mutation accumulation

Goyal S, Balick DJ, Jerison ER, Neher RA, Shraiman BI & Desai MM 2012 Dynamic mutation-selection balance as an evolutionary attractor. Genetics 191:1309-1319.

• back and compensatory mutations become more common as a population declines in fitness
• the probability that a random mutation is beneficial increases with decreasing absolute fitness

• Muller's ratchet plays a crucial role in determining the structure of genetic variation in asexual populations or on short distance scales in the genomes of sexual organisms
• these expectations must be revised if populations exist instead in the dynamic state in which both beneficial and deleterious mutations fix, without any continuous net degradation or growth of fitness
• even though no change in fitness occurs, signatures of both positive and negative selection are likely to be found in patterns of molecular evolution, as has been suggested by earlier studies
• this state is the natural null expectation for the effects of mutations and purifying selection on patterns of genetic variation
• efforts to look for positive selection that represent "true" adaptation should look for deviations from this situation