Two years ago I reviewed the evidence on inbreeding in pedigree dogs. Inbreeding can result in reduced fertility both in litter size and sperm viability, developmental disruption, lower birth rate, higher infant mortality, shorter life span, increased expression of inherited disorders and reduction of immune system function. The immune system is closely linked to the removal of cancer cells from a healthy body and, indeed, reduction of immune system function increased the risk of full-blown tumours. These well-documented cases in domestic dogs confirm what is known from many wild populations of other species. It comes as no surprise, therefore, that a variety of mechanisms render inbreeding less likely in the natural world. One such is the choice of unfamiliar individuals as sexual partners.
Despite all the evidence, the story is more complicated than at first appears and this is where the explanation for what happens has a certain beauty. While inbreeding is generally seen as being undesirable, the debate has become much more nuanced in recent years. Purging of the genes with seriously damaging effects can carry obvious benefits. This can happen when a population is inbred.Outcrossing, which is usually perceived as advantageous, does carry the possibility that the benefits of purging are undone by introducing new harmful genes into the population. Furthermore a population adapted to one environment may not do well if crossed with a population adapted to another environment. So a balance is often struck between inbreeding and outbreeding
When the life history of the species demands careful nurturing of the offspring, the parents may go to a lot of trouble to mate with the best partner possible. A mate should be not too similar to oneself but not too dissimilar either. Thirty years I found that Japanese quail of both sexes preferred partners that were first cousins. Subsequent animal studies have suggested that an optimal degree of relatedness is most beneficial to the organism in terms of reproductive success. A study of a human Icelandic population also pointed to the same conclusion. Couples who were third or fourth cousins had a larger number of grandchildren than more closely related or more distantly related partners. Much evidence from humans and non-human animals suggests that the choice of a mate is dependent on experience in early life, with individuals tending to choose partners who are a bit different but not too different from familiar individuals, who are usually but not always close kin.
The role of early experience in determining sexual and social preferences bears on a well-known finding that humans are extremely loyal to members of their own group. They are even prepared to give up their own lives in defence of those with whom they identify. In sharp contrast, they can behave with lethal aggressiveness towards those who are unfamiliar to them. This suggests then a hopeful resolution to the racism and intolerance that bedevils many societies. As people from different countries and ethnic backgrounds become better acquainted with each other, they will be more likely to treat them well, particularly if the familiarity starts at an earlier age. If familiarity leads to marriage the couples may have fewer grandchildren, but that may be a blessing on an over-populated planet. This optimistic principle, generated by knowledge of how a balance has been struck between inbreeding and outbreeding, subverts biology, but it does hold for me considerable beauty.