An extreme test of mutational meltdown shows mutational firm up instead.

Traditionally, the accumulation of new deleterious mutations in populations or species in low numbers is expected to lead to a reduction in fitness and mutational meltdown, but in this study the opposite was observed. Beginning with a highly inbred populations of Drosophila melanogaster, new mutations that accumulated in experiments of two females and two males or of one female and one male each generation for 52 generations did not cause a decline in progeny numbers over time. Only two lines went extinct among 52 tested lines. In three of four experiments there was a significant increase in progeny numbers over time (mutational firm up), which had to be due to new beneficial, compensatory, overdominant, or back mutations.