Tuesday, November 15, 2005

Is Natural Selection a Tautology?   posted by DavidB @ 11/15/2005 03:54:00 AM

This question was raised in some recent comments, and I want to discuss it more fully than was appropriate to a comments box.

In population genetics, fitness is usually defined by reference to reproductive success. The details vary - some authors define fitness by the absolute number of offspring, some by reference to the population average, and so on - but roughly speaking, the fittest organism (or gene) is the one with the greatest number of offspring. It may therefore be said that the ‘reproduction of the fittest’ is a tautology: the fittest individuals necessarily have the most offspring, because that is how fitness is defined.

This is apparently regarded by Creationists as some kind of knockout blow for the theory of evolution by natural selection. Oddly enough, they also argue that natural selection is inadequate, impossible, and so on, which would be absurd if they really believed it was a tautology. No-one (except the late Willard van Orman Quine) spends much time arguing whether all bachelors are unmarried.

But the point I want to emphasise is that even if we define fitness by reference to reproductive success (which is convenient, but not essential, in population genetics), this in no way implies that the theory of evolution by natural selection (TENS) is a tautology.

TENS involves at least the following seven empirical facts or generalisations:

1. Individual organisms differ in reproductive success. This is not a tautology: it is an empirical matter of fact. It would logically be quite possible for organisms all to reproduce equally (or to live for ever without reproducing, like angels - though Genesis 6: 2-4 may cast some doubt on the last point).

2. Differences in reproductive success are associated with phenotypic traits. Traits associated with superior reproductive success (in a given environment, etc.) may be described as adaptive. It is not a tautology that adaptive traits exist. It would be logically possible that differences in reproductive success are merely a matter of chance. In this case there would be no way of predicting whether a trait would increase in frequency from one generation to the next. After a succession of many generations, purely by chance, some traits might increase or decrease in frequency, but this would be genetic drift, not natural selection. Probably a great many genes, and some phenotypic traits, have increased in frequency by genetic drift, but TENS maintains that adaptive traits are usually due to natural selection. (By chance, drift may also sometimes promote an adaptive trait, but this is unlikely to be important except in small populations.)

3. Some adaptive traits are heritable. They tend to be reproduced by genetic inheritance, even over many generations. This is not a tautology. It would be logically possible that offspring did not share their parents’ adaptive traits, or that these would disappear after a few generations. One of the objections Darwin himself had to face was that favourable traits would be ‘washed out’ quickly by interbreeding. Darwin had no conclusive answer to this, but modern genetics has shown that genetic material is very durable, and not destroyed or ‘blended’ by interbreeding.

4. From time to time there are variations (mutations, in a broad sense), in genetic material. This is not a tautology.

5. Some mutations increase adaptiveness. This is not a tautology.

6. There is no inbuilt limit to the amount of cumulative genetic change. This is not a tautology.

7. Perhaps most important of all, mutations have no tendency to occur in directions favourable to adaptive traits. It is often said (and I have recently said it myself) that mutation is random with respect to adaptativeness, but this can be misleading. It is highly likely that the majority of mutations reduce adaptiveness, as there are more ways of reducing it than increasing it. But a minority of mutations increase it, and these are the ones that are favoured by natural selection. This is not a tautology. It would be logically possible that mutations occur preferentially in adaptive directions. Many non-Darwinian theories - Lamarckism, orthogenesis, ‘directed mutation’ of all kinds - have maintained that this actually happens. If this were true, TENS would either be false or reduced in importance. Indeed, there have recently been claims that in some circumstances directed mutation does occur. It remains to be seen whether this is confirmed, and if so whether it is really important or just another minor ‘tweak’ to evolutionary theory.

I conclude that while population genetics may contain some tautological statements (as John Maynard Smith once remarked, any theory involving two lines of algebra will contain tautologies), the theory of evolution by natural selection is by no means a tautology, and rumours of its death are, as usual, greatly exaggerated.