Does Population Genetics Theory Explain?

Population genetics theory isn't explanatory. Perhaps this is obvious to some. It's obvious to me, for instance. But I don't think it's obvious to everyone, at least not to every philosopher. Take the notorious papers by Mohan Matthen and Andre Ariew (2002) and Denis Walsh, Tim Lewens, and Andre Ariew (2002). In these papers, WALM, as I'll call them, apparently argue that population genetics theory issues statistical explanations of evolutionary phenomena by virtue of its purely statistical structure. But if population genetics theory doesn't explain, then, obviously, it doesn't issue statistical explanations.

Population genetics theory is a family of mathematical structures the aim of which, as Richard Lewontin (1974, p. 12) puts it, is the "study of the origin and dynamics of genetic variation within populations." These models are used to track changes and equilibria of gene frequencies in populations. Indeed, in population genetics evolution just is change in gene frequencies in a population from one generation to the next. The mathematical structures include variables, parameters, and rules (or laws) of transformation for transitioning through the states of a model-system over (generational) time. The rules are the rules of genetics, mating, migration, selection, drift, mutation, etc., and the variables and parameters hold values of the states of the system, i.e., for allele frequencies, population size, selection pressure, etc., against the background of the relevant rules. This is rough, but close enough. (Lewontin 1974  has a bit more on the formalisms of population genetics.)

How might these structures be explanatory? Philosophers of science have spent a lot of time trying to understand the nature of scientific explanation. And I think they've (we've) said lots of interesting things. A very basic point philosophers have made clear is that explanation is not the same thing as description. To describe some phenomenon is, roughly, just to state a set of facts about it. To describe, for example, the length of a flagpole's shadow is just to state, among other things, that it is X meters long. To explain the length of the shadow, however, is to do more. It is to say why the shadow is X meters long. In terms of population genetics, one may describe the distribution of gene frequencies in a population. But it is another thing to explain why the distribution is as it is. Like I said at the outset of this post, population genetics theory can't do this. Let's go back and think about the implications of my previous posts, here and here, on the nature of selection as a way of establishing this point.

Lewontin (1970) characterizes evolution by natural selection in the following way: In order for evolution by natural selection to occur, there must be phenotypic variation, differential fitness, and heritability of fitness. I critically discussed these conditions, arguing that they aren't sufficient for evolution by natural selection (others have as well). Indeed, Lewontin's conditions  are quite consistent with evolution by drift. In a comment to an earlier post, Richard Lawler pointed me to Sean Rice's critical discussion of the Price Equation in his 2004 book, Evolutionary Theory. The Price Equation is meant to be a precise statement about the covariance between fitness and phenotype and, as such, a precise statement about evolution by natural selection. Yet, Rice shows mathematically what I argued for conceptually. The Price Equation is insufficient as a description of evolution by natural selection. It is quite consistent with evolution by drift. Now, Lewontin's characterization is an informal gloss on the mathematics of population genetics. So, where Lewontin's informal characterization fails I expect the formal one to fail as well. I think Rice shows that the formal characterization fails as well.

If population genetics theory fails to distinguish between evolution by selection and evolution by drift, then it isn't explanatory. And it isn't because it doesn't track information that is crucially relevant to explaining why some population's distribution of gene frequencies is the way it is. What information is that? Rice says that the Price Equation fails because it doesn't say "anything about what causes the covariance between fitness and phenotype" (2004, p. 168). In one of my previous posts, I said that Lewontin's conditions fail because they don't include the interaction between variants and their environment that biases their abilities to survive and reproduce. Rice and I are saying essentially the same thing.

Something more is needed to explain that a population has evolved by natural selection rather than drift. Robert Brandon, in his 1990 book, Adaptation and Environment, sets out five criteria for a complete adaptation explanation (p. 165). One must have

1. evidence that selection has occurred;
2. an ecological explanation of the fact that some types are better adapted than others;
3. evidence that the trait in question is heritable;
4. information about the structure of the population, including both demic structure and the structure of the selective environment;
5. phylogenetic information concerning what has evolved from what.

