Online research in philosophy

The latter half of the twentieth century has been marked by debates in evolutionary biology over the relative significance of natural selection and random drift: the so-called “neutralist/selectionist” debates. Yet John Beatty has argued that it is difficult, if not impossible, to distinguish the concept of random drift from the concept of natural selection, a claim that has been accepted by many philosophers of biology. If this claim is correct, then the neutralist/selectionist debates seem at best futile, and at worst, meaningless. I reexamine the issues that Beatty raises, and argue that random drift and natural selection, conceived as processes, can be distinguished from one another.

In addressing phenotypic evolution, this article reconsiders natural selection, random drift, developmental constraints, and internal selection in the new extended context of evolutionary developmental biology. The change of perspective from the "evolution of phenotypes" toward an "evolution of ontogenies" (evo-devo perspective) affects the reciprocal relationships among these different processes. Random drift and natural selection are sibling processes: two forms of post-productional sorting among alternative developmental trajectories, the former random, the latter nonrandom. Developmental constraint is a compound concept; it contains even some forms of natural ("external" and "internal") selection. A narrower definition ("reproductive constraints") is proposed. Internal selection is not a selection caused by an internal agent. It is a form of environment-independent selection depending on the level of the organism's internal developmental or functional coordination. Selection and constraints are the main deterministic processes in phenotypic evolution but they are not opposing forces. Indeed, they are continuously interacting processes of evolutionary change, but with different roles that should not be confused.

In addressing phenotypic evolution, this article reconsiders natural selection, random drift, developmental constraints, and internal selection in the new extended context of evolutionary developmental biology. The change of perspective from the "evolution of phenotypes" toward an "evolution of ontogenies" (evo-devo perspective) affects the reciprocal relationships among these different processes. Random drift and natural selection are sibling processes: two forms of post-productional sorting among alternative developmental trajectories, the former random, the latter nonrandom. Developmental constraint is a compound concept; it contains even some forms of natural ("external" and "internal") selection. A narrower definition ("reproductive constraints") is proposed. Internal selection is not a selection caused by an internal agent. It is a form of environment-independent selection depending on the level of the organism's internal developmental or functional coordination. Selection and constraints are the main deterministic processes in phenotypic evolution but they are not opposing forces. Indeed, they are continuously interacting processes of evolutionary change, but with different roles that should not be confused.

Evolutionary processes such as natural selection and random drift are commonly regarded as causes of population-level change. We respond to a recent challenge that drift and selection are best understood as statistical trends, not causes. Our reply appeals to manipulation as a strategy for uncovering causal relationships: if you can systematically manipulate variable A to bring about a change in variable B, then A is a cause of B. We argue that selection and drift can be systematically manipulated to produce different kinds of population-level change. They should therefore be regarded as causes.

How do fitness and natural selection relate to other evolutionary factors like architectural constraint, mode of reproduction, and drift? In one way of thinking, drawn from Newtonian dynamics, fitness is one force driving evolutionary change and added to other factors. In another, drawn from statistical thermodynamics, it is a statistical trend that manifests itself in natural selection histories. It is argued that the first model is incoherent, the second appropriate; a hierarchical realization model is proposed as a basis for a statistical treatment. It emerges that natural selection does not cause evolution; it just is evolution. The theory incorporates relations of statistical correlation, but not the kind of causation found in fundamental physical processes.

The statistical interpretation of the Theory of Natural Selection claims that natural selection and drift are statistical features of mathematical aggregates of individual-level events. Natural selection and drift are not themselves causes. The statistical interpretation is motivated by a metaphysical conception of individual priority. Recently, Millstein, Skipper, and Dietrich (2009) have argued (a) that natural selection and drift are physical processes, and (b) that the statistical interpretation rests on a misconception of the role of mathematics in biology. Both theses are contested.

Among the liveliest disputes in evolutionary biology today are disputes concerning the role of chance in evolution--more specifically, disputes concerning the relative evolutionary importance of natural selection vs. so-called "random drift". The following discussion is an attempt to sort out some of the broad issues involved in those disputes. In the first half of this paper, I try to explain the differences between evolution by natural selection and evolution by random drift. On some common construals of "natural selection", those two modes of evolution are completely indistinguishable. Even on a proper construal of "natural selection", it is difficult to distinguish between the "improbable results of natural selection" and evolution by random drift. In the second half of this paper, I discuss the variety of positions taken by evolutionists with respect to the evolutionary importance of random drift vs. natural selection. I will then consider the variety of issues in question in terms of a conceptual distinction often used to describe the rise of probabilistic thinking in the sciences. I will argue, in particular, that what is going on here is not, as might appear at first sight, just another dispute about the desirability of "stochastic" vs. "deterministic" theories. Modern evolutionists do not argue so much about whether evolution is stochastic, but about how stochastic it is.

1. Drift and selection can be distinguished conceptually. 2. Selection and drift are physical, biological phenomena. 3. Drift and selection can occur simultaneously in a population. 4. Selection and drift should be characterized as processes (see #1), not outcomes. 5. Distinguishing between selection and drift empirically is difficult, but is (sometimes) not impossible. 6. Selection and drift are population-level causal processes.

We distinguish dynamical and statistical interpretations of evolutionary theory. We argue that only the statistical interpretation preserves the presumed relation between natural selection and drift. On these grounds we claim that the dynamical conception of evolutionary theory as a theory of forces is mistaken. Selection and drift are not forces. Nor do selection and drift explanations appeal to the (sub-population-level) causes of population level change. Instead they explain by appeal to the statistical structure of populations. We briefly discuss the implications of the statistical interpretation of selection for various debates within the philosophy of biologythe `explananda of selection' debate and the `units of selection' debate.

In “Two Ways of Thinking About Fitness and Natural Selection” (Matthen and Ariew [2002]; henceforth “Two Ways”), we asked how one should think of the relationship between the various factors invoked to explain evolutionary change – selection, drift, genetic constraints, and so on. We suggested that these factors are not related to one another as “forces” are in classical mechanics. We think it incoherent, for instance, to think of natural selection and drift as separate and opposed “forces” in evolutionary change – that it makes sense to say, for instance, that selection contributed 80% to the actual evolutionary history of the human eye, and drift only 20%. We proposed instead a statistical view of the Theory of Evolution, a view in which fitness is not a cause of evolution, but rather a measure of growth. We also argued for a “hierarchical realization model” for thinking about the relationship between evolutionary factors such as those mentioned above, and suggested that in a “fully specified model”, as we call it below, there is no distinction between natural selection and evolution.