On page 59, we discussed the common notion that inbreeding can be bad for health because it increases the probability of receiving two copies of a deleterious recessive allele. Inbreeding depression, which is the negative effects of inbreeding on health outcomes, is in fact quite well studied, and Alvarez et al. (2009) show that the level of inbreeding within the Spanish Hapsburg royal dynasty became very high, leading to quite low survival amongst their young, and Charles II of Spain actually having not one but two rare genetic disorders. The dynasty eventually died out. Why did they inbreed so much? There is a tension here between the economic imperative to keep lands and power concentrated within the family, and the genetic imperative to outbreed. The Hapsburgs clearly erred on the side of the economic imperative.

What may be less familiar to you is the idea that too much outbreeding can be bad for you. Breeding with a reasonably similar partner is likely to keep together complexes of coadapted genes (basically, combinations of alleles that work well together), and so the optimal mate is one who is not so close as to cause problems of inbreeding depression, but close enough for a reasonable amount of genetic similarity. Outbreeding depression, the loss of health due to too much genetic dissimilarity between parents, is well studied in non-human species. Recently, Helgason et al. (2008) looked at the history of the Icelandic population. It’s a small, stable population with excellent records of everyone’s family history. They showed that the marriages which produced the greatest number of surviving grandchildren were those between third- or fourth- cousins. Any more closely related than this, and there were detectable problems of inbreeding depression, but if the parents were not related at all, the couple’s reproductive success was less than if they were slightly related. Many societies practice cousin marriage. The traditional interpretation of this is that it is to do with keeping economic assets together, but it may be genetically adaptive too, especially if the cousins are distant.

Alvarez, G. et al. (2009). The role of inbreeding in the extinction of a European dynasty, Plos One 4:e5174 (Freely available online). [DOI: 10.1371/journal.pone.0005174]

Helgason, A. et al. (2008). An association between the kinship and fertility of human couples. Science 319: 813-6.