sexual reproduction sexual reproduction sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete (haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote that develops into an organism composed of cells with two sets of chromosomes (diploid). this is typical in animals, though the number of chromosome sets and how that number changes in sexual reproduction varies, especially among plants, fungi, and other eukaryotes. in the first stage of sexual reproduction, meiosis, the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). during fertilisation, haploid gametes come together to form a diploid zygote, and the original number of chromosomes is restored. sexual reproduction is the most common life cycle in multicellular eukaryotes, such as animals, fungi and plants. sexual reproduction also occurs in some unicellular eukaryotes. sexual reproduction does not occur in prokaryotes, unicellular organisms without cell nuclei, such bacteria and archaea. however, some processes in bacteria, including bacterial conjugation, transformation and transduction, may be considered analogous to sexual reproduction in that they incorporate new genetic information. some proteins and other features that are key for sexual reproduction may have arisen in bacteria, but sexual reproduction is believed to have developed in an ancient eukaryotic ancestor. in eukaryotes, diploid precursor cells divide to produce haploid cells in a process called meiosis. in meiosis, dna is replicated to produce a total of four copies of each chromosome. this is followed by two cell divisions to generate haploid gametes. after the dna is replicated in meiosis, the homologous chromosomes pair up so that their dna sequences are aligned with each other. during this period before cell divisions, genetic information is exchanged between homologous chromosomes in genetic recombination. homologous chromosomes contain highly similar but not identical information, and by exchanging similar but not identical regions, genetic recombination increases genetic diversity among future generations. during sexual reproduction, two haploid gametes combine into one diploid cell known as a zygote in a process called fertilization. the nuclei from the gametes fuse, and each gamete contributes half of the genetic material of the zygote. multiple cell divisions by mitosis (without change in the number of chromosomes) then develop into a multicellular diploid phase or generation. in plants, the diploid phase, known as the sporophyte, produces spores by meiosis. these spores then germinate and divide by mitosis to form a haploid multicellular phase, the gametophyte, which produces gametes directly by mitosis. this type of life cycle, involving alternation between two multicellular phases, the sexual haploid gametophyte and asexual diploid sporophyte, is known as alternation of generations. the evolution of sexual reproduction is considered paradoxical, because asexual reproduction should be able to outperform it as every young organism created can bear its own young. this implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation. this 50% cost is a fitness disadvantage of sexual reproduction. the two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. however, one definite advantage of sexual reproduction is that it increases genetic diversity and impedes the accumulation of genetic mutations. sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates for sexual reproduction. it has been described as "a powerful evolutionary force that does not exist in asexual populations". evolution the first fossilized evidence of sexual reproduction in eukaryotes is from the stenian period, about 1.05 billion years old. biologists studying evolution propose several explanations for the development of sexual reproduction and its maintenance. these reasons include reducing the likelihood of the accumulation of deleterious mutations, increasing rate of adaptation to changing environments, dealing with competition, dna repair, masking deleterious mutations, and reducing genetic variation on the genomic level. all of these ideas about why sexual reproduction has been maintained are generally supported, but ultimately the size of the population determines if sexual reproduction is entirely beneficial. larger populations appear to respond more quickly to some of the benefits obtained through sexual reproduction than do smaller population sizes. maintenance of sexual reproduction has been explained by theories that work at several levels of selection, though some of these models remain controversial. however, newer models presented in recent years suggest a basic advantage for sexual reproduction in slowly reproducing complex organisms. sexual reproduction allows these species to exhibit characteristics that depend on the specific environment that they inhabit, and the particular survival strategies that they employ. sexual selection in order to reproduce sexually, both males and females need to find a mate. generally in animals mate choice is made by females while males compete to be chosen. this can lead organisms to extreme efforts in order to reproduce, such as combat and display, or produce extreme features caused by a positive feedback known as a fisherian runaway. thus sexual reproduction, as a form of natural selection, has an effect on evolution. sexual dimorphism is where the basic phenotypic traits vary between males and females of the same species. dimorphism is found in both sex organs and in secondary sex characteristics, body size, physical strength and morphology, biological ornamentation, behavior and other bodily traits. however, sexual selection is only implied over an extended period of time leading to sexual dimorphism. animals further information: reproductive system § animals, fertilization in animals, and animal sexual behavior arthropods this section is an excerpt from arthropod § reproduction and development. aphid giving birth to live young from an unfertilized eggharvestmen mating a few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. however, individuals of most species remain of one sex their entire lives. a few species of insects and crustaceans can reproduce by parthenogenesis, especially if conditions favor a "population explosion". however, most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable. the ability to undergo meiosis is widespread among arthropods including both those that reproduce sexually and those that reproduce parthenogenetically. although meiosis is a major characteristic of arthropods, understanding of its fundamental adaptive benefit has long been regarded as an unresolved problem, that appears to have remained unsettled. aquatic arthropods may breed by external fertilization, as for example horseshoe crabs do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. all known terrestrial arthropods use internal fertilization. opiliones (harvestmen), millipedes, and some crustaceans use modified appendages such as gonopods or penises to transfer the sperm directly to the female. however, most male terrestrial arthropods produce spermatophores, waterproof packets of sperm, which the females take into their bodies. a few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely to be successful. the nauplius larva of a penaeid shrimp most arthropods lay eggs, but scorpions are ovoviviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care. newly born arthropods have diverse forms, and insects alone cover the range of extremes. some hatch as apparently miniature adults (direct development), and in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first moult. many insects hatch as grubs or caterpillars, which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body. dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws. crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages. insects an australian emperor dragonfly laying eggs, guarded by a male 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. mammals