These five conditions capture what I and Rice are looking for (and then some). And they aren't satisfied by population genetics theory. That is, they aren't satisfied by the formal structures of population genetics. They are satisfied out of attempts to relate a system described by a population genetics model and a natural system. That's where evolutionary explanations come from. Now, you won't always find Brandon's five criteria met in a data paper from Evolution (for example). And in those cases, it's fair to say, I think, that such papers aren't complete adaptation explanations. But they are papers that get close to Brandon's criteria. As Roberta Millstein has discussed in a forthcoming paper in The British Journal for the Philosophy of Science, Nathan Rank and Elizabeth Dalhoff's (1992, 2000, 2002) studies of the montane willow leaf beetle are as close as one might hope for. And, in particular, those studies exemplify the requirement that there is an ecological explanation that some types are better adapted than others --that is, the requirement that I and Rice are specifically after.

I think one implication of the argument here is that WALMs view that evolutionary explanations are purely statistical because evolutionary theory is purely statistical can't be right. Population genetics theory can't distinguish between evolution by natural selection and evolution by drift. More information is needed. And at least some of that information, crucial information directly relevant to explaining that some population evolved by natural selection, is causal in nature. Evolutionary explanations are causal explanations (albeit probabilistic). Such explanations aren't part and parcel of the theory. But they are part and parcel of relating theories to the world.

There's more to say about the implications of the argument here. It's now crucial to articulate a full blown view of evolutionary explanation that takes into account the (presumed) insights here. Some philosophers have already begun, in various ways, e.g., Frederic Bouchard and Alex Rosenberg (2004), Ken Reisman and Patrick Forber  (forthcoming), and Roberta Millstein (forthcoming). I'll revisit these next steps in subsequent posts.

References

Bouchard, Frederic and Alex Rosenberg (2004), "Fitness, Probability, and the Principles of Natural Selection", British Journal for the Philosophy of Science 55: 693-712.

Brandon, Robert (1990), Adaptation and Environment. Princeton: Princeton University Press.

Dahlhoff, Elizabeth P. and Nathan Egan Rank (2000), "Functional and Physiological Consequences of Genetic Variation at Phosphoglucose Isomerase: Heat Shock Protein Expression Is Related to Enzyme Genotype in a Montane Beetle", Proceedings of the National Academy of Sciences 97: 10056-10061.

Lewontin, R. C. (1970), "The Units of Selection", Annual Review of Ecology and Systematics 1: 1-18.

Lewontin, R. (1974), The Genetic Basis of Evolutionary Change. New York: Columbia University Press.

Matthen, Mohan and Andre Ariew (2002), "Two Ways of Thinking About Fitness and Natural Selection." Journal of Philosophy 99: 55-83.

Millstein, R. (forthcoming), "Natural Selection as a Population-Level Causal Process", British Journal for the Philosophy of Science.

Rank, Nathan Egan (1992), "A Hierarchical Analysis of Genetic Differentiation in a Montane Leaf Beetle Chrysomela Aeneicollis (Coleoptera: Chrysomelidae)", Evolution 46: 1097-1111.

Rank, Nathan Egan and Elizabeth P. Dahlhoff (2002), "Allele Frequency Shifts in Response to Climate Change and Physiological Consequences of Allozyme Variation in a Montane Insect", Evolution 56: 2278-2289.

Reisman, Ken and Patrick Forber (forthcoming), "Manipulation and the Causes of Evolution", Philosophy of Science.

Rice, S. (2004), Evolutionary Theory. Sunderland, MA: Sinauer and Associates.

Stephens, Christopher (2004), “Selection, Drift, and the 'Forces' of Evolution”, Philosophy of
Science 71: 550-570.

Walsh, Denis M., Tim Lewens, and André Ariew (2002), "The Trials of Life: Natural Selection and Random Drift." Philosophy of Science 69: 452-73.