08 December 2009

Weasels, clouds and biomorphs, part II

Back in September I wrote about the silly preoccupation on the part of various anti-evolutionists with the so-called Weasel program, a simple exercise created more than 20 years ago by Richard Dawkins to illustrate the efficacy of cumulative selection in evolutionary scenarios. My main point was that the Weasel program had one very simple purpose (comparing "single-step selection" – which is purely random – to cumulative selection) and constitutes a trivial fraction of the argument in Dawkins' The Blind Watchmaker.

One might think that Dawkins' basic message – that random one-step flying-together of a Shakespearean phrase or a hemoglobin molecule is impossibly unlikely compared to cumulative selection of intermediate stages – is so elementary that no intelligent person would need to consider it more than once. (Once seems like a lot to me in this case, but never mind.) And yet the error (if that's what it is) is shockingly common. (It forms one pillar of poor Cornelius Hunter's whole enterprise, for example.)

Dawkins understood this problem when he wrote The Blind Watchmaker, so before he unveiled the fascinating program that forms the heart of his case for the power of selection, he took one last stab at making the basic outline clear by going back to clouds. Wait...clouds? (Fans of Hamlet are already sighing blissfully; those who don't get the connection between weasels and clouds should read either Act III, Scene II of Hamlet or Chapter 3 of The Blind Watchmaker.) Yep. Dawkins pointed back at what makes cumulative selection work: the things that are evolving must be able to generate related offspring. And that's what clouds can't do.

Clouds are not capable of entering into cumulative selection. There is no mechanism whereby clouds of particular shapes can spawn daughter clouds resembling themselves. If there were such a mechanism, if a cloud resembling a weasel or a camel could give rise to a lineage of clouds of roughly the same shape, cumulative selection would have the opportunity to get going. Of course, clouds do break up and form 'daughter' clouds sometimes, but this isn't enough for cumulative selection. It is also necessary that the 'progeny' of any given cloud should resemble its 'parent' more than it resemble any old 'parent' in the 'population'... It is further necessary that the chances of a given cloud's surviving and spawning copies should depend on its shape.
The Blind Watchmaker, pages 50-51, italics in the original
Hence the Weasel program.

But I noted last time that Dawkins spent a tiny amount of time and text on the Weasel program, and that he declared it to be "misleading in important ways." The most important, by far, is this: the selection that drove the Weasel program was goal-directed. A better simulation of evolution would be one in which selection is more capricious, more "in the moment." (Survival can be capricious; reproduction happens rather decisively "in the moment.")

Dawkins came up with just such a program, and I mentioned it in the previous post. It's a wonderfully simple simulation of the basic aspects of selection-driven evolution: it includes development, reproduction, genes, and selection, and generates "organisms" with shapes instead of a phrase in all caps. We'll look at that program in the next and final post. But if you want to play with a modern version, you'll find plenty of nice implementations out there. So much more fun than studying...or grading.

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