Muller's ratchet
Muller's ratchet is a population genetical model that describes the random accumulation of sightly deleterious mutations in finite populations with limited amounts of recombination.
If Muller's ratchet operates long enough it can lead to genomic decay that is strong enough to drive a population to extinction. The rate of deleterious mutation accumulation from Muller's ratchet is difficult to predict.
Muller's ratchet gets its name from the fact that the loss of fitness
in the fittest individuals of a population due to the accumulation of
deleterious mutations cannot be reversed, if there is no recombination
and no back mutation (like in an asexual population with an infinite genome).

