Mingjun Zhang (University of Pennsylvania)
In the sixth edition of the Origin, Darwin wrote that, “I am convinced that Natural Selection has been the most important, but not the exclusive, means of modification” (Darwin 1872, 4). The idea that natural selection is the most important, if not the only important, driving factor of evolution is further developed and crystalized in the various views under the name of adaptationism. However, it is not always clear what exactly it means to talk about relative importance in the relevant debate. In this paper, I distinguish three senses of relative importance and use this distinction to reexamine the various claims of adaptationism. I give examples of how these different senses of relative importance are applied in different adaptationist claims, and discuss some possible issues in their application.
The first sense: A factor is more important than others if the proportion of phenomena in a domain explained or caused by this factor is greater than the proportion of those explained or caused by other factors. I call it relative importance based on relative frequency. The famous debate between Fisher and Wright about the role of natural selection can be understood as a debate about the relative importance of natural selection in this sense, in which they disagree about the relative frequency of genetic variation within and between populations caused by natural selection and other factors like drift. However, philosophers like Kovaka (2017) have argued that there is no necessary connection between relative frequency and relative importance.
The second sense: A factor is more important than others if it can explain special phenomena in a domain and help answer the central or most important questions within it. I call it relative importance based on explanatory power. This kind of relative importance is involved in the view of explanatory adaptationism formulated by Godfrey-Smith (2001). According to this view, natural selection is the most important evolutionary factor because it can solve the problems of apparent design and/or adaptedness, which are the central problems in biology. However, some biologists may deny that there are “central questions” in biological research. Even if there are central questions in biology, apparent design and adaptedness may not be the only ones.
The third sense: A factor is more important than others if it has greater causal efficacy in the production of a phenomenon than others. For example, the gravity of the Moon is a more important cause of the tides on the Earth than the gravity of the Sun because the Moon has a bigger influence on the tidal height on the Earth. I call it relative importance based on causal efficacy. Orzack and Sober (1996) understand adaptationism as the view that selection is typically the most important evolutionary force. Here they use relative importance in the third sense because their formulation involves the comparison of causal efficacy between selection and other factors in driving evolution. The main issue is how to measure the causal efficacy of different factors.
