Purging
As Purging (of English. To purge : clean, purify) is called in population genetics , the phenomenon that in inbred populations under certain conditions despite strong inbreeding to a reduction or even elimination of inbreeding depression may occur. Purging usually occurs in connection with a genetic bottleneck and can occur when there is strong inbreeding in a population in connection with strong selection pressure for fitness. For this, the reproduction rate must be high enough to obtain at least the smallest survivable population size under the increased selection .
background
Inbreeding depression is the phenomenon that with increasing inbreeding a reduction in fitness (fertility, infection resistance, lifespan, etc.) can be determined. Since inbreeding always leads to an increase in homozygosity , this can only happen if the responsible alleles are inherited at least in part in a dominant-recessive manner.
Partial dominance model
The partial dominance model assumes that inbreeding depression arises because, due to the increased degree of homozygosity of the inbred population, recessive alleles with a negative effect on fitness are more frequently formed phenotypically than is the case in non-inbred populations. This model is considered the most likely today.
Over-dominance model
The overdominance model assumes that the increased homozygosity is responsible for the loss of fitness regardless of the alleles present. So far it has not been refuted, but most experts consider it less likely than the partial dominance model.
Scenarios
After a genetic bottleneck, there are basically three possible scenarios for further development in an inbred population:
Increased inbreeding depression
After the bottleneck, inbreeding depression increases in the population and subsequently remains constant at a high level. This can lead to population extinction.
Partial purging
After the bottleneck, inbreeding depression increases, but subsequently decreases to a lower value. However, this value is still higher than in the population before the bottleneck.
Complete purging
After the bottleneck, the inbreeding depression increases, but subsequently falls within a few generations to the value of the population in front of the bottleneck or even falls below it. This happens when high inbreeding is combined with strong selection for fitness: the selection removes the disadvantageous alleles from the population. Because of the increased homozygosity caused by inbreeding, fewer alleles survive this selection in its heterozygous form than is the case in a non-inbred population. This in turn leads to an elimination of inbreeding depression from the population until, after hundreds of generations, new disadvantageous recessive alleles accumulate due to mutations.
Examples of purging
- Golden hamsters kept as pets or laboratory animals
- Peromyscus polionotus
- Chillingham cattle
- Species gazelles
- Icelandic horses
- dingo
- David deer
literature
- Crnokrak / Barrett (2002): "Purging the Genetic Load: A Review of the Experimental Evidence". Evolution 56 (12): 2347-58
- Lacy / Ballou (1998): "Effectiveness of Selection in Reducing the Genetic Load in Populations of Peromyscus polionotus During Generations of Inbreeding". Evolution 52 (3): 900-909
- Templeton / Reed (1984): "Factors Eliminating Inbreeding Depression in a Captive Herd of Speke's Gazelle". Zoo Biology 3 : 177-199
- N. Wilmsen Thornhill (ed.): "The Natural History of Inbreeding and Outbreeding: Theoretical and Empirical Perspectives". University of Chicago Press, 1993; ISBN 0-226-79855-0