Deleterious Mutations Hitchhike to Fixation

Figure WN23.9. In an asexual population, deleterious mutations hitchhike to fixation along with favorable mutations. (A) In a balance between mutation and selection, most genomes may carry several deleterious mutations (crosses), and the average fitness of the population is e^–U. A beneficial mutation (circle) that arises in a genome with an average number of deleterious mutations (lower genome at right, X) will have an initial advantage but, as it accumulates additional mutations, may eventually be eliminated (B). A beneficial mutation only has a chance of fixing in the population if its overall fitness, including the load of deleterious mutations it starts out with, is higher than the fittest class (e.g., Y). However, it will then carry deleterious mutations to fixation (in example Y, three are fixed).