic variation within single populations, whereas inbreeding species distribute relatively more of their genetic variation among rather than within populations (Brown, 1979). Because ferns differ from seed plants in several life-history characteristics, especially in their life cycle and potential for large amounts of interpopulational gene flow via spore dispersal, it is unclear whether species of ferns follow these generalizations. Therefore, we address the following topics in homosporous ferns: (1) levels of genetic variation within populations of ferns, (2) the genetic structure of populations of ferns, and (3) the genetic structure of species of ferns. For all three topics we review the patterns of genetic variation that have been observed in ferns and then discuss factors that may contribute to these patterns. Electrophoretic data from the literature will be used to provide estimates of genetic variation. Data from the literature will be supplemented by our published and previously unpublished data for North American species of Polystichum. LEVELS OF GENETIC VARIATION WITHIN POPULATIONS OF FERNS Typical measures used to quantify the amount of genetic variation present in natural populations are P, the proportion of loci that are polymorphic in the population, A, the mean number of alleles per locus in a population, H, mean heterozygosity in a population (that is, heterozygosity expected at HardyWeinberg equilibrium), and Hobs, observed heterozygosity in the population. Measures of genetic variation for most of the diploid fern species that have been analyzed are presented in Table 1. Mean values for each measure of genetic variation cover a broad range. P ranges from 0.067 in Polystichum lonchitis to 0.635 in Pellaea andromedifolia. A ranges from 1.08 in P. lonchitis to 2.62 in Bommeria hispida. Hobs ranges from 1.08 in P. lonchitis to 0.221 in P. andromedifolia.