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Inbreeding avoidance, or the inbreeding avoidance hypothesis, is a concept in evolutionary biology that refers to the prevention of the deleterious effects of inbreeding. The inbreeding avoidance hypothesis posits that certain mechanisms develop within a species, or within a given population of a species, as a result of natural and sexual selection in order to prevent breeding among related individuals in that species or population. Although inbreeding may impose certain evolutionary costs, inbreeding avoidance, which limits the number of potential mates for a given individual, can inflict opportunity costs.〔Waser PM, Austad SN, Keane B (1986) When should animals tolerate inbreeding? American Naturalist, 128, 529?537.〕 Therefore, a balance exists between inbreeding and inbreeding avoidance. This balance determines whether inbreeding mechanisms develop and the specific nature of said mechanisms.〔Archie, E. A., HOLLISTER‐SMITH, J. A., Poole, J. H., Lee, P. C., Moss, C. J., Maldonado, J. E., ... & Alberts, S. C. (2007). Behavioural inbreeding avoidance in wild African elephants. Molecular Ecology, 16(19), 4138-4148.〕 Inbreeding results in inbreeding depression, which is the reduction of fitness of a given population due to inbreeding. Inbreeding depression occurs via one of two mechanisms. The first mechanism involves the appearance of disadvantageous traits via the pairing of deleterious recessive alleles in a mating pair’s progeny. When two related individuals mate, the probability of deleterious recessive alleles pairing in the resulting offspring is higher as compared to when non-related individuals mate. The second mechanism relates to the increased fitness of heterozygotes. Many studies have demonstrated that homozygous individuals are often disadvantaged with respect to heterozygous individuals.〔Crnokrak P and Roff DA. (1999). Inbreeding depression in the wild. Heredity 83, 260–270; d-oi:10.1038/sj.hdy.6885530 〕 For example, a study conducted on a population of South African cheetahs demonstrated that the lack of genetic variability among individuals in the population has resulted in negative consequences for individuals, such as a greater rate of juvenile mortality and spermatozoal abnormalities.〔O’Brien SJ, Roelke ME, Marker L. Genetic basis for species vulnerability in the cheetah. Science. 1985;227(4693):1428–1434.〕 When heterozygotes possess a fitness advantage relative to a homozygote, a population with a large number of homozygotes will have a relatively reduced fitness, thus leading to inbreeding depression. Through these described mechanisms, the effects of inbreeding depression are often severe enough to cause the evolution of inbreeding avoidance mechanisms.〔Pusey A, Wolf M (1996) Inbreeding avoidance in animals. Trends Ecol Evol 11: 201?206.〕 ==Mechanisms== Inbreeding avoidance mechanisms have evolved in response to selection against inbred offspring. Inbreeding avoidance occurs in nature by at least four mechanisms: kin recognition, dispersal, extra-pair/extra-group copulations, and delayed maturation/reproductive suppression.〔〔 Of note, these mechanisms are not mutually exclusive and more than one can occur in a population at a given time. ===Kin Recognition=== Kin recognition is the mechanism by which individuals identify and avoid mating with closely related conspecifics. There have been numerous documented examples of instances in which individuals are shown to find closely related conspecifics unattractive. In one set of studies, researchers formed artificial relative and non-relative mate-pairs (artificial meaning they preferentially paired individuals to mate for the purposes of the experiments) and compared the reproductive results of the two groups. In these studies, paired relatives demonstrated reduced reproduction and higher mating reluctance when compared with non-relatives.〔〔Simmons, L.W. (1989) Kin recognition and its influence on mating preferences of the field cricket, Gryffus bimaculatus (de Geer), Anim. Behav. 38,68-77〕〔Krackow, S. and Matuschak, B. (1991) Mate choice for non-siblings in wild house mice: evidence from a choice test and a reproductive test, Ethology 88,99-108〕〔Bollinger, E.K. et al. (1991) Avoidance of inbreeding in the meadow vole (Microtus pennsykanicus), .I Mammal. 72, 419-421〕 For example, in a study by Simmons in field crickets, female crickets exhibited greater mating latency for paired siblings and half-siblings than with non-siblings.