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Of those definitions, 4 involve sexual dimorphism (sex differences) and 2 refer only to males.However, the essence of sexual selection as Darwin defined it is selection through competition for mates.Therefore, as is so often the case, in his description and definition of sexual selection, Darwin identified most of the issues that occupy us today.Bateman (1948) hypothesized that males compete for mates and females do not because reproductive success in females is limited by the resources available for egg production, that is, the female gain curve plateaus, whereas the reproductive success of males is limited only by access to females, and the male gain curve is proportional to the number of mates (or eggs) available.Sexual selection is a term that has meant different things to different people.In a recent review, Tim Clutton-Brock (2004) listed 9 different definitions of sexual selection and the list is not exhaustive (see Table 1).Table 2 lists factors that have been considered to be important in determining the strength and direction of sexual selection by producing differences between the sexes (or sexual roles) in variance in reproductive success and consequently skewed breeding sex ratio (BSR).The great difficulty is to determine which differences between the sexes are causes of sexual selection and which are effects.

The skewed breeding sex ratios associated with sequential hermaphroditism have long been recognized as contributory to sexual selection.

Sexual selection has come to be seen as a keystone of Charles Darwin's theory of evolution by natural selection, being the exception that proves the rule that evolution proceeds through differential reproduction (see Ghiselin 1969b for discussion).

Famously, Darwin developed the theory of sexual selection to account for certain traits such as the weapons used in male-male competition (for example, a stag's antlers) or the ornaments used to attract members of the opposite sex (for example, a peacock's tail) which seemed to be very important in obtaining mates but unimportant otherwise.

Darwin considered sexual selection to be limited to higher animals (from arthropods on up) on two grounds; first, “it is almost certain that these animals have too imperfect senses and much too low mental powers to feel mutual rivalry, or to appreciate each other's beauty or other attractions (Darwin 1871, p 321),” and “In the lowest classes the two sexes are not rarely united in the same individual, and therefore secondary sexual characters cannot be developed” (Darwin 1871, p 321).

Darwin, then, saw hermaphroditism as incompatible with sexual selection both because of a lack of opportunity for evolution of sexual dimorphism and a lack of capacity for mate choice and/or direct competition for mates in many invertebrates.

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