12 September 2005

Drawing careful conclusions

Well, when I said that the last post would stir up some controversy, I wasn't wrong. In the comments section, someone has already accused me of "political correctness" for refusing to draw the conclusion that the two alleles reported in the study are being selected due to their effects on brain size. This accusation was predictable, and it is wrong.

Political correctness has absolutely nothing to do with the proper interpretation of this study. If I thought that the only explanation for the positive selection was the effect of the mutation on brain size, I would have no problem whatsoever in saying that. At present, however, we simply don't know what the exact effect of the mutation is.

We know that the gene is involved in brain size determination, but that does not mean that the mutation in question has anything at all to do with increasing or decreasing brain size. It doesn't mean that the allele results in better brain function. It doesn't mean that people with this allele are smarter than those without it. When the researchers report that an allele of this gene has been subjected to positive selection, and that the gene therefore continues to evolve adaptively, it means just that and nothing more. Determining why the allele is positively selected requires that we study the effects of the allele.

Here's an example that might help illustrate things: the sickle cell trait. I have not run the statistics, but I'm reasonably confident that the allele for this trait shows the effects of positive selection in some populations. Yet the allele does not have a postive effect on the normal functioning of the gene. In fact, the effect of this allele is extremely negative in homozygotes. The positive selection stems from the effect that this trait has in increasing resistance to a disease, not in its effects on the oxygen-carrying functions of hemoglobin.

For all we know, the situation with either or both of the alleles discussed in these two studies is exactly the same - they might actually have a negative impact on brain size, but be positively selected for other reasons. It is also possible that the reason that these alleles have been positively selected is because they result in an enormous increase in intelligence. But we don't know that, either.

Restricting the conclusions that you draw from a study to those supported by what the study actually demonstrates is not political correctness. It's just responsible science.
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