How Sexual Selection Came To Be Recognized
Charles Darwin proposed that all living species were derived from common ancestors. The primary mechanism he proposed to explain this fact was natural selection: that is, that organisms better adapted to their environment would benefit from higher rates of survival than those less well equipped to do so. However he noted that there were many examples of elaborate, and apparently non-adaptive, sexual traits that would clearly not aid in the survival of their bearers. He suggested that such traits might evolve if they are sexually selected, that is if they increase the individual's reproductive success, even at the expense of their survival (Darwin 1871).
Darwin noted that sexual selection depends on the struggle between males to access females. He recognized two mechanisms of sexual selection: intrasexual selection, or competition between members of the same sex (usually males) for access to mates, and intersexual selection, where members of one sex (usually females) choose members of the opposite sex. The idea of cumbersome traits evolving to aid males in competition during aggressive encounters was readily accepted by scientists shortly after Darwin's publication. However, the idea of female mate choice was received with ridicule, and was not seriously reconsidered until nearly 80 years later (Cronin 1991). In the 40 years since, there has been much progress in our understanding of how sexual selection operates.
Which Sex is Under Stronger Selection?
Sex roles are defined by differences in gametes: females produce relatively few, highly nutritious (usually non-motile) gametes, whereas males produce comparatively abundant, smaller, motile gametes. Because only a single gamete of each type is required to produce an offspring, there will be an excess of male gametes that will not fertilize any eggs. This asymmetry leads to Bateman's principle, whereby female reproduction is primarily limited by their access to resources to nourish and produce these large gametes, whereas male reproduction is mainly limited by access to females (Bateman 1948). Therefore males typically compete among themselves for access to females, whereas females tend to be choosy and mate only with preferred males.
In sexually reproducing species, every offspring has one father and one mother, so the average reproductive success is equal for both males and females. A successful male can potentially sire many offspring. If a male gains a disproportionate share of reproduction, he will take away reproductive opportunities from other males, leading to a high reproductive variance among males. A successful female, on the other hand, will not take away reproductive opportunities from other females, leading to a smaller variance in reproductive success. The higher the reproductive variance, the stronger the effects of sexual selection (Figure 1). Strong sexual selection typically results in sexually dimorphic traits that are exaggerated, or more elaborate, in the sex with highest reproductive variance (Figure 1).
Males and females in a population have the same average reproductive success (R. S., black bars) but they may differ in the reproductive variance among members of each sex (shown in red). Differences in the selection gradient will result in sexual dimorphism. (A) When males are subject to stronger sexual selection than females, males will evolve secondary sexual characters that result in marked differences between the sexes. Peacocks do not provide any parental care, and some males are more successful than others who may never reproduce, leading to marked dimorphism. (B) When males contribute to offspring care, the selection gradient is lower and the sexes will be monomorphic. Many seabirds are monogamous and raise offspring together and the sexes are indistinguishable. (C) When males provide all the parental care, the selection gradient can be reversed and females may have to compete for access to males, leading to reverse sexual dimorphism. Red-necked phalaropes compete for access to males who provide all the parental care. Females are larger and more aggressive than males. (Courtesty of Arthur Grosset)
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