The rapid diversification of fertilization molecules in mammals (Clark NL, et al., 2006) is not likely to be explained by selection for species isolation, because the complex male-female interactions before copulation, and internal female barriers such as reduced sperm transport, probably make interactions between heterospecific eggs and sperm very rare (Gomendio et al., 1997). Both sperm proteins and egg coat zona pellucida glycoproteins show rapid divergence because of selection (Swanson et al., 2003), and the ZP2 and ZP3 egg glycoproteins are among the 10% most different proteins between rodents and humans (Clark NL, et al., 2006). Presumably SAC or CFC was involved in the divergence of these molecules.
Darwin did not apply his principle of sexual selection to plants, and this extension was a long time in coming (Willson and Burley, 1983; Andersson, 1994; Delph and Havens, 1997). Competition between males leading to sexual selection can occur both before and after the plant equivalent of insemination (arrival of pollen on the stigma). There is ample opportunity for females to exercise postcopulatory choice in processes such as pollen germination, growth of the pollen tube to reach the ovule, and maturation (versus abortion) of the resulting seeds and their fruit. Some otherwise paradoxical traits of females, like the production of inhibitors of pollen grain germination at the site where pollen grains must germinate (the stigma) until it is loaded with pollen, and initiation of many more seeds and fruit than will eventually mature, may be female-imposed mechanisms that impose “rules of the game” for male-male competition (Delph and Havens, 1997). The consistent finding that a female’s offspring show greater vigor when the competition among pollen grains from a single donor plant is more intense indicates a payoff for female selectivity that is compatible with CFC explanations (and contrary to those of SAC). Plant reproductive proteins are as yet only incompletely investigated, and evolutionary patterns of the genes and molecules involved cannot be checked (Clark NL, et al., 2006).
Two widespread traits make postcopulatory selection in plants likely to differ from that in most animals. Many plants are hermaphroditic, and their largely passive roles in pollination can lead to a common post-copulatory problem that is largely absent in animals: avoidance of self-fertilization. Some of the diversity in postcopulatory traits in plants is probably related to selection favoring avoidance of fertilization with self-pollen (Kao and Tsukamoto, 2004). Species-specific diversity may also result from selection to avoid hybridization between species. For instance, specificity in the structural complexity of the outer cell wall of the pollen grain is apparently responsible for its ability to adhere to the conspecific