cross pollen pool. Because the selfers were morphologically quite different from the outcrossers in Mimulus and much less so in E. paniculata and I. purpurea, it may be that differences in the extent of pollen discounting are related to differences in floral morphology. This would be consistent with the observation that pollinator movement within multiple-flowered influorescences led to observable differences in the degree of pollen discounting in other experiments on E. paniculata (Harder and Barrett, 1995).
Just as the forces favoring evolution of self-fertilization, reproductive assurance, and automatic selection are well-known, so also is the primary force opposing its spread, inbreeding depression. Thomas Knight pointed out more than 200 years ago that the selfed progeny of garden peas are less vigorous and fertile than are outcrossed progeny (Knight, 1799). The impact that inbreeding depression has on the evolution of self-fertilization is, however, more complex than might be expected. The fate of a variant causing an increase in the rate of selfing depends not only on the magnitude of inbreeding depression, but also on the genetic basis of inbreeding depression, and on the magnitude of the difference in selfing rates that the variant induces (see Uyenoyama et al., 1993 for a detailed review). The complexity arises because different family lineages within a population may exhibit different degrees of inbreeding depression. Because selection among family lines is an important component of natural selection in partially self-fertilizing populations (see above), inbred families (those with a high frequency of alleles promoting self-fertilization) may show less inbreeding depression than less inbred families. If the extent of the association between family inbreeding depression and mating system is strong enough, selfing variants may spread even in the face of high population inbreeding depression (Holsinger, 1988).
The extent of associations between genetic variants affecting the mating system and levels of inbreeding depression in natural populations is not known. If the genomic rate of mutations to recessive or nearly recessive lethals is sufficiently high, levels of inbreeding depression are relatively insensitive to selfing rates (Lande et al., 1994), which will cause even families that differ substantially in their selfing rate to have similar levels of inbreeding depression. The relationship between within-population mating system differences and inbreeding depression also may be weak if mating system differences are polygenically controlled (Schultz and Willis, 1995). Experimental results are mixed. In Lobelia siphilitica (Campanulaceae) no differences in inbreeding depression could be found between females, which must outcross, and hermaphrodites, which self to some extent (Mutikainen and Delph, 1998), whereas in Gilia achilleifolia (Polemoniaceae) individuals with anthers and stigmas well separated (more outcrossing) have greater amounts of inbreeding depression than