COMPUTING SCIENCE
Experimental Lamarckism
Brian Hayes
Costs and Benefits
The rise and fall of melanism in the peppered moth was
unquestionably a Darwinian event, brought about by natural selection
acting on random mutations. Inheritance of acquired characteristics
was impossible simply because there were no acquired characteristics
to inherit. A moth has no way to change its color over the course of
its lifetime, even if it could somehow figure out that making the
change would be advantageous. And if the moth cannot adjust its
color, it obviously cannot transmit any adjustments to its descendants.
But in imagination—or in the computer—we can rerun the
experiment without the constraints of insect physiology. We can
create chameleon moths that sense the color of their environment and
adjust their own color to match. I shall refer to this adjustment
process as learning, although it needn't imply any kind of
cognitive capacity; the term is meant to encompass any adaptation
within the lifetime of an organism.
Would moths that learn have an advantage over those that don't? It
seems like a sure bet—and yet if adjustable camouflage is such
an obvious asset, why don't all prey species have it? A likely
answer is the no-free-lunch theorem. Learning has a cost, which in
some cases may outweigh the benefits. At a minimum there is a
complexity cost: Sensing the state of the environment and responding
to it requires metabolic machinery that a simpler organism could do
without. Building and maintaining that machinery incurs an energy
cost; resources that might have gone into growth and reproduction
have to be diverted into learning. Thus a creature that does a lot
of learning could be expected to have a slower reproductive cycle
than one with more hard-wired traits. (H. sapiens takes 20
or 30 years to accomplish what E. coli can do in 20 or 30
minutes.) Thus adjustable camouflage might reduce mortality, but the
price would be reduced fertility.
The cost-benefit analysis for Lamarckism is similar. In a Darwinian
world, any acquired improvements cannot cross the generation gap. A
smart moth born with white wings might darken gradually to match a
sooty environment, but the moth's offspring would be white again
(barring mutations). The moth's acquired pigment is no more
heritable than a suntan. Lamarckism creates a link between learning
and genetics. A moth that adjusts its color during its lifespan will
give birth to offspring that share at least some of this adjustment.
Is this shortcut advantageous? Again it would seem so. The young
moths are hatched with protective coloration already in place. But,
as with learning, maintaining the Lamarckian mechanism imposes a
metabolic cost, so that lowering the death rate limits the birth
rate. The balance between these two effects determines whether
Lamarckian inheritance pays off. Finding the point of balance is the
aim of the computer simulation.
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