Mating system
The primary mating systems in plants are outcrossing (cross-fertilisation), autogamy (self-fertilisation) and apomixis (asexual reproduction without fertilization, but only when arising by modification of sexual function).
In polygynandry, each of the males may assist one female; if all adults help rear all the young, the system is more usually called "communal breeding".
[9] Compared to other vertebrates, where a species usually has a single mating system, humans display great variety.
However, males are more likely to practice polygamy because their reproductive success is based on the amount of offspring they produce, rather than any kind of benefit from parental investment.
This shows that human sexual behavior is unusually flexible since, in most animal species, one mating system dominates.
The temptation to draw conclusions about what is "natural" for human sexual behavior from observations of animal mating systems should be resisted: a socio-biologist observing the kinds of behavior shown by humans in any other species would conclude that all known mating systems were natural for that species, depending on the circumstances or on individual differences.
Some clues can be taken from human anatomy, which is essentially unchanged from the prehistoric past: Some have suggested that these anatomical factors signify some degree of sperm competition, although others have provided anatomical evidence to suggest that sperm competition risk in humans is low;[10][13] Monogamy has evolved multiple times in animals, with homologous brain structures predicting the mating and parental strategies used by them.
Mating systems can also have large impacts on the genetics of a population, strongly affecting natural selection and speciation.
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.
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.
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.
[27] 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.
Ajon et al.[31] showed that UV-induced cellular aggregation mediates chromosomal marker exchange with high frequency in S. acidocaldarius.
[citation needed] Protists are a large group of diverse eukaryotic microorganisms, mainly unicellular animals and plants, that do not form tissues.
Based on a phylogenetic analysis, Dacks and Roger[33] proposed that facultative sex was present in the common ancestor of all 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.
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.
[35] Other protists for which evidence of mating and sexual reproduction has recently been described are parasitic protozoa of the genus Leishmania,[36] Trichomonas vaginalis,[37] and acanthamoeba.
