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COMPUTING SCIENCE

Experimental Lamarckism

Brian Hayes

The More Things Change...

If slow change favors Darwinian selection, a rapidly fluctuating environment is where learning proves its worth. What is a little less obvious is that evolution becomes irrelevant here.

Figure 3. Gene for LamarckismClick to Enlarge Image

Even in the absence of learning, natural selection is helpless when change is faster than a generation time. If bark color can go from black to white in a week, and moths live several months, then the genotype can't keep up. Light-colored specimens might be favored one week, and produce more offspring than dark-colored moths, but the genes for paleness would be maladaptive by the time these descendants reproduced.

Turning on the harvard gene does nothing to restore the efficacy of natural selection; on the contrary, learning further decouples the genotype from the phenotype. When learning is rewarded, it becomes so efficient that there is little selection pressure on the genotype. Learning provides any newborn moth with excellent camouflage in a day or two, and so survival is essentially independent of the color gene.

What happens if Lamarckian inheritance is turned on in this rapid-change regime? Not much. If Lamarckism is assessed the same penalty as learning, the vanderbilt gene is disadvantageous, and the distribution of values slides toward the low end of the scale. This result is not hard to fathom. In the model the sole benefit of Lamarckian inheritance is being born pre-adapted to the color of the environment. But if that environment is changing rapidly, the benefit won't last long. Furthermore, in a population dominated by fast learners, most of the newborn moths would come to match their background in a few days anyway, even without the Lamarckian head start.

To put it another way: Learning is a valuable survival skill every day of your life, whereas Lamarckism helps only on the first day. This formulation suggests a way to quantify the worth of the vanderbilt gene. If your expected lifespan is L, then you should be willing to pay about 1/L as much for a Lamarckian legacy as you would pay for learning. In the model, learners live more than 100 days, so Lamarckism should be worth less than 1 percent of the price of learning. If the penalties are adjusted accordingly—making the vanderbilt inheritance tax less than 1 percent of the harvard tuition—Lamarckism ought to spread through the population. In my experiments I could not see this effect clearly. Even at a cost of zero I couldn't be sure whether the vanderbilt gene was growing in frequency or merely drifting neutrally, but it may be that I wasn't patient enough to wait for the trend to become apparent.








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