POPULATION STRUCTURE AND LOCAL SELECTION IN IMPATIENS PALLIDA (BALSAMINACEAE), A SELFING ANNUAL

The genetic structure of populations plays a dominant role in evolution. Levels of selection (Wade, 1978), the evolutionary consequences of interactions among conspecifics (Wilson, 1979; Wade, 1980) and the potential for differentiation (Slatkin, 1981) and speciation (Wright, 1980) are all influenced by population structure. Documentation of the factors which contribute to population structure is essential to an understanding of the process of evolution within populations. In addition, the amount of genetic variation within populations, and the extent to which local populations differ in absolute genetic variation influence the evolutionary potential of populations and species. Wright (1946, 1969) emphasized the importance of mating system on population structure, and demonstrated that self-fertilization and inbreeding promote genetic subdivision of populations. Because of the great diversity of breeding systems in plants, from almost complete selfing to outcrossing, breeding systems have a major impact on the structure of plant populations (Baker, 1953; Allard et al., 1968; Jain, 1975). Local populations of selfers may have less genetic variability than outcrossers (Solbrig, 1972; Jain, 1976; Gottlieb, 1977), but the genetic variation maintained by selfers is often far greater than expected (Imam and Allard, 1965; Kannenberg and Allard, 1967; Allard et al., 1968; Brown, 1979). Restricted recombination in selfing species results in high between-population variation (Wright, 1946; Jain, 1975) and extreme geographic differentiation (Harlan, 1945; Imam and Allard, 1965; Jain and Marshall, 1968; Hillel et al., 1 973a; Rick and Fobes, 1975). Although local population differentiation is well documented for outbreeders (Gregor, 1930; Bradshaw, 1959; Cook, 1962; Weil and Allard, 1964; Schaal, 1975), the reduced pollen flow of selfers increases their potential for between-population differentiation (Wright, 1969; Levin and Kerster, 1974). Because small effective population size is characteristic of many plant species (Levin and Kerster, 1974; Beattie and Culver, 1979), genetic drift is expected to be a major differentiating force. However, there is limited evidence for drift in natural plant populations (see Schall, 1975), and the relative importance of drift and selection continue to be debated (Bradshaw, 1972; Jain and Rai, 1974). In that mating system has a marked effect on the potential for differentiation through drift, a critical objective in plant microevolutionary studies is to determine the contributions of mating system and local selection to population structure. There are many examples of population differentiation in plants (see above), but few studies have examined population structure, local genetic variation and selection intensities in the context of plant mating system (see Antonovics, 1968). In this paper, I quantify -the extent of differentiation and local adaptation in Impatiens pallida (Balsaminaceae), a selfing annual. The goals of this research were to 1) determine the withinand between-population components of variation in quantitative characters, including several components of fitness, 2) quantify and compare the genetic variation present in local populations, and 3) experimentally determine the role of selection in population differentiation.