Evolving Thoughts

Evolution, culture, philosophy and chocolate! John Wilkins' continuing struggle to come to terms with impermanence... "Humanus sum, nihil humanum a me alienum puto" - Terence

Friday, November 05, 2004

Kinematics versus dynamics in evolution

Walsh' et alia's characterisation of evolution as a statistical process operating over realworld trial events is, to my mind, incomplete. It is, I think, true that evolution is not a dynamic theory - selection occurs over many different physical substrates, as Sober himself noted. So does gravity, but the modern theory of gravity applies a single physical explanation, in terms of deformation of spacetime, no matter what the makeup of the masses are. But no such single mechanism applies to evolution, or even to all cases of selection.

For example, "selection" and "fitness" can apply equally to a case in which a beetle increases its fitness by a mutated enzyme that allows it to metabolise a previously toxic fungus, as to a tree that is able to increase its reproductive investment by growing slightly longer seed "wings" than its neighbours. Very little is common physically to these two cases even if the fitness or selection coefficient is in all respects identical.

But is evolution just a statistical observation? I think not - evolutionary explanations in terms of selection (and also of drift, for I do agree that they are both the same kind of process in this respect) involve rates of change. It involves complex behaviours of systems of organisms in their environment. It involves mathematical descriptions that have greater and wider application than the immediate context in which the model was developed.

There is a distinction in physical theory between a dynamic model, which indicates the forces active in a particular case under explanation, and a kinematic model which merely describes them. Mendel's model of inheritance is in this respect a kinematic account, as he did not describe the underlying physical processes, but rather the ways in which traits, phenomenal, even epiphenomenal, aspects of inheritance, behaved.

A kinematic model is a precursor to a fuller explanation. It is not merely a statistical description of a state, or even a sequence of states - but it is not a full explanation of the underlying physical processes either. It is, as someone I now cannot recall once said, a promissory note for an explanation. It is a necessary step in the development of the understanding of a domain.

It may even be that the same model, the same mathematics, can apply in various domains. For example, epidemiological models apply to computer viruses as well as Ebola, and to demographic trends in culture as much as the progress of a pathogen. A Universal Darwinism such as Dawkins and others desire is, in the end, just a large-scale kinematic expectation. If culture is a process of Darwinian evolution, it is not because ideas have DNA, RNA or enzymatic expression.

I think therefore that an intermediate view is possible. Sober's force-theory can be recast as a theory of propensities or tendencies, and retain all the virtues of the force model. Walsh, Ariew and Lewens can reject the dynamic model of evolution and still not have to undercut the motivations for it in explanation. And we can learn something generally true about scientific, and in particular biological, explanation.

Tuesday, November 02, 2004

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...

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