trons associated with two nuclear genes in Leavenworthia (Brassicaceae) illustrate the dramatic impact mating systems can have on the genetic structure of plant species. In Mimulus both allozyme and nucleotide sequence diversity in a selfing species are only one-fourth that of a closely related outcrossing species (Fenster and Ritland, 1992). The lower nucleotide diversity in cpDNA might not be expected, because it is maternally inherited, but in highly selfing species background selection against deleterious alleles at nuclear loci can substantially reduce diversity in both nuclear and cytosolic genomes (Charlesworth et al., 1995; Charlesworth et al., 1997). In Leavenworthia populations of selfers are composed almost entirely of a single haplotype at each of the two loci studied, and each population is characterized by a different haplotype. In outcrossers, on the other hand, individuals belonging to the same population are only a little more similar to one another than were individuals belonging to different populations (Liu et al., 1998, 1999). In addition, balancing selection appears to be responsible for maintaining an electrophoretic polymorphism at the locus encoding phosphoglucose isomerase in outcrossing species of Leavenworthia (Filatov and Charlesworth, 1999). Thus, selfers may have lower individual fitness than outcrossers, because they are genetically uniform at this locus.
The consequences of asexual reproduction are in some ways similar to those of selfing. In a strictly asexual population there is no exchange of genes among family lines, just as there is none within a completely selfing population. In contrast to selfers, however, asexual genotypes reproduce themselves exactly, except for differences caused by somatic mutation. Thus, the frequency of heterozygotes can be large in asexual populations even if the number of genotypes found is quite small, especially because many apogamous or agamospermous plants are derived from products of hybridization (Manton, 1950; Stebbins, 1950; Grant, 1981). Agamospermous Crepis in western North America, for example, are polyploids derived from hybridization between different pairs of seven narrowly distributed diploid progenitors (Babcock and Stebbins, 1938), and local populations are composed of relatively few genotypes. Moreover, most of the genetic diversity in the entire set of agamospermous species, which are facultatively sexual, is attributable to multiple origins rather than sexual recombination (Whitton, 1994; Holsinger et al., 1999).
Although asexual populations are virtually guaranteed to have many fewer genotypes than sexual populations with similar allele frequencies, the number of genotypes within a population can still be quite large. Allozyme studies revealed between 15 and 47 clones in populations of the salt-marsh grass Spartina patens on the east coast of North America (Silander, 1984) and 13–15 clones of the daisy Erigeron annuus (Hancock and Wilson, 1976). When the number of genotypes per population is large,