Methodological problems in evolutionary biology. X. Natural selection without selective agents.

On a common view of evolution, natural selection is the major force that produces evolutionary change. Selection is thought to operate on different types (genotypes or phenotypes) in populations so as to generate differential reproductive survival of these types. This should engender changes in population composition. The conception of selection as a 'force' should be considered as a convenient shorthand that easily misleads us. Selection is not a factor over and above items such as temperature regimes, predators, and so forth. These items do causal work in evolutionary processes. The term 'selection' is merely an placeholder for them. Differential reproductive survival thus appears to depend on particular environmental items that influence different types in different ways. Such items are properly regarded as the selective agents. On the face of it, selection processes must always be due to the operation of such agents. I argue that this is a mistaken assumption. Processes of selection may well occur in the absence of selective agents. That is because environmental factors may contribute to differential reproductive survival even if they do not affect different genotypes or phenotypes in different ways. Considering the role of the environment in selection, we should distinguish between selective agents and contributive agents.