Evolution of experimental "mutator" populations of Drosophila melanogaster.

The theory of evolution predicts that the rate of adaptation of a population is a function of the amount of genetic variation present in the population. This has been experimentally demonstrated in Drosophila populations in which genetic variability was increased either by mass hybridization of two gene pools, or by X-irradiation.--Mutator genes increase the spontaneous mutation rates of their carriers. We have now studied the effects of a third-chromosome mutator gene, mt, on the rate of adaptation of laboratory populations. Initially, experimental and control populations had similar genetic constitutions except for the presence of absence of the mt gene. The populations were maintained for 20-25 generations by "serial transfer" under conditions of very intense selection.--The number of flies produced per unit time remained constant throughout the experiment in the experimental as well as in the control populations. However, in the mutator-carring populations the average longevity of the flies (and consequently the average population size) gradually decreased. Under the experimental conditions natural selection is unable to counteract completely the increased input of deleterious mutations due to the mt gene.