The elusive synthesis

Much of evolutionary theory, and in particular population genetics, has typically been concerned with the consequences of fitness differences. Population-genetic models specify the changes in populations of genes and organisms that are the consequences of fitness differences at the initial state of the system. These models probe the effects of population structure, genetic variation, migration, mating systems, and fitness differentials on evolutionary trajectories. There has been the development of models with more realistic assumptions about genotype-phenotype relations; models of multispecies interactions, models of the effects of frequency dependence, and models of fitness effects that prevent the establishment of strategies that would be stable were they established. The ecological parameters of these models-demographic growth rates, carrying capacity, and fitness values-are assumed by the models rather than explained within them. Fitness would not be an explanatory or even a predictive property of organisms or genes. Evolutionary theory needs source laws explaining the bases of fitness differences and consequence laws explaining their upshot. Some fitness differences among organisms are not specifically tied to their environmental situations: selection will penalize genes that compromise basic metabolic or reproductive functions in almost any circumstances. Understanding the interface between ecology and evolution will be hard enough without having to take into account indirect effects on and from ecological ensembles. © 2005