Sexual selection in plants

This discussion of plant reproduction is convenient for the next subject: sexual selection. Sexual selection was another brainchild of Darwin (1871) who realized that characters that made organisms successful at acquiring a mate might be selected for, and that this can account for some of the extraordinary exaggerated traits of organisms, such as antlers of deer and long tails and colourful plumage of pheasants. The first person to explicitly model female choice as the exaggerating process was Fisher, who imagined runaway selection (Chapter 1). Females would initially prefer some males more than others and these males would then gain greater reproductive success. Over time the preference and the male trait would become coupled and increase in magnitude until countered by some cost of the male trait.A second more recent set of models, collectively known as good gene models, assume that female choice is costly and that therefore it should not be arbitrary. Instead, females should be selected to choose males that provide them or their offspring with some kind of advantage in natural selection. The particular traits females choose should in fact be correlated with some fitness advantage in natural selection.

Are there any plant traits that might have evolved through sexual selection? The obviously exaggerated plant reproductive traits are large showy flowers and long pistils, which may have evolved by male-male competition. In the former case, one can imagine male flowers competing with each other for pollinators to remove pollen. The phenomenon of fleur-du-male, whereby male flowers are overproduced and then aborted, may well be a sexually selected phenomenon: a plant makes itself as showy as possible to attract as many pollinators as possible into the vicinity. For example, many Agave mckelveyama flowers are aborted before fruit initiation, and these are always functionally male, and give high nectar rewards (Sutherland 1987). Therefore some flowers, indeed, seem to serve merely to attract pollinators and have no fruiting function. Queller (1983) studied Asclepias exaltata and showed that inflorescence size is positively related to pollen removal, while seed set is unaffected. This therefore suggests that the showy trait benefits male function much more than female function.

Despite the possible role of sexual selection in flower evolution, plants in general do not work through the showy media of vision and sound that animals do, so we should expect the effects of sexual selection to be more cryptic. In particular, we might expect evolution of competition between pollen. Pollen grains are known to inhibit each other's growth in some species, and in other species pollen grain size is related to fertilization success. Female choice and male-male competition are likely to interact through pollen competition, as the female tissues provide the arena for pollen competition. In peach trees, the base of the style reduces after pollination and this increases competition between pollen grains by reduction in both resource flow and space (Herrero and Hormaza 1996). Pollen competitiveness also sometimes reflects male quality: pollen tube growth rate in violets increases with the seed production of the donor in violets, and this translates into offspring vigour (Skogsmyr and Lankinen 2000).

The interactions between pollen and pollen, and pollen and sporophyte therefore provide intriguing examples of sexual selection at the boundary of male-male competition and female choice. Much more clearly defined examples of female choice occur in seed abortion. The last section has already shown how this can be selective according to male genotype, and also increase offspring vigour. Once again though, the effects of this choice are relatively cryptic to human eyes.

It is clear that the biological differences between plants and animals sometimes provide us with evolutionary outcomes that are relatively unique to one kingdom or the other. It is equally clear,however, that theory developed for applications in one kingdom can be usefully translated across to the other and enhance understanding. While nobody can deny the success of behavioural ecology, the term itself has limited the application of its findings to different taxa. Evolutionary ecology of course has no taxonomic boundaries and it is on this wider stage that the full implications of theory will be realized.

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