Mutational meltdown

In evolutionary genetics, mutational meltdown is a sub class of extinction vortex in which the environment and genetic predisposition mutually reinforce each other. Mutational meltdown (not to be confused with the concept of an error catastrophe) is the accumulation of harmful mutations in a small population, which leads to loss of fitness and decline of the population size, which may lead to further accumulation of deleterious mutations due to fixation by genetic drift. This loss of fitness is drift load, and it is a target of conservation genetics.

A population experiencing mutational meltdown is trapped in a downward spiral and will go extinct if the phenomenon lasts for some time. Usually, the deleterious mutations would simply be selected away, but during a mutational meltdown, the number of individuals not reproducing (e.g. by suffering an early death) is too large relative to the overall population size so that mortality exceeds the birth rate.

The same circumstances that create mutational meltdown also create a stronger phenomenon of "mutational drought" on the comparably long timescale of allele fixation. Mutational drought describes a shortage of new beneficial mutations, which are needed both to counter deleterious fixations and to adapt to a changing environment.

Explanation

The mechanism behind mutational meltdown is that a spontaneous deleterious mutation is introduced and after some time is eventually fixed into the population. This, in turn, leads to an accumulation in small populations where the growth rate as well as the population size both decrease.