Qualitative behavior of fitness and stability versus neutrality exponent at fixed GC and population size.

At stability is close to the lethal threshold without any penalization for the fitness. In the small regime stability increases with , but the penalization for low stability decreases even more, with the net effect of a decrease in fitness. At intermediate both stability and fitness increase with and stability reaches a maximum that depends on . Finally, at large stability decreases with , since the differences in fitness produced by a given difference in stability become smaller and cannot be fixed against the entropic effect of mutations, while fitness tends to the maximum possible value .