〔 In another set of studies, researchers allowed individuals to choose their mates from conspecifics that lie on a spectrum of relatedness. In this set, individuals were more likely to choose non-related over related conspecifics.〔〔〔Keane, B. (1990) The effect of relatedness on reproductive success and mate choice in the white-footed mouse, Peromyscus leucopus, Anim. Behav. 39,264-273〕 For example, in a study by Krackow et al., male wild house mice were set up in an arena with four separate openings leading to cages with bedding from conspecifics. The conspecifics exhibited a range of relatedness to the test subjects, and the males significantly preferred the bedding of non-siblings to the bedding of related females.〔 Studies have shown that kin recognition is more developed in species in which dispersal patterns facilitate frequent adult kin encounters.〔 There is a significant amount of variation in the mechanisms used for kin recognition. These mechanisms include recognition based on association or familiarity, an individual’s own phenotypic cues, chemical cues, and the MHC genes. In association/familiarity mechanisms, individuals learn the phenotypic profiles of their kin and use this template for kin recognition.〔 Many species accomplish this by becoming “familiar” with their siblings, litter mates, or nestmates. These species rely on offspring being reared in close proximity to achieve kin recognition. This is called the Westermarck effect.〔Wolf, A.P. Westermarck Redivivus. Annual Review of Anthropology 22: 157-175, 1993 〕 For example, Holmes and Sherman conducted a comparative study in Arctic ground squirrels and Belding’s ground squirrels. They manipulated the reared groups to include both siblings and cross-fostered nestmates and found that in both species the individuals were equally aggressive toward their nestmates, regardless of kinship.〔 Holmes WG, Sherman PW (1982) The ontogeny of kin recognition in two species of ground squirrels. American Zoologist, 22,491?517.〕 In certain species where social groups are highly stable, relatedness and association between infants and other individuals are usually highly correlated.〔〔Pusey, A.E. (1990) Mechanisms of inbreeding avoidance in nonhuman primates, in Pedophilia: Biosocial Dimensions (Feirman, J.R., ed.), pp. 201-220, Springer-Verlag〕 Therefore, degree of association can be used as a meter for kin recognition. Individuals can also use their own characteristics or phenotype as a template in kin recognition. For example, in one study, Mateo and Johnston had golden hamsters reared with only non-kin then later had them differentiate between odors of related and non-related individuals without any postnatal encounters with kin. The hamsters were able to discriminate between the odors, demonstrating the use of their own phenotype for the purpose of kin recognition.〔Mateo JM, Johnston RE (2000) Kin recognition and the ‘armpit effect’: evidence of self-referent phenotype matching. Proceedings of the Royal Society of London. Series B, Biological Sciences, 267, 695?700.〕 This study also provides an example of a species utilizing chemical cues for kin recognition. The major histocompatibility complex genes, or MHC genes, have been implicated in kin recognition.〔Jerram L. Brown and Amy Eklund The American Naturalist Vol. 143, No. 3 (Mar., 1994), pp. 435-461 Published by: The University of Chicago Press〕 On idea is that the MHC genes code for a specific pheromone profile for each individual, which are used to discriminate between kin and non-kin conspecifics. Several studies have demonstrated the involvement of the MHC genes in kin recognition. For example, Manning et al. conducted a study in house mice that looked at the species's behavior of communal nesting, or nursing one's own pups as well as the pups of other individuals. As Manning et al. state, kin selection theory predicts that the house mice will selectively nurse the pups of their relatives in order to maximize inclusive fitness. Manning et al. demonstrate that the house mice utilize the MHC genes in the process of discriminating between kin by preferring individuals who share the same allelic forms the MHC genes.〔Manning CJ, Wakeland EK, Potts WK. Communal nesting patterns in mice implicate MHC genes in kin recognition. Nature. 1992;360:581–583. doi: 10.1038/360581a0〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Inbreeding avoidance」の詳細全文を読む スポンサード リンク
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