Re: DNA sequence space

From: Dr. David Campbell <amblema@bama.ua.edu>
Date: Fri Sep 30 2005 - 14:13:34 EDT

> Ah, need to be careful. Again, NS guides nothing, all it does is kill
> off the bad designs.

It guides in two ways:

Mutations are random in the sense that the probability of any
particular mutation occurring is expressed by a probabilistic rather
than a deterministic model (as far as we can measure-this doesn't prove
anything about God's determining of the results). However, the
probability that mutation X versus mutation Y occurs may differ
significantly. Past natural selection would affect what parts of the
genome are more or less prone to mutation. Also, certain conditions
may favor increased or decreased mutation rates in general. E.g., heat
shock proteins help stabilize other proteins and thus hide the effects
of mutations. If the cell encounters significant stress, the heat
shock proteins may take up other damage control functions, allowing the
previously hidden mutations to have an effect.

Natural selection also constrains the portion of the sequence space
that is reachable by mutations. E.g., regions of the sequence space
that automatically kill the organism that reaches them will not be
further explored once they are reached. An analogy would be if I were
to attempt to find the door of this room by walking around randomly
with my eyes closed. The walls and furniture prevent me from trying
certain routes. Thus, my search will not be a full random search of
the space of the room.

> At any given point, variation must be unguided,
> so it must land on a functional nearby point. There is no such thing
> as "selection pressure," though this metaphor is commonly used.

Selection pressure cannot prompt particular mutations to occur, as far
as we know. Rather, it is the pressure that will favor or work against
the effects of a particular mutation if it does occur. E.g., during a
volcanic eruption there is selective pressure favoring tolerance of
high temperatures. This pressure largely has the effect of killing
anything that doesn't escape, but if something happens to be nearby and
heat tolerant, it may survive beter than most of its neighbors.

> NS does not encourage or draw out the variation in any way. In fact,
> every little bit of variation that is supposed to occur over time
> (such as via mutation) must occur on its own. The bat, cheetah and
> human all were created soley by a series of
> unguided variations.

Once a mutation occurs, natural selection can encourage its spread in a
population. In turn, this increased abundance provides more chances
for further mutations.

Unguided is subject to the previously discussed caveats about the
nature of divine action-"without inherent direction detectable by
biology" versus "God is too dumb and/or too powerless to determine the
outcome without using a miracle" (which is implied by the claim that
God could not work through evolution but not by the claim that He
sometimes used miracles in the creation of living things).

-- 
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA
Received on Fri Sep 30 14:14:58 2005

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