<P> Before copulation, intrasexual selection - usually between males - may take the form of male - to - male combat . Also, intersexual selection, or mate choice, occurs when females choose between male mates . Traits selected by male combat are called secondary sexual characteristics (including horns, antlers, etc .), which Darwin described as "weapons", while traits selected by mate (usually female) choice are called "ornaments". Due to their sometimes greatly exaggerated nature, secondary sexual characteristics can prove to be a hindrance to an animal, thereby lowering its chances of survival . For example, the large antlers of a moose are bulky and heavy and slow the creature's flight from predators; they also can become entangled in low - hanging tree branches and shrubs, and undoubtedly have led to the demise of many individuals . Bright colourations and showy ornamenations, such as those seen in many male birds, in addition to capturing the eyes of females, also attract the attention of predators . Some of these traits also represent energetically costly investments for the animals that bear them . Because traits held to be due to sexual selection often conflict with the survival fitness of the individual, the question then arises as to why, in nature, in which survival of the fittest is considered the rule of thumb, such apparent liabilities are allowed to persist . However, one must also consider that intersexual selection can occur with an emphasis on resources that one sex possesses rather than morphological and physiological differences . For example, males of Euglossa imperialis, a non-social bee species, form aggregations of territories considered to be leks, to defend fragrant - rich primary territories . The purpose of these aggregations is only facultative, since the more suitable fragrant - rich sites there are, the more habitable territories there are to inhabit, giving females of this species a large selection of males with whom to potentially mate . </P> <P> After copulation, male--male competition distinct from conventional aggression may take the form of sperm competition, as described by Parker in 1970 . More recently, interest has arisen in cryptic female choice, a phenomenon of internally fertilised animals such as mammals and birds, where a female can get rid of a male's sperm without his knowledge . </P> <P> Finally, sexual conflict is said to occur between breeding partners, sometimes leading to an evolutionary arms race between males and females . Sexual selection can also occur as a product of pheromone release, such as with the stingless bee, Trigona corvina . </P> <P> Female mating preferences are widely recognized as being responsible for the rapid and divergent evolution of male secondary sexual traits . Females of many animal species prefer to mate with males with external ornaments - exaggerated features of morphology such as elaborate sex organs . These preferences may arise when an arbitrary female preference for some aspect of male morphology--initially, perhaps, a result of genetic drift--creates, in due course, selection for males with the appropriate ornament . One interpretation of this is known as the sexy son hypothesis . Alternatively, genes that enable males to develop impressive ornaments or fighting ability may simply show off greater disease resistance or a more efficient metabolism, features that also benefit females . This idea is known as the good genes hypothesis . </P>

Ultimately natural selection will always make a species