Re: [asa] Exaptation

From: David Campbell <pleuronaia@gmail.com>
Date: Tue Jun 23 2009 - 13:14:47 EDT

Whether a large change is likely to be helpful or harmful depends very
strongly on a) what the change is in and b) what the environment is

Obviously, messing with some component that is essential to daily
survival is unlikely to be beneficial and very likely to be harmful.
However, change in something that is far from the "goal" has a fairly
good chance of getting closer to the goal.

In the fossil record, there are examples both of stasis-type patterns
and gradual change. (Note, however, that the original examples of
stasis were cases in which the change could be seen in fossils-there
were long intervals of no change punctuated by brief _observed_
intervals of rapid change.) What would favor one or the other? The
old handyman principle of "if it ain't broke, don't fix it" applies
well. Under ordinary conditions, there are a lot of competitors and
the environment's putting some degree of pressure on the organism.
Maintaining an already successful approach is likely to be more
successful than searching for other alternatives, unless some new
development appears that makes the current approach unsuccessful. In
other words, there is selection for not changing. Things not changing
over a long interval of time is no problem for evolution-it just means
they found something that worked and stuck with it. However, if
things are either really easy or really hard, then there's more
opportunity for change. If conditions are very easy (e.g., opening of
new opportunities because of a particular evolutionary innovation, the
extinction of competitors, or an environmental change), the set of
"fit enough" phenotypes will be much larger than when the pressure is
on, and various options may get explored evolutionarily. If
conditions are very hard, then your competitors are having a hard
time, too, and any sort of change might help. A good example of this
is the appearance of high mutation rate mutants in bacteria that were
mutated so they couldn't use lactose and then given lactose. Normally
a high mutation rate is bad, but if what you've got doesn't work, it
can be good. Another situation that may promote change is if there is
some sort of specialized pressure selecting against average or
uniformity. For example, the variety of shell patterns in the land
snail Cepaea nemoralis is thought to reflect pressure from birds and
other visual predators. If most of the snails look alike, birds can
learn to target that pattern. As a result, that pattern declines and
the initially rarer patterns are more successful, until the birds
learn them....

Another factor overlooked in exaptation or adaptation is the fact that
the initial steps are probably rather preliminary. We think of the
origin of feathers as a big deal because they are essential to bird
flight, but they probably started out not notably all that different
from mammal or pterosaur fur and only later developed into something
that would make a good wing. Only in hindsight does the step seem so
big.

There are some mechanisms where genetic changes could be covered for a
while and then appear in cumulative effect. Recessive alleles and
gene duplications provide ways where a genetic difference can exist
without affecting the phenotype (or without affecting it as much)
until something happens to bring them out into the open (e.g.,
inheriting two copies of the mutant; gene conversion, recombination,
or other event causing the loss of one version of the gene). Another
comes from heat-shock proteins and related forms. Many of these have
multiple functions. Under ordinary conditions, they help proteins
fold properly, and can compensate for some mutations. However, they
also play a role in cellular defense against stress (such as high
temperature). If they are called up in a stress response, they leave
behind their posts of ordinary protein-folding assistance, and
mutations that were compensated for can emerge into prominence.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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Received on Tue Jun 23 13:15:50 2009

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