Text 87, 101 rader
Skriven 2004-09-17 16:37:00 av Guy Hoelzer (1:278/230)
Ärende: Re: Replicating What?
=============================


in article cidugl$tul$1@darwin.ediacara.org, Anthony Cerrato at
tcerrato@optonline.net wrote on 9/16/04 10:59 PM:

> "Guy Hoelzer" <hoelzer@unr.edu> wrote in message
> news:ciaksj$311n$1@darwin.ediacara.org...
>> in article ci9ngq$2o58$1@darwin.ediacara.org, Anthony
>> Cerrato at tcerrato@optonline.net wrote on 9/15/04 8:35 AM:
>> 
>>> Yes, to both you and Tom. Another way of talking 'bout it though, other
than
>>> in terms of raw materials, is what's replicated is the blueprint for the
>>> structure which will be the organism (house), including (stapled to) the
>>> work flow-sheets (work-plans) for building it. Note, this automatically
>>> includes any accidents (mutations, external physical damage etc.) which
>>> occur to the blueprint, or the work-staff, prior to, and during, the
>>> building process. Thus, workmen spilling their morning coffee on the plans
>>> is already pre-accounted for (and even expected at times.)
>> 
>> This is a useful metaphor in many contexts, but the genome is clearly not a
>> blueprint because it does not contain structural plans or any where near the
>> information needed to build an organism.  You might say that the combination
>> of the genome with existing cellular machinery encodes specific RNA
molecules
>> and regulatory mechanisms of gene expression, but we will never be able to
>> read an unknown genome in isolation (i.e., without using comparative
genomics
>> from organisms with known phenotypes) and predict what the adult organism
>> will look like because too much of the information contained in the
>> organismal structure is not there in the genome.
> 
> OK, you're saying you can't use the blueprint to build a new
> house without using an old house as the office. :)

No.  IMHO the vast majority of the information (structure) in the adult
phenotype is generated through the process of self-organization during
development.  It is not strictly in the genome or any other sort of
pre-existing template.

> Hmm...the infinite recursion here bothers me a little though...chicken and
the
> egg? It begs for an answer to the question, how was the first animal in the
> genetic line built...? Perhaps using the blueprint for a smaller "starter
> house" or flimsy summer bungalow or hut? But, still ad infinitum...  :)

In my opinion there is nothing like a "blueprint" used at all in organismal
development, so the question of where the first blueprint came from is a bad
question.  There is a nice, simple argument for this view in the book by
Camazine et al. (2001) called "Self-Organization in Biological Systems."
Consider, for example, that your body generates over 100 million different
antibodies at any one moment, given a genome with about 30,000 genes.  Of
course the genome would have to encode everything else about the organism,
too, if it were really a blueprint.  My gut feeling is that your genome
doesn't contain enough information to encode your eyes, let alone the rest
of you.  It merely puts into place lots of different kinds of components
that participate in the dynamics of a growing complex system, and the
information is generated through the interactions of those components.  I
also expect that a substantial fraction of the necessary components (perhaps
the vast majority?) come from sources other than the genome.  As I wrote
before, the "blueprint" metaphor has been useful, but I think we know way
more than enough to leave it behind us at this point.

If you find my view to be implausible, then I would ask you why natural
selection would result in the unnecessary encoding of information in the
genome, which would be relatively inflexible in the face of unpredictable
environmental variation, when it would be far more efficient to take
advantage of the opportunities for automatic information generation through
physical tendencies for self-organization?  I expect that fighting those
tendencies through rigid and expansive genomic encoding would be highly
maladaptive.
 
> I might modify the original analogy to add that this is
> further analogous to the need, with blueprints, for the
> workers to have the reading and measurement skills, and
> proper raw materials, prior to executing the building
> process. In any event, you must admit, no analogy whatsoever
> is perfect--of necessity. Must go--sudden desire for fried
> eggs and chicken! to heck with the specific order!

I hope your meal was tasty!

I agree that metaphors of the "blueprint" variety always fall apart as we
learn more.  The hope is that we move on to metaphors that are closer to
Truth, even though we can't know exactly where Truth lies.  I hope the view
I articulated above seems less metaphorical and more mechanistic, although
you (and many others, I'm sure) might remain skeptical of the idea of a
"self-organizing system," which represents a mechanistic approach to
understanding the balance between bottom-up and top-down (e.g., entrainment)
forces in determining structural dynamics.

Regards,

Guy
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