When Not to Avoid Inbreeding

Abstract

Avoidance of incestuous matings is widely reported across many animal taxa, and the adaptive value of such behavior is explained through inbreeding depression. However, an old and somewhat neglected theoretical result predicts that inbred matings offer another, positive effect on the inclusive fitness of parents: an individual who mates with a relative will help that relative to spread genes identical by descent. This benefit can be substantial, if the additional mating achieved by the relative does not harm his mating success otherwise, and in the context of selfing in plants the phenomenon is well known. Here, we develop a model that derives expected values of inbreeding tolerance, that is, the magnitude of inbreeding depression that is required to make individuals avoid inbreeding, for different animal life histories and parental investment patterns. We also distinguish between simultaneous and sequential mate choice, and show that inbreeding tolerance should often be remarkably high in the latter scenario in particular, although egalitarian parental care will lead to lower tolerance. There is a mismatch between theory and data: the almost complete lack of cases where individuals prefer to mate incestuously is at odds with a large overlap between the predicted range of inbreeding tolerance and estimates of inbreeding depression found in nature. We discuss four different solutions to this enigma, and suggest that inbreeding tolerance, where it is found, should not always be attributed to a simple constraint that has prevented finding any other mate.

Are flies kind to kin? The role of intra- and inter-sexual relatedness in mediating reproductive conflict

“…This is because the reproductive success of one's relatives also results in 'indirect' fitness benefits owing to the increased transmission of alleles inherited from a common ancestor [7]. Thus, according to kin selection theory, it is hypothesized that individuals should act more altruistically towards their relatives to maximize their inclusive fitness, as long as the net benefit(s) outweigh costs [7,8].…”

“…Several theoretical studies have attempted to identify how the predictions of kin selection theory may interact with those of sexual conflict theory [8][9][10]. According to these models, there are circumstances in which altruistic behaviour towards relatives should occur, both in intra-sexual interactions [9,10] and in inter-sexual interactions [8].…”

“…According to these models, there are circumstances in which altruistic behaviour towards relatives should occur, both in intra-sexual interactions [9,10] and in inter-sexual interactions [8]. When competing against same-sex relatives, one adaptive strategy is to reduce the amount of harm inflicted on these 'rivals', thereby maximizing potential inclusive fitness benefits via the enhanced success of relatives and the shared reproductive resource [10].…”

“…When competing against same-sex relatives, one adaptive strategy is to reduce the amount of harm inflicted on these 'rivals', thereby maximizing potential inclusive fitness benefits via the enhanced success of relatives and the shared reproductive resource [10]. When the potential for mating exists between relatives, it is further hypothesized that there exists an optimal level for inbreeding, where it can be beneficial to mate with a relative [8]. In these situations, where an individual benefits from the increased transmission of alleles owing to shared ancestry with a related mate, a reduction in the level of inter-sexual harm on this relative and thus a direct gain (owing to lifetime reproductive success (LRS) of a related mate) is also predicted to occur [8].…”

“…When the potential for mating exists between relatives, it is further hypothesized that there exists an optimal level for inbreeding, where it can be beneficial to mate with a relative [8]. In these situations, where an individual benefits from the increased transmission of alleles owing to shared ancestry with a related mate, a reduction in the level of inter-sexual harm on this relative and thus a direct gain (owing to lifetime reproductive success (LRS) of a related mate) is also predicted to occur [8].…”