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.
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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