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From at least the time of Darwin (1876) botanists have tried to understand why there are plants that are primarily self-fertilizing even though the progeny produced by self-fertilization(whether of normally outcrossing or normally selling plants) are often less vigorous than the progeny produced by cross-fertilization (see, eg, Schoen 1983). The explanation commonly proposed has been that the breeding system of a species is a compromise between the conflicting demands of" immediate f-ss" and" long-range fitness"(Mather 1943). The ecological conditions in wnich a given species is found are supposed to determine whether it is predominantly self-fertilized or predominantly outcrossed (Stebbins 1950, 1957; Grant 1958).

That there is variation in outcrossing rates between species is common knowledge. Studies documenting intraspecific variation in outcrossing rates are rarer, but they do exist, eg, the studies of …