RISK OF POPULATION EXTINCTION FROM FIXATION OF DELETERIOUS AND REVERSE MUTATIONS

A model is developed for alternate fixations of mildly deleterious and wild-type alleles arising by forward and reverse mutation in a finite population. For almost all parameter values, this gives an equilibriu m load that agrees closely with the general expression derived from di ffusion theory. Nearly neutral mutations with selection coefficient a few times larger than 1/(2N(e)) do the most damage by increasing the e quilibrium load. The model of alternate fixations facilitates dynamica l analysis of the expected load and the mean time to extinction in a p opulation that has been suddenly reduced from a very large size to a s mall size. Reverse mutation can substantially improve population viabi lity, increasing the mean time to extinction by an order of magnitude or more, but because many mutations are irreversible the effects may n ot be large. Populations with initially high mean fitness and small ef fective size, N-e below a few hundred individuals, may be at serious r isk of extinction from fixation of deleterious mutations within 10(3) to 10(4) generations.