Genetic purging
Genetic purging is the increased pressure of natural selection against deleterious alleles prompted by inbreeding.
[1] Purging occurs because deleterious alleles tend to be recessive, which means that they only express all their harmful effects when they are present in the two copies of the individual (i.e., in homozygosis).
It is important to note that it is the gene pool itself that undergoes purging, as deleterious alleles are removed through the selection against unfit offspring—not their parents.
This purging occurs because inbred individuals who do not carry such harmful alleles have higher survival rates and greater reproductive success, thereby passing on their genes more effectively.
Over generations, this process not only eliminates deleterious alleles but also homogenizes beneficial ones within the population through continued inbreeding.
It would avoid ambiguity to use "purifying selection" in that general context, and to reserve "purging" to its more strict meaning defined above.
Deleterious alleles segregating in populations of diploid organisms have a remarkable trend to be, at least, partially recessive.
Purging can reduce the average frequency of deleterious alleles across the genome below the value expected in a non-inbred population during long periods.
However this requires some information on the magnitude of the deleterious effects that are hidden in the heterozygous condition but become expressed in homozygosis.
Then inbreeding depression occurs at a rate δ, due to (partially) recessive deleterious alleles that were present at low frequencies at different loci.
[1] It depends upon the "purging coefficient" d, which represents the deleterious effects that are hidden in heterozygosis but exposed in homozygosis.
The average "purged inbreeding coefficient" can be approximated using the recurrent expression There are also predictive equations to be used with genealogical information.
However, for non-lethal deleterious alleles, the efficiency of purging would be smaller, and it can require larger populations to overcome genetic drift.
[8][9] When a previously stable population undergoes inbreeding, if nothing else changes, natural selection should consist mainly of purging.
The effects of purging were first noted by Darwin[10] in plants, and have been detected in laboratory experiments and in vertebrate populations undergoing inbreeding in zoos or in the wild, as well as in humans.
Furthermore, if inbreeding is due to a reduction in population size, selection against new deleterious mutations can become less efficient, and this can induce additional fitness decline in the medium-long term.
Endangered populations use to undergo inbreeding due to their reduced numbers, and purging can play a relevant role in determining their extinction risk and the success of conservation strategies.
