Genetic purging

Genetic purging is the reduction of the frequency of a deleterious allele occurred by an increased efficiency of natural selection that is prompted by inbreeding.

Purging occurs because many deleterious alleles only express all their harmful effect in homozygosis. During inbreeding, as related individuals mate, they produce offspring that are more likely to be homozygous, so that these deleterious alleles appear more often in homozygous individuals that are less fit and that pass fewer copies of their genes to future generations. This allows natural selection to purge the deleterious alleles.

Purging reduces both the overall number of recessive deleterious alleles and the decline of mean fitness caused by inbreeding (the inbreeding depression for fitness).

The term "purge" is sometimes used for selection against deleterious alleles in a general way. 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. This means that, when they occur in homozygosis (double copies), they reduce fitness by more than twice than when they occur in heterozygosis (single copy). In other words, part of their potential deleterious effect is hidden in heterozygosis but expressed in homozygosis, so that selection is more efficient against them when they occur in homozygosis. Since inbreeding increases the probability of being homozygous, it increases the fraction of the potential deleterious effect that is expressed and, therefore, exposed to selection. This causes some increase in the selective pressure against (partially) recessive deleterious alleles, which is known as purging. Of course, it also causes some reduction in fitness, which is known as inbreeding depression.

Purging can reduce the average frequency of deleterious alleles across the genome below the value expected in a non-inbred population. Although this reduction usually does not compensate for all the negative effects of inbreeding, it has several beneficial consequences for fitness. A consequence is the reduction of the so-called inbreeding load. This means that, after purging, further inbreeding is expected to be less harmful. But the most immediate consequence is the reduction of the actual inbreeding depression of fitness: due to purging, mean fitness declines less than would be expected just from inbreeding and, after some initial decline, it can even rebound up to almost its value before inbreeding.

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