Sexual dimorphism, a difference in form or behavior between females and males, is common. The traits like the enormous tail feathers of peacock that appear to be opposed by natural selection. To explain these puzzling traits, Darwin invoked sexual selection. Sexual selection is differential reproductive success resulting from variation in mating success.

The members of the sex experiencing strong sexual selection typically compete among themselves over access to mates. This competition may involve direct combat, gamete competition, infanticide, or advertisement.

The members of the sex whose reproductive success is limited by resources rather than mating is typically choosy. This choosiness by female may get benefits, such as food or better genes for its offsprings, or it may be the result of a preexisting sensory bias. Sexual selection is distinct from natural selection in that the two forces often are at odds, as when a conspicuous mating display increases both the probability of finding a mate and the probability of being eaten by a predator.

Theory of sexual selection has been extended to plants also. Though plants "neither sing nor dance", they do exhibit female choice and male competition as intensely as animals do. Competition among male gametes can take several forms in plants and two major strategies are gamete inundation and gamete exclusion and pre-empting strategies. The kind of Swayamvara is seen in a number of forms ranging from dance competition (male peacocks) to tug of horns (male ungulates) and perhaps to the competition of archery skills in human beings.


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