13 August 2005

Predictions by Darwin – II – Retrodictions

I think that the image of predictions as critical experiments in the mold of Einstein and the 1919 solar eclipse expedition lead us to ignore other valid means by which scientists become confident about theories.

My guess is that the most common form of scientific prediction is retrodiction. These are predictions that take the form: “If the theory is correct then certain events will have had to have happened, so we’ll look for evidence that they did.” These should be familiar to fans of CSI: “If the gun was fired at close range, then gunpowder residue should have been left on the clothing, so let’s go check.”

Much of what happens when a theory is presented and justified is retrodiction. Indeed, it is often presented as a comparison of the retrodictive powers of the new theory and its competitor. Thus, Darwin presents data on the geographic distribution of species, and notes that, for similar groups, the degree of similarity is higher for groups that are closer in space or for which geographic barriers are lower. He then says that this is a logical result if these groups are slowly diverging from a common ancestor, as the more distant are more likely to have preserved variations from the ancestor rather than breeding with and blending back in to the central population. Recast into the retrodictive form: if the theory is correct, then such divergence will have had to have happened, and there it is. On the other hand, if the different groups were all specially created, it would be hard to see why such a correlation of divergence with distance should occur. Thus, in the comparison of retrodictions, descent with modification wins over special creation.

Of course, retrodictions included in the initial justification of the theory don’t have the punch of predictions resolved by future experiments. Again, we have that cinematic bias. But the fact is that Origin of Species is full of successful predictions, in the form of retrodictions, and given the fact that Darwin was pretty honest about including data that were clearly problematic for the theory (most famously the matter of the completeness of the fossil record), that we should not be discounting all these successes.

There are other forms of scientific prediction that relate to Darwin's work. I shall discuss them in the next post in this series.


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