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stigma in species with “dry” stigmas (Zinkl et al., 1999; Clark NL, et al., 2006). But crucial data (see previous section) are generally not available to discriminate among the inbreeding avoidance, species isolation, CFC, and SAC explanations for generating these traits.

Second, female plants may be more likely than female animals to be able to reap indirect payoffs from screening among pollen grains. A large fraction of the genome of a pollen grain is expressed during the growth of the pollen tube [up to ≈⅔ of the expressed genome in a mature plant; Mascarenhas (1990)]. In contrast, the genome of a sperm cell is largely silent (Pizzari and Foster, 2008). More vigorous pollen tube growth tends to correlate with more vigorous growth by the resulting offspring (Rocha and Stephenson, 1990; Delph and Havens, 1997), thus favoring female abilities to select among pollen tubes.

A further potentially intense filter of males in plants is the often substantial rate of abortion of zygotes before maturation of seed and fruit. One yet to be explored possibility is that sexual selection on males promoted genetic imprinting as a mechanism to reduce the chances that the male’s offspring would be aborted.

CONCLUDING THOUGHTS

The traits just reviewed share a strong trend: rapid divergent evolution. There are reasons to suppose that sexual selection has been important in many cases, but it is possible that no single explanation accounts for all cases or rapid divergence. Selection for species isolation mechanisms may have had an important role in the evolution of abalone lysins, but species isolation has probably been of little importance in producing the widespread rapid divergent evolution of male genitalia or mammal sperm and egg proteins.

It is impressive to see the long shadow that one of Darwin’s few omissions had in the history of studies of sexual selection. Despite the resulting delays, Darwin’s theory of sexual selection is now inspiring progress in explaining new findings and directing research in various fields that involve postcopulatory male-female interactions.

ACKNOWLEDGMENTS

I thank F. Ayala and J. Avise for inviting me to participate in the colloquium and M.-J. West-Eberhard, H. Lessios, and 2 referees for useful comments. This work was supported by the Sminthsonian Tropical Research Institute and the Universidad de Costa Rica.



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