Figure 1.

(a) Mutation-selection balance and mutational meltdown: a fast-growing wild-type population accumulates deleterious mutations. If the mutation rate is too large, the population fitness declines continuously. (b) Experimental system used by Lavrentovich et al. (4): yeast wild-type cells (blue) stochastically convert to a slower-growing mutant type (yellow) at a slow, tunable rate. To see this figure in color, go online.