Sexual selection in plants

SummaryDarwin's theory of evolution by natural selection provided an immediately convincing explanation for the close fit between form and function in nature that had previously only been explicable in terms of supernatural design. Traits evolved in a way that improved their bearer's chances of survival and its success at producing offspring. But what could be said of exaggerated ornamental traits such as the long and lurid tail feathers of many male birds and the ferocious looking mandibles and horned protuberances of various male insects, which were almost certain to compromise their bearer's survival? To explain these traits, Darwin proposed the theory of sexual selection, first in ‘Origin of Species’ and then, at greater length, in ‘The Descent of Man’. In a nutshell, he argued that certain traits (secondary sex characters) will be favoured not because they improve survivorship or fecundity (i.e., by natural selection), but because they improve an individual's mating success. This basic idea has been broadly accepted by zoologists, but it has been contentious when applied to plants, not least because they are often hermaphrodites. In this Primer, we explain the application of sexual-selection ideas to both dioecious and hermaphroditic plants. We point out that, far from being irrelevant to their study, sexual selection to increase male mating success can be interpreted as a major selective force in the evolution of floral diversity (Figure 1).