Animals, plants, and microbes have evolved diverse forms of sexual reproduction, which is the mixing of genetic material from different parents. Almost all organisms on Earth engage in some kind of sex.
Males and females are distinguished by the size of the gametes they make: males make small gametes, and females big ones. Many species are hermaphroditic. In species with separate males and females, sexual dimorphism ranges from minimal to extreme.
Sexual selection, which is selection caused by competition among individuals of the same sex for mates, leads to the evolution of exaggerated secondary sexual traits that increase mating success but usually decrease survival. Sexual selection acts on males much more often than on females. Males can often increase their fitness by mating with more females, but females typically do not benefit from mating with more males.
Some unusual species show sex role reversal. Here sexual selection acts on females because the operational sex ratio is female-biased: more females are available to mate than males.
One of the two major modes of sexual selection is male-male competition, which occurs by male combat, sperm competition, infanticide, and other mechanisms.
The second major mode of sexual selection is female choice. Female mating preferences evolve as the result of direct benefits that females receive from their mates, pleiotropic effects of preference genes, and the good genes mechanism. Some preferences result from perceptual biases and apparently did not originate by either direct benefits or good genes. A final mechanism for the evolution of female preferences is Fisher’s runaway process.
Sexual selection on plants favors flowers that increase pollinator visitation, production of greater quantities of pollen, and pollen that outcompetes other pollen in fertilizing ovules.
The sex ratio can evolve in many species. In most situations, selection favors producing equal numbers of males and females. Exceptions occur in organisms such as fig wasps where selection favors those families that produce the largest numbers of daughters.
The rarity of asexual reproduction is a puzzle because several factors give it an evolutionary advantage over sexual reproduction. The biggest of these is the twofold cost of males suffered by sexual species. Other advantages to asexual reproduction include re-productive assurance and escape from sexually transmitted diseases.
Recombination gives sexual reproduction several advantages that compensate for its disadvantages and thereby explain why it is so common. The Red Queen hypothesis suggests that sex is favored in changing environments. Recombination is also favored because it reduces selective interference, which is a general term that includes clonal interference, the ruby-in-the-rubbish effect, and Muller’s ratchet.
Inbreeding depression frequently causes the evolution of mechanisms that prevent self-fertilization and mating between close relatives.