Inbreeding in nature: brothers and sisters, do not unite!

Abstract

Inbreeding, or mating among related individuals, causes problems with viability and fertility in small captive populations. Young inbred ungulates often die in zoos, and the eggs of inbred fruit flies frequently fail to hatch in the laboratory. But what is the role of inbreeding in nature? Does it really affect patterns of population change in the wild, and could it even cause extinctions? These questions were the subject of hot debate in the 1970s. By the late 1980s, many ecologists seemed to agree that other ecological factors were more important than inbreeding in reducing population size and in extinguishing populations. However, we could now be witnessing the revival of interest in inbreeding as a factor to contend with. Whereas theoretical studies have demonstrated how the accumulation of mildly deleterious alleles can lower the fitness of small populations, experiments, both in the laboratory and in the wild, have demonstrated significant effects of inbreeding on the density of several animal and plant species.Observations in 1998 suggested that inbreeding increases the risk of population extinction in the butterfly Melitaea cinxia.1xInbreeding and extinction in a butterfly metapopulation. Saccheri, I et al. Nature. 1998; 392: 491–494Crossref | Scopus (1008)See all References1 Some of the same authors now report experimental evidence2xExperimental confirmation that inbreeding depression increases extinction risk in butterfly populations. Nieminen, M et al. Am. Nat. 2001; 157: 237–244Crossref | PubMed | Scopus (106)See all References2 that not only confirms the earlier results, but also demonstrates when and how inbreeding exerts its harmful effect. Working on small populations of Melitaea cinxia in the laboratory, Nieminen et al. found that inbreeding lowered the proportion of eggs hatched and decreased the survival of over-wintering larvae. The larvae of this butterfly spin communal winter nests, but inbreeding made them spin nests full of holes, which failed to protect them from the elements. And when Nieminen et al. introduced in- and outbred populations into the field, they found that inbred populations had a much higher risk of extinction than did outbred ones.This study suggests that inbreeding in Melitaea cinxia may reduce local survival to the point of local extinction. And as the species occurs as a set of classical metapopulations, where the extinction of small local populations is counterbalanced by the colonization of empty sites, an increased risk of local extinction might ultimately threaten the survival of the species at a landscape level.The work of Nieminen et al. is clearly a case study on a single species with many special features. In their study region, Melitaea cinxia typically occurs as very small local populations, where inbreeding between close relatives is more the rule than an exception. This should keep us from generalizing the current results too broadly. But by identifying inbreeding as a key agent in the dynamics of at least this species, Nieminen et al. do offer an eye opener. With more and more species being split into smaller and more fragmented populations, perhaps inbreeding might soon be a threat to the survival of many wild populations?