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COMPUTING SCIENCE

Experimental Lamarckism

Brian Hayes

Looking for Lamarck in All the Wrong Places

If Lamarckism has no value in a static environment and only the slimmest of marginal benefits in a fast-changing environment, it's natural to wonder if there might be some intermediate condition where the utility of Lamarckian inheritance is maximized. This would be a condition where change is quick enough to make learning worthwhile, but not so rapid as to make genetics irrelevant. I have surveyed a broad range of rates of environmental variation looking for this point of optimality, without reaching any firm conclusion. The level of statistical fluctuations in the output of the model suggests that much longer runs and larger populations would be needed to settle the question. I do feel confident in saying there is no level of variation where Lamarckian inheritance is worth as much as learning is, or even half as much. But there may be a range of variation rates where a species could benefit from a Lamarckian mechanism if it cost no more than a tenth of what learning costs.

Even where the model's answers are clear, they are at best preliminary and provisional. The model is too simple to capture much detail about the lives of real organisms. On the other hand, it's not simple enough to explore the entire space of parameter values. Another reason for caution is that the model sometimes behaves in ways I don't understand. For example, in certain mixed-population experiments the Lamarckian gene is driven toward the lowest possible values, indicating it is unfavorable and "wants" to be zero, yet at the same time the Lamarckian breed drives its competitors to extinction. What does that mean?

It would be interesting to test the model on problems other than moth camouflage, especially problems where the environment is not a passive background but can react and evolve on its own. One realm where Lamarckian mechanisms look particularly attractive is the immune system. Every child must reinvent immunity to measles and chickenpox and other diseases. It's done through a miniature Darwinian process of generating many random antibodies and selecting those that recognize a pathogen. Wouldn't it be better to pass on the selected antibody genes to later generations, so that babies would be born pre-immunized? Edward J. Steele of the University of Wollongong in Australia argues that something like this does go on in the immune system, through the agency of reverse transcriptase. But Steele has won few converts. It's hard to be a believer in genetically transmitted immunity when your parents had chickenpox and you had chickenpox and your children get chickenpox.

If Steele's mechanism could exist, would it prevent disease? The obvious drawback is resistance: A generation born with a high titer of antibody would exert powerful selective pressure on the pathogen, so that mutants with slightly different surface markers would proliferate. Thus every generation would have to come up with a new defense anyway, and Lamarckian inheritance would be rendered superfluous. But this is speculation; a model might well reveal subtler effects.

In closing, I want to say a word about cultural evolution, which is often described as a Lamarckian process. Suppose it were truly Lamarckian: Suppose some neurogenetic innovation allowed your children to be born already knowing everything you know. What a boon to humanity! What a head start! No one would ever again have to spend all those years learning the alphabet and the multiplication tables and the conjugation of Latin verbs and the law of cosines and the preamble to Evangeline and the date of the Battle of Hastings and how to ride a bicycle. But the more items I add to this list—let's not forget the state capitals or the nine orders of angels or the 20 amino acids or the recipe for mom's meatloaf—the more I'm struck by the fundamental problem of Lamarckism. Which acquired traits do you choose to pass on?

© Brian Hayes





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