Genetic load

Genetic load is the reduction in the average fitness of a population due to the presence of deleterious alleles. Every population carries a burden of mutations that are harmful to fitness — some mildly, some severely — and these mutations persist because natural selection cannot purge them instantly. The genetic load represents the gap between the population's actual fitness and the fitness it would achieve if all deleterious alleles were eliminated.

The concept was formalized by J.B.S. Haldane and Hermann Joseph Muller in the mid-twentieth century. Haldane showed that the cost of selection — the reproductive excess required to eliminate a deleterious allele — imposes a limit on the rate of adaptive evolution. Muller extended this to argue that small populations are particularly vulnerable to genetic load because genetic drift can allow deleterious alleles to reach fixation, a process he called mutational meltdown.

Sources and Dynamics

Genetic load accumulates from several sources. Mutational