THE APPORTIONMENT OF GENETIC VARIATION WITHIN AND AMONG POPULATIONS OF DESMODIUM NUDIFLORUM

A major aspect of the study of plant evolution has been the determination and comparison of the genetic structures of species. Features such as the levels of genetic variability, the factors which influence levels of variability, and the apportionment of genetic variability within and among populations all strongly affect the evolutionary potential of species. Measurements of differences in these characteristics have been used to assess the different evolutionary paths species have taken. From a number of studies of plant species there is evidence that levels of detectable variability vary widely from almost none in Typha (Masburn et al., 1978) to relatively high levels of variability in Stephanomera exigua ssp. carotifera (Gottlieb, 1975). The level of genetic variability is in some cases associated with variation in edaphic factors (Babbel and Selander, 1974), with karyotype (Levin, 1974), with geographical variation (Levin, 1977), or with population structure (Schwaegerle and Schaal, 1979). While many studies have examined genetic variability among populations of a species, less attention has been focused on the apportionment of genetic variability within individual populations. Genetic substructuring of plant populations may be a consequence of either local differentiation due to different selective values (Jain and Bradshaw, 1966), or gene flow restriction due to self-fertilization, nonrandom pollinator behavior or localized seed dispersal. Extensive work has shown that gene dispersal in many plant species is very restricted (see review by Levin and Kerster, 1974). Such limited gene flow in a single DoDulation may lead to genetic subdivision into subpopulations, each of which is characterized by different gene frequencies (Wright, 1943a). Studies of several plant species (e.g., Linanthus parryae, Wright, 1943b; Liatris cylindracea, Schaal, 1974) have indicated that populations of some plant species may be subdivided. The purpose of the current study is to determine the levels of genetic substructuring within populations of Desmodium nudiflorum (L) DC (Leguminosae) and to compare the apportionment of genetic variation within and among populations of the species. Desmodium nudifiorum is an herbaceous plant which occurs locally in black oak-white oak communities throughout the eastern deciduous forest. Extant populations of D. nudiflorum are restricted in much of the range to well established, secondary successional forests located in public and private nature preserves. Extant populations are geographically isolated, although the species was certainly more common before the clearing of the eastern forests. Populations of D. nudifiorum rarely exceed 200 individuals and the plant density rarely exceeds one or two plants per meter square. The species is perennial, reaching an estimated maximum age in excess of ten years. Adult plants of D. nudiflorum range in height from 14 to 29 cm and produce up to 109 flowers, with a mean of 50 flowers per plant (Smith, 1971). Twenty-two percent of the flowers produce fruit; each fruit contains an average of 2.8 seeds. Outcrossing is predominant in D. nudiforum with bumblebees as the major pollinators. Asexual reproduction does not occur. The seeds of D. nudiflorum are caDable of dis-