Inbreeding depression is a very strong danger to small populations. As relatives with
similar DNA interbreed, more and more deleterious mutations begin to be expressed,
effectively crippling the population, and often driving it to extinction. However, small
populations are often unable to avoid inbreeding, meaning other mechanisms must be
employed to counter its destructive effect.
For an inbred population, there are a couple of ways to prevent inbreeding depression
from crippling the population according to the prevailing idea.. The most obvious way is to simply increase heterozygosity. However, this is not always simple. Other members of the same species may not be available to mate with, either due to isolation, extinction, or some other reason. Further, mutation only increases genetic load, it does not increase it. It would seem a dark fate awaits an inbred population in this condition.
According to evolutionary dogma, there is a third alternative. Natural selection can act to purge the worst alleles from the population. Essentially, as the deleterious alleles become concentrated, they
begin to be expressed more frequently. As they are expressed, they become more visible to,
and therefore vulnerable to the action of natural selection. Thus in the long term,
inbreeding depression can sometimes benefit a population.
There is some evidence that purging selection can be effective. In a lab study of Drosophila
melanogaster, Bouzat and Swindell (2006), determined that lines that were strongly
purged had one third less inbreeding depression than lines that did not. The purged lines
were kept incredibly small, ten pairs, for just short of twenty generations and still showed
less inbreeding depression than non-purged lines. These results seem to indicate that
purging can significantly reduce the effects of inbreeding depression.
D. melanogaster is not the only insect in which inbreeding depression can be lessened by
purging. The well-known ladybug (Harmonia axyridis) was subject to a study on the
potential effects of purging in an invasive population. When subjected to a bottleneck that
purged deleterious mutations, the resultant ladybug population was more fit than the
parent population (Facon et al, 2011). Inbreeding depression was very strong in native
populations, but in invasive populations that underwent a bottleneck, inbreeding
depression was completely absent.
Insects are not the only organisms in which purging selection has been demonstrated.
Holstein-Friesian horses are a breed of horse that is fairly heavily inbred and has
undergone long term selection for milk production. This purging selection has removed
some, though not all of the genetic load on the Holstein-Friesian breed when it comes to
milk production (McParland et al, 2009). However, the same study found that purging
selection had been ineffective at increasing fertility. This is likely due to purging selection
being applied less strongly to this trait.
There have been a few studies that have questioned the role of purging selection in
stopping inbreeding depression. Boakes et al (2007) studied captive populations of various
zoo animals and found that purging selection was, on average, very weak across the 119
populations studied, decreasing inbreeding depression by less than one percent. However,
determining an average across so many types of taxa, when the populations have existed for varying times and have different starting genetic loads seems spurious. Purging selection may simply not have been needed in some of the populations yet, or some of the populations may not be expressing enough deleterious alleles for purging selection to act. Further, fourteen populations did experience purging selection. As such, purging selection seems a solid escape hatch for a population undergoing inbreeding depression, in at least some cases.
The problem is, selection is too weak to remove enough mutations. Let me explain what I mean. Even if we grant that purging selection can remove the worst alleles from an inbred population (which we shouldn’t, more on that in a moment), selection can only remove the worst alleles. That means other alleles that are deleterious, but not relatively the worst, will persist in building up. The only way to get rid of all the alleles that are bad is kill off every single animal that has them. In other words, commit genocide on the entire population.
Further, the whole idea of purging selection assumes selection can see alleles. It can’t. All it sees is the phenotype. If the phenotype does not manifest the deleterious alleles, selection can’t get rid of them. That’s why purging selection cannot remove all, or even enough deleterious alleles to deal with genetic load: it can’t see the mutations.
Boakes EH, Wang J, Amos W. 2007. An investigation of inbreeding depression and purging
in captive pedigreed populations. Heredity. 98:172-182.
Bouzat JL, Swindell WR. 2006. Reduced inbreeding depression due to historical
interbreeding in Drosophila melanogaster: evidence for purging. J Evol Biol. 19(4):1257-
Facon B, Hufbauer RA, Tayeh A, Loiseau A, Lombaert E, Vitalis R, Guillermaud T, Lundgren
JG, Estoup A. 2011. Inbreeding depression is purged in the invasive insect Hamonia
axyridis. Curr Biol. 21(5):424-427.
McParland S, Kearney F, Berry DP. 2009. Purging of inbreeding depression within the Irish
Holstein-Friesian population. Genet Sel Evol. 41(16).