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Evolution and Philosophy

A Good Tautology is Hard to Find

Copyright © 1997 by John Wilkins

Summary: The claim that evolutionary theory is a tautology rests on a misunderstanding of the theory. Fitness is more than just survival.




The simple version of the so-called 'tautology argument' is this:

Natural selection is the survival of the fittest. The fittest are those that survive. Therefore, evolution by natural selection is a tautology (a circular definition).

The real significance of this argument is not the argument itself, but that it was taken seriously by any professional philosophers at all. 'Fitness' to Darwin meant not those that survive, but those that could be expected to survive because of their adaptations and functional efficiency, when compared to others in the population. This is not a tautology, or, if it is, then so is the Newtonian equation F=ma [Sober 1984, chapter 2], which is the basis for a lot of ordinary physical explanation.

The phrase 'survival of the fittest' was not even Darwin's. It was urged on him by Wallace, the codiscoverer of natural selection, who hated 'natural selection' because he thought it implied that something was doing the selecting. Darwin coined the term 'natural selection' because had made an analogy with 'artificial selection' as done by breeders, an analogy Wallace hadn't made when he developed his version of the theory. The phrase 'survival of the fittest' was originally due to Herbert Spencer some years before the Origin .

However, there is another, more sophisticated version, due mainly to Karl Popper [1976: sect. 37]. According to Popper, any situation where species exist is compatible with Darwinian explanation, because if those species were not adapted, they would not exist. That is, Popper says, we define adaptation as that which is sufficient for existence in a given environment. Therefore, since nothing is ruled out, the theory has no explanatory power, for everything is ruled in.

This is not true, as a number of critics of Popper have observed since (eg, Stamos [1996] [note 1]). Darwinian theory rules out quite a lot. It rules out the existence of inefficient organisms when more efficient organisms are about. It rules out change that is theoretically impossible (according to the laws of genetics, ontogeny, and molecular biology) to achieve in gradual and adaptive steps (see Dawkins [1996]). It rules out new species being established without ancestral species.

All of these hypotheses are more or less testable, and conform to the standards of science. The answer to this version of the argument is the same as to the simplistic version - adaptation is not just defined in terms of what survives. There needs to be a causal story available to make sense of adaptation (which is why mimicry in butterflies was such a focal debate in the teens and twenties). Adaptation is a functional notion, not a logical or semantic a priori definition, despite what Popper thought.

The current understanding of fitness is dispositional . That is to say, fitness is a disposition of a trait to reproduce better than competitors. It is not deterministic. If two twins are identical genetically, and therefore are equally fit, there is no guarantee that they will both survive to have equal numbers of offspring. Fitness is a statistical property. What 'owns' the fitness isn't the organism, but the genes. They will tend to be more often transmitted so far as what they deliver is better 'engineered' to the needs of the organisms in the environment in which they live. And you can determine that, within limits, by 'reverse engineering' the traits to see how they work [Dennett 1995: chapter 8].

Moreover, fitness exists over and above the properties of the individual organisms themselves. There are three debated ways to construe this. Fitness can be a relation of genes to other genes. Fitness can be a supervenient property - that is, it can be a property of very different physical structures (of ants, aardvarks and artichokes) [Sober 1984]. Or fitness can be seen as an emergent property, a property of systems of a certain complexity and dynamics [Depew and Weber 1995]. Whether fitness is a genetic, organismic or system property is a hot topic in modern philosophy of biology. I think the system interpretation is the way to approach it [Weber and Depew 1996, Depew and Weber 1995].

Recently, there have been attacks on the very notion of adaptive explanation by some evolutionary biologists themselves (eg, Gould and Lewontin [1979]). These fall into two camps - those who think adaptation is not enough to explain diversity of form, and those who think that adaptive explanations require more information than one can obtain from either reverse engineering or the ability to generate plausible scenarios. The reason given for the former is a kind of argument from incredulity - natural selection is not thought to be a sufficient cause, and that macroevolution (evolution at or above the level of species) is a process of a different kind than selection within species. Arguments about parsimony (Ockham's Razor) abound.

Arguments for the second view - that selective explanations need supplementing - rest not on the causal efficacy of selection (which is not denied) but on the problems of historical explanation [Griffith 1996]. In order to explain why a species exhibits this trait rather than that trait, you need to know what the null hypothesis is (otherwise you can make a selective explanation for both a case and its opposite equally well). Perhaps it has this trait because its ancestors had it and it has been maintained by selection. Perhaps it has it because it would be too disruptive of the entire genome and developmental machinery to remove it. Perhaps it has it for reasons to do with genetic drift, simple accident, or whatever. In order to make a good scientific explanation, says Griffiths, you must know a fair bit about the phylogeny of the species, its environmental distribution, and how the processes that create the trait work at the level of genes, cells and zygotes.

This leads us to the question of what a scientific explanation really is; indeed, it opens up the question of what science is, that it is so different from other intellectual pursuits like backgammon, theology or literary criticism.




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