Figure 1.

The Charnov-Charlesworth model, illustrated for selection on two traits subject to functional constraint. Functional constraint prohibits phenotypes above and to the right of the blue line. Fitness contours are shown as red ellipses, with highest fitness at the center. The population is initialized with trait values far from constraint (I). Selection then moves the population closer to the optimum, but functional constraint prevents the population from evolving past the blue line and it settles in mutation-selection balance at (II). Mutations with negative pleiotropy can be close to neutral and maintain trait variation along the line of functional constraint, while mutations affecting only one trait tend to reduce fitness and are purged.