A neutral theory predicts multigenic aging and increased concentrations of deleterious mutations on the mitochondrial and Y chromosomes.
Population genetic forces have molded the constitution of the human genome over evolutionary time, and some of the most important parameters are the initial frequency of the allele, p, the effective population size, Ne, and the selection coefficient, s. There is considerable agreement among evolutionary gerontologists that the amplitude of -s is small for alleles that are Deleterious In Late Life (DILL), and thus DILL traits are effectively neutral and should be fixed in the human population in relationship to Ne and p. Even higher rates of fixation of deleterious mutations are predicted to occur in the two nonrecombinant genomes in humans, i.e., the Y chromosome and the mitochondrial genome, as a consequence of their lower Ne than autosomes, and the predicted higher rate of fixation of deleterious alleles on the Y may explain the reduced average life span of males vs. females. The high probability of fixation of neutral and mildly deleterious mutations in the mitochondrial genome explains in part its fast rate of evolution, the high observed frequency of mitochondrial disease in relationship to this genome's small size, and may be the underlying reason for the transfer of mitochondrial genes over evolutionary time to the nucleus. The predicted higher concentration of deleterious mutations on the mitochondrial genome could have some leverage to cause more dysfunction than that predicted by mitochondrial gene number alone, because of the essential role of mitochondrial gene function in multisubunit complexes, the coupling of mitochondrial functions, the observation that some mtDNA sequences facilitate somatic mutation, and the likelihood of deleterious mutations either increasing the production of or the sensitivity to mitochondrial ROS.