The fitness of a rare mutant is plotted relative to the resident . The production rate of the mutant cell is , where and are plotted on the and axes respectively. The resident production rate is shown as a red dot and varies (a) , (b) , (c) and (d) . The mutant () with maximum fitness is shown as a black dot. The contour where is marked by a black dashed line (b and c). The fitness difference is always negative in (a) and always positive in (d). Other parameters used: .