Drift load

Drift load, a type of genetic load, is the decline in fitness in a population due to the fixation of deleterious mutations by genetic drift. Drift load generally decreases as a function of effective population size, and is a mechanism of inbreeding depression.

If all individuals in a population become homozygous for one allele, that allele is fixed, and any other alleles at that locus are lost from that population by the random evolutionary process of genetic drift. As fixation of deleterious alleles occurs at more loci and variability is lost, drift load increases. With increasing drift load, populations become less fit with each generation, a mutational meltdown, which can lead to an extinction vortex.

Strategies to address drift load in endangered species is a goal of conservation genetics.

For realistic values of ${\displaystyle N_{e}}$, the effective population size, and ${\displaystyle \lambda _{D}}$, the mean effect on fitness of all fixed deleterious mutations, the loss of fitness in each generation due to drift load, ${\displaystyle \Delta {W_{D}}}$, can be calculated:

${\displaystyle \Delta {W_{D}}\cong {\frac {U_{D}}{8{N_{e}^{2}}\lambda _{D}}}}$

Where the total number of new deleterious mutations per diploid zygote is ${\displaystyle U_{D}}$.

This demonstrates that more fixed deleterious alleles in a population lead to greater reductions in fitness, unless counteracted by large effective populations or weakly deleterious effects overall.

Factors that decrease drift load, or counteract its negative effects on population fitness, include polyploidy, gene flow and purifying selection. Drift load increases more rapidly in small populations, so factors that tend to maintain large populations such as high reproductive rates and low levels of biotic and abiotic stress will reduce the incidence of drift load.