Are there laws of evolution?

I’ve just come across a very interesting essay by Eugene Koonin. He asks an intriguing question: does evolution, like physics, have universal mathematical laws?

To answer it, he pulls out some fascinating patterns that keep turning up no matter what organism we look at. For example, genes that are more active tend to evolve more slowly, and the number of genes in gene families follows a power law distribution. He points out that such patterns can emerge out of simple models of the behaviour of genes and genomes. Just like physical laws. (Ha. Take that, you physicists who tell us that biology is not “really” a science!)

Koonin makes an important point using his example laws: we shouldn’t see everything about living things as an adaptation. The “adaptationist” attitude seems to be an old problem in evolutionary biology – he cites Stephen Jay Gould’s complaints about the same issue voiced back in 1979. Evolution includes not just adaptation, but also many historical leftovers and random elements. Sometimes, things just happen because that’s what follows from the physics of the system. Accordingly, Koonin’s example models produce evolution’s observed laws without incorporating any selection.

Well, some of them at least. The gene expression level/evolutionary rate relationship is actually all about selection if you pay attention to the description of the model (or check Drummond and Wilke, 2008, which proposed the model). In this model, the relationship comes about due to the cost of errors in protein synthesis – which can kill cells, so it probably qualifies as a selective pressure…* (That doesn’t make the resulting pattern any less a law, just changes the reason it’s a law.)

Of course, as Koonin reminds the reader, none of this means that selection is not important. It’s just that we should first test whether something is the result of selection before starting to concoct adaptive explanations for it.

All in all, this was a very interesting read, made even better by the fact that PLoS included a reviewer’s comment and Koonin’s response with it. It’s a cool little glimpse under the hood of peer review, and an opportunity to see a different expert viewpoint on the same subject. I think it would be great to see more of that, to be honest.

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*Incidentally, I wonder if that means that the “law” doesn’t apply to genes that don’t encode proteins, like ribosomal RNA.

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References

Koonin EV (2011) Are there laws of genome evolution? PLoS Computational Biology 7:e1002173

Drummond DA, Wilke CO (2008) Mistranslation-induced protein misfolding as a dominant constraint on coding-sequence evolution. Cell 134:341-352
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