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COMPUTING SCIENCE

Experimental Lamarckism

Brian Hayes

The Backdrop to Evolution

When I first started up the model, I got an immediate reminder of a fundamental principle of evolutionary biology: No organism evolves in isolation. Evolution only makes sense as an interaction between the organism and its environment. I should not have needed a reminder—after all, the driving force in the peppered moth story was environmental change—but in fact I had given too little attention to the backdrop against which the moths play out their drama.

What matters most about the background is not its specific color but the rate at which the color changes. In a static environment, learning is useless; there's nothing to learn. Darwinian mutation and selection can match an unchanging background just as closely as learning can, and so organisms unburdened by the overhead of learning will be favored. Running the model with an unchanging environment illustrates this effect clearly. Starting with random values of the harvard gene, the distribution shifts within a few dozen generations to favor the lowest values—those that produce the least learning but also incur the least penalty. In a mixed population of nonlearning Darwinian moths and learners, the outcome is even more dramatic. The Darwinians take over the population and drive the learners to extinction. And if learning is disadvantageous in these circumstances, then Lamarckism must also be unfavorable, since organisms that don't learn acquire nothing to bequeath their offspring.

The rate of environmental change does not have to be exactly zero to favor Darwinians. The rate merely has to be low enough to ensure that change is insignificant within the lifetime of an individual. Even a world with large and abrupt environmental transitions can penalize learners if the upheavals are separated by long interludes of stasis. Learners are better equipped to deal with the upsets, but they are wiped out by the faster-breeding Darwinians during the periods of calm.

Watching the simulations in action gives a new perspective on the relation between learning and life cycle. The idea that learning takes so much time and energy that it delays reproductive maturity is only half the story. The other half is that only a long-lived organism has any use for learning. Bacteria can rely on Darwinian evolution to fine-tune their metabolism to seasonal changes in temperature; as individuals they don't need to learn about hot and cold. Large mammals, on the other hand, would get no benefit from winter genes and summer genes, because they must cope with both seasons.




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