Selection, drift and the forces of evolution

In 1984, Elliot Sober published his classic book The Nature of Selection (MIT Press: Cambridge MA). In it, Sober set up evolutionary theory as a theory of forces. Something that changes may be acted upon by a force, or it may drift, and if something is static, then it may be due to opposing forces canceling each other out, or by a lack of forces. Selection, of course, is a force in this account of the theory. He says:

"In evolutionary theory, the forces of mutation, migration, selection, and drift constitute causes that propel a population through a sequence of gene frequencies. To identify the causes of the current state . . . requires describing which evolutionary forces impinged." (1984, 141, quoted in Walsh et al., 453)

Recently, a paper was published which challenged this account (Walsh et al 2002). According to them, Sober's theory is a dynamical theory, while drift is a statistical process. But drift occurs via the same trial events of evolution - births, deaths and reproduction - as selection does. These are the actual physical processes going on. So if drift is a statistical process so is selection. They therefore conclude that physically speaking, drift and selection are the same kind of process, and that evolution is a statistical theory that appeals to the nature of populations.

"The central explanatory concept in natural selection theory is fitness. In fact there are two distinct concepts of fitness in play in evolutionary theory and it is important to distinguish their respective roles. Individual fitness is a causal/dispositional property of an individual organism, its propensity to survive and reproduce in a given environment . . . . Trait fitness is a statistical property of a trait type. It is a function of the mean and variance of the fitnesses of individuals within a population . . . . It is easy to overlook the differences between these two varieties of fitness, yet the differences are crucial." (460)

According to them, individual fitness is the substrate, as it were, of evolution, but natural selection explains trait fitness; that is, the fitness of a type within a population. Selection is a sorting process, an "ensemble-level" phenomenon (463) that is a property of a sequence of the trial events. It is, as philosophers of mind would express it, an epiphenomenal property.

Now this is rather interesting, as Sober himself noted in the original book that fitness is a supervenient property. This is another term of art in the philosophy of mind, due to Jaegwon Kim. It is often expressed in terms of possible worlds - which make my head spin - but Sober expresses it more simply - it is a property that can be realized by many physical substrates, but if any two objects have the same physical state, they have the same supervenient property. The difference between a supervenient and an epiphenomal property here is that epiphenomena are causally ineffectual. They are caused by, but cause no, physical processes. Supervenient properties, on the other hand, are able to have a causal role.

If fitness is a supervenient property of traits, then selection can be a causally effective process. If it is an epiphenomenal property, it cannot.

In our next entry, we will consider the dynamical and statistical nature of selection and drift in more detail. For now, let us note that selection and mind seem to run on parallel paths. Perhaps this is a result of a common cause - maybe the concepts are superveniently the same...

Reference
Walsh, Denis M; Lewens, Tim; Ariew, André, "The Trials of Life: Natural Selection and Random Drift", Philosophy of Science, 2002; 69(3): 452-473