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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