dition known as heterostyly. Short anthers are found in flowers with long stigmas and vice versa. Darwin (1877) described the classic example of this system in Primula veris. In that species as in many others, morphological differences are associated with compatibility differences that allow pollen derived from short anthers to germinate only on short stigmas and pollen derived from long anthers to germinate only on long stigmas (see also Barrett, 1992). In other species of flowering plants and in all gymnosperms sexual functions are separate from one another. Either pollen and ovules are produced in different structures on the same plant (monoecy) or they are produced on different plants (dioecy). The separation of sexual functions also may be associated with physiological and ecological differences between the sexes, as in Siparuna grandiflora in which females and males have different patterns of distribution within populations (Nicotra, 1998).
The genetic structure of a species comprises the identity and frequency of genotypes found within populations and the distribution of genotypes across populations. The reproductive system has long been recognized as a predominant influence on the genetic structure of plant species. Asexual progeny are genetically identical to the individuals that produced them, except for differences caused by somatic mutation. Selfed progeny may differ from their parent as a result of segregation at heterozygous loci, but selfing usually produces far fewer genotypes among offspring than outcrossing. As a result, fewer genotypes usually are found in populations in which either form of uniparental reproduction is common than in those in which outcrossing is the norm.
Among sexually reproducing species, selfers have populations with a smaller and more variable effective sizes (Schoen and Brown, 1991) and with less exchange of alleles among individuals within and among populations. As a result, selfing species are usually more homozygous than close relatives and have fewer genotypes per population than outcrossers. They also typically have fewer polymorphic loci and fewer alleles per polymorphic locus than closely related outcrossers (Brown, 1979; Gottlieb, 1981). In addition, the diversity found within selfing species is more a result of differences among populations than of differences among individuals within populations. Over 50% of the allozyme diversity found in selfers is attributable to differences among populations, whereas only 12% is attributable to differences among populations in outcrossers (Hambrick and Godt, 1989).
Allozyme and restriction site analyses of chloroplast DNA (cpDNA) in Mimulus (Scrophulariaceae) and nucleotide sequence analyses of in-