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System of sexual reproduction in a population

A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to animals, the term describes which males and females mate under which circumstances. Recognised systems include monogamy, polygamy (which includes polygyny, polyandry, and polygynandry), and promiscuity, all of which lead to different mate choice outcomes and thus these systems affect how sexual selection works in the species which practice them. In plants, the term refers to the degree and circumstances of outcrossing. In human sociobiology, the terms have been extended to encompass the formation of relationships such as marriage.

Monogamy has evolved multiple times in animals, with homologous brain structures predicting the mating and parental strategies used by them. These homologous structures were brought about by similar mechanisms. Even though there have been many different evolutionary pathways to get to monogamy, all the studied organisms express their genes very similarly in the fore and midbrain, implying a universal mechanism for the evolution of monogamy in vertebrates. While genetics is not the exclusive cause of mating systems within animals, it is influential in many animals, particularly rodents, which have been the most heavily researched. Certain rodents’ mating systems—monogamous, polygynous, or socially monogamous with frequent promiscuity—are correlated with suggested evolutionary phylogenies, where rodents more closely related genetically are more likely to use a similar mating system, suggesting an evolutionary basis. These differences in mating strategy can be traced back to a few significant alleles that affect behaviors that are heavily influential on mating system, such as the alleles responsible for the level of parental care, how animals choose their partner(s), and sexual competitiveness, among others, which are all at least partially influenced by genetics. While these genes may not perfectly correlate with the mating system that animals use, genetics is one factor that may lead to a species or population reproducing using one mating system over another, or even potentially multiple at different locations or points in time. Mating systems can also have large impacts on the genetics of a population, strongly affecting natural selection and speciation. In plover populations, polygamous species tend to speciate more slowly than monogamous species do. This is likely because polygamous animals tend to move larger distances to find mates, contributing to a high level of gene flow, which can genetically homogenize many nearby subpopulations. Monogamous animals, on the other hand, tend to stay closer to their starting location, not dispersing as much. Because monogamous animals don’t migrate as far, monogamous populations which are geographically closer together tend to reproductively isolate from each other more easily, and thus each subpopulation is more likely to diversify or speciate from the other nearby populations as compared to polygamous populations. In polygamous species, however, the male partner in polygynous species and female partner in polyandrous species often tend to spread further to look for mates, potentially to find more or better mates. The increased level of movement among populations leads to increased gene flow between populations, effectively making geographically distinct populations into genetically similar ones via interbreeding. This has been observed in some species of rodents, where generally promiscuous species were quickly differentiated into monogamous and polygamous taxa by a prominent introduction of monogamous behaviors in some populations of that species, showing the swift evolutionary effects different mating systems can have. Specifically, monogamous populations speciated up to 4.8 times faster and had lower extinction rates than non monogamous populations. There are many possible reasons for this, one of the possibilities being that there is greater genetic variation in families because most offspring in a family will have either a different mother or father. This reduces the potential harm done by inbreeding, as siblings will be less closely related and more genetically diverse. Additionally, because of the increased genetic diversity among generations, the levels of reproductive fitness are also more variable, and so it is easier to select for positive traits more quickly, as the difference in fitness between members of the same generation would be greater. When many males are actively mating, polyandry can decrease the risk of extinction as well, as it can increase the effective population size. Increased effective population sizes are more stable and less prone to accumulating deleterious mutations due to genetic drift. and thus transformation is clearly a bacterial adaptation for DNA transfer. In order for a bacterium to bind, take up and recombine donor DNA into its own chromosome, it must first enter a special physiological state termed natural competence. In Bacillus subtilis about 40 genes are required for the development of competence and DNA uptake. The length of DNA transferred during B. subtilis transformation can be as much as a third and up to the whole chromosome. Transformation appears to be common among bacterial species, and at least 60 species are known to have the natural ability to become competent for transformation. The development of competence in nature is usually associated with stressful environmental conditions, and seems to be an adaptation for facilitating repair of DNA damage in recipient cells.

In several species of archaea, mating is mediated by formation of cellular aggregates. Halobacterium volcanii, an extreme halophilic archaeon, forms cytoplasmic bridges between cells that appear to be used for transfer of DNA from one cell to another in either direction. When the hyperthermophilic archaea Sulfolobus solfataricus and Sulfolobus acidocaldarius are exposed to the DNA damaging agents UV irradiation, bleomycin or mitomycin C, species specific cellular aggregation is induced. Aggregation in S. solfataricus could not be induced by other physical stressors, such as pH or temperature shift, The earliest eukaryotes were likely protists. Mating and sexual reproduction are widespread among extant eukaryotes. Based on a phylogenetic analysis, Dacks and Roger proposed that facultative sex was present in the common ancestor of all eukaryotes. However, to many biologists it seemed unlikely until recently, that mating and sex could be a primordial and fundamental characteristic of eukaryotes. A principal reason for this view was that mating and sex appeared to be lacking in certain pathogenic protists whose ancestors branched off early from the eukaryotic family tree. However, several of these protists are now known to be capable of, or to recently have had, the capability for meiosis and hence mating. To cite one example, the common intestinal parasite Giardia intestinalis was once considered to be a descendant of a protist lineage that predated the emergence of meiosis and sex. However, G. intestinalis was recently found to have a core set of genes that function in meiosis and that are widely present among sexual eukaryotes. These results suggested that G. intestinalis is capable of meiosis and thus mating and sexual reproduction. Furthermore, direct evidence for meiotic recombination, indicative of mating and sexual reproduction, was also found in G. intestinalis. Other protists for which evidence of mating and sexual reproduction has recently been described are parasitic protozoa of the genus Leishmania, Trichomonas vaginalis, and acanthamoeba. Protists generally reproduce asexually under favorable environmental conditions, but tend to reproduce sexually under stressful conditions, such as starvation or heat shock.

Both animal viruses and bacterial viruses (bacteriophage) are able to undergo mating. When a cell is mixedly infected by two genetically marked viruses, recombinant virus progeny are often observed indicating that mating interaction had occurred at the DNA level. Another manifestation of mating between viral genomes is multiplicity reactivation (MR). MR is the process by which at least two virus genomes, each containing inactivating genome damage, interact with each other in an infected cell to form viable progeny viruses. The genes required for MR in bacteriophage T4 are largely the same as the genes required for allelic recombination. Examples of MR in animal viruses are described in the articles Herpes simplex virus, Influenza A virus, Adenoviridae, Simian virus 40, Vaccinia virus, and Reoviridae.