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COMPUTING SCIENCE

Experimental Lamarckism

Brian Hayes

A curious young elephant got its nose stretched by a crocodile, with the result that elephants everywhere now carry a trunk. What this story tells us is that Rudyard Kipling was a thoroughgoing Lamarckian—a believer in the inheritance of acquired characteristics. As it happens, Lamarckian ideas were already in disrepute when Kipling wrote his "Just So" stories. The German biologist August Weismann, in a remarkably Kiplingesque experiment, had shown that chopping off a rat's tail did not lead to the birth of tailless ratlets. Experimental protocols have gotten more sophisticated since then, but the verdict is the same: There's no sign of Lamarckian inheritance anywhere in the kingdoms of life.

But why not? A few years ago Colin McGinn wrote (in a review of a book by Daniel Dennett): "Why have Lamarckian organisms never evolved? Surely a mutation which made the genes responsive to changes of phenotype ('learning') would have selectional advantage, and there seems to be no physical impossibility in such a set-up. Wouldn't natural selection favour a physiological mechanism that allowed learned characteristics to be passed genetically to offspring?"

These are good questions. One way of answering them is to note that the molecular pathways needed for Lamarckian inheritance just don't exist. Within the context of life-as-we-know-it, there's no way for the elephant's nose to talk to the elephant's genes—especially the germ-line genes. The "central dogma" of molecular biology says that information flows from DNA to RNA to protein, not the other way around. A Lamarckian feedback loop would seem to require some mechanism by which the proteins of the phenotype could alter the DNA of the genotype.

The trouble with such an answer is that it invites a further annoying question: Why is it that such feedback loops have never evolved? Given all that has evolved in the way of genetic detours and shortcuts—plasmids, transposons, retroviruses, prions—it seems a bit arbitrary to declare this one pathway out of bounds. The case of retroviruses is particularly provocative, since they produce an enzyme (reverse transcriptase) that violates the central dogma, copying information from RNA back into DNA.

Here's another possible reason for the absence of Lamarckian inheritance in nature: Maybe it's just not worth the bother. Many authors seem to take for granted that a genetic means of passing on learned traits would be beneficial if it could exist. They assume Lamarckism would make for a smoother and quicker kind of evolution than Darwin's blindfolded selection of random variations. But what are the true costs and benefits of Lamarckism? Perhaps the reason we see no Lamarckian organisms is not that nature cannot invent the necessary apparatus but rather that the result is maladaptive. Lamarckism could be a trick that nature has tried and discarded.

I have attempted to investigate this issue through some simple computer simulations. Specifically, I've addressed the following question: If you were offered a Lamarckian capability, how much should you be willing to pay for it, when the price is exacted in the form of some compensating detriment to fitness? My experiments in free-market genetics are too crude to yield a definitive answer, but I can report that within the rather narrow bounds of this one model, I've been unable to find any situation where the benefits of Lamarckism would justify paying more than a small price.





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