Why do plant species need to promote cross-fertilization rather than self-fertilisation?

Answer 1

Self- and Cross-Fertilization in Plants. I. Functional Dimensions, by David G. Lloyd and Daniel J. Schoen © 1992 The University of Chicago Press.

AbstractMany functional-ecological, morphological, and physiological-factors affect the occurrence of selffertilization. Six modes of self-pollination are distinguished. These differ in whether they utilize specialized flowers, whether they involve the transfer of pollen within or between flowers, whether they are autonomous or mediated by vectors, and their timing relative to opportunities for outcrossing. The various modes of selfing are subject to different structural constraints. Prepotency, the preferential success of cross-pollen in achieving fertilizations when it competes with self-pollen, influences the frequency of selffertilization in some species. The amount of self-fertilization may depend on environmental conditions and the vector species visiting each flower and may vary among the flowers of one plant. To gain information on the prevalence of autonomous self-pollination, 66 species for which the degrees of selfcompatibility and autofertility (seed set in isolation) have been published were surveyed. Partially selfincompatible species (in which the seed set is lower after self-pollination than after separate outcrosses) have on average lower autofertility than self-compatible species (in which self- and cross-pollinations succeed equally well), but some partially self-incompatible species have considerable autofertility and some self-compatible species have none. A number of features of floral morphology and phenology are associated with high Autofertility Indices.