<Li> As the 1: 1 sex ratio is approached, the advantage associated with producing males dies away . </Li> <Li> The same reasoning holds if females are substituted for males throughout . Therefore 1: 1 is the equilibrium ratio . </Li> <P> Insect species make up more than two - thirds of all extant animal species . Most insect species reproduce sexually, though some species are facultatively parthenogenetic . Many insects species have sexual dimorphism, while in others the sexes look nearly identical . Typically they have two sexes with males producing spermatozoa and females ova . The ova develop into eggs that have a covering called the chorion, which forms before internal fertilization . Insects have very diverse mating and reproductive strategies most often resulting in the male depositing spermatophore within the female, which she stores until she is ready for egg fertilization . After fertilization, and the formation of a zygote, and varying degrees of development, in many species the eggs are deposited outside the female; while in others, they develop further within the female and are born live . </P> <P> There are three extant kinds of mammals: monotremes, placentals and marsupials, all with internal fertilization . In placental mammals, offspring are born as juveniles: complete animals with the sex organs present although not reproductively functional . After several months or years, depending on the species, the sex organs develop further to maturity and the animal becomes sexually mature . Most female mammals are only fertile during certain periods during their estrous cycle, at which point they are ready to mate . Individual male and female mammals meet and carry out copulation . For most mammals, males and females exchange sexual partners throughout their adult lives . </P>

The reproductive process of the specialized cells used for human reproduction is called what