The Collaborative International Dictionary
Sexual \Sex"u*al\, a. [L. sexualis, fr. sexus sex: cf. F. sexuel.] Of or pertaining to sex, or the sexes; distinguishing sex; peculiar to the distinction and office of male or female; relating to the distinctive genital organs of the sexes; proceeding from, or based upon, sex; as, sexual characteristics; sexual intercourse, connection, or commerce; sexual desire; sexual diseases; sexual generation.
Sexual dimorphism (Biol.), the condition of having one of the sexes existing in two forms, or varieties, differing in color, size, etc., as in many species of butterflies which have two kinds of females.
Sexual method (Bot.), a method of classification proposed by Linn[ae]us, founded mainly on difference in number and position of the stamens and pistils of plants.
Sexual selection (Biol.), the selective preference of one
sex for certain characteristics in the other, such as
bright colors, musical notes, etc.; also, the selection
which results from certain individuals of one sex having
more opportunities of pairing with the other sex, on
account of greater activity, strength, courage, etc.;
applied likewise to that kind of evolution which results
from such sexual preferences.
In these cases, therefore, natural selection seems
to have acted independently of sexual selection.
--A. R. Wallace.
n. A type of natural selection, where members of the sexes acquire distinct forms either because the members of one sex choose mates with particular features or because in the competition for mates among the members of one sex only those with certain traits succeed.
Sexual selection is a mode of natural selection where members of one biological sex choose mates of the other sex to mate with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). These two forms of selection mean that some individuals have better reproductive success than others within a population, either from being more attractive or preferring more attractive partners to produce offspring. For instance in the breeding season sexual selection in frogs occurs with the males first gathering at the water's edge and making their mating calls: croaking. The females then arrive and choose the males with the deepest croaks and best territories. Generalizing, males benefit from frequent mating and monopolizing access to a group of fertile females. Females have a limited number of offspring they can have and they maximize the return on the energy they invest in reproduction.
The concept was first articulated by Charles Darwin and Alfred Russel Wallace who described it as driving speciation and that many organisms had evolved features whose function was deleterious to their individual survival, and then developed by Ronald Fisher in the early 20th century. Sexual selection can lead typically males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds such as birds of paradise and peafowl, or the antlers of deer, or the manes of lions, caused by a positive feedback mechanism known as a Fisherian runaway, where the passing on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect. Although the sexy son hypothesis indicates that females would prefer male offspring, Fisher's principle explains why the sex ratio is 1:1 almost without exception. Sexual selection is also found in plants and fungi.
The maintenance of sexual reproduction in a highly competitive world is one of the major puzzles in biology given that asexual reproduction can reproduce much more quickly as 50% of offspring are not males, unable to produce offspring themselves. Many non-exclusive hypotheses have been proposed, including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species.