Rates and effects of deleterious mutations and their evolutionary consequences

Abstract In a highly influential paper, Muller (1950) put forward the idea that deleterious mutations affecting the heterozygote appear at a high rate in populations, thus, imposing a reduction in fitness (mutational load) that could become unbearable. As shown by Haldane (1937), this load is mainly determined by the rate of deleterious mutations and is practically independent of their severity, although it is modulated by their degree of dominance. It took a long time until adequate experimental data for Drosophila viability were first obtained by Mukai (1964), who estimated a high genomic rate of mutations (haploid rate A > = 0.3, the number of deleterious mutations appearing per gamete and generation) showing very little dominance (average coefficient of dominance E(h) = 0.4, where h = 0, 0.5, and 1 denotes recessive, additive and dominant gene action of mutations, respectively) and mild average effects (average selection coefficient E(s) < 007E 0.03, the relative reduction in fitness of the homozygous mutant).