Fig. 5.

Understanding somatic evolution in terms of fitness and phenotypic heterogeneity. A Example of a fitness-phenotype relationship, showing a highly nonlinear behaviour. Optimal fitness is associated with an intermediate phenotype. The fastest rate of evolution is predicted around phenotypes that produce the steepest change in fitness. B Wider phenotypic variation is associated with higher rates of evolution. C Experiments that compare a control phenotype with a pharmacologically or genetically inactivated phenotype may miss important information about fitness. In this example, optimal fitness was attained at an intermediate phenotype and would not be detected with the canonical experimental approach. D The relationship between a phenotype and fitness can change in response to constraints placed by the environment. In this example, low pHe inhibits glycolysis, which forces the cell to rely more on oxidative phosphorylation (OXPHOS); this manifests as a steeper fitness-OXPHOS curve and therefore a higher rate of evolution