DNA Information and evolution...

From: Adam Crowl (qraal@hotmail.com)
Date: Mon Jan 17 2000 - 02:38:46 EST

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    Hi ASA

    Information in DNA and its relationship to an organism is an interesting
    issue with some deep implications for our understanding of evolution. Here's
    my rough outline of why we may be so confused about it...

    Massie wrote

    >Really, can you count the bytes of information in
    >the eye
    >for example and then estimate the number of bytes of information that
    >could
    >be generated per generation and then the number of generations.
    >Something
    >to hang this on, just something.

    To go from bytes in the eye, or in the brain [another example of organismic
    complexity] there is no easy relationship between DNA and cellular
    organisation. The human eye contains some 100 million receptors and a layer
    of preprocessing edge-detecting systems which send organised information to
    the visual cortex.

    The human genome only contains some 300 - 90 million base-pairs that
    actually code for proteins - the rest is tied up in repeating segments,
    non-functional pseudo-genes and marker. timers and non-protein coding stuff
    that is useful. Yet the human body has over a thousand trillion cells with
    256 types of cells. How does such a small amount of infomation translate
    into the incredible complexity of eyes, brains, nerves, muscles and so
    forth? The underspecification is a factor of more than a million, perhaps
    even a billion or more considering the amount of specification a single
    cells type and place would require. What's true for humans is true for
    almost all animals - underspecified by BIG factors... I think round worms
    [nematodes] are the most complex FULLY specified animals.

    The miracle of the How is, I suspect, analogous to how a few lines of code
    can produce the immense complexity of fractal images... ALGORITHMIC
    COMPRESSION. And the cells are self-replicating "nano-computers" that bring
    it about by running the "body program" that they all share. The input is the
    time and generation number of the cells and the extra-cellular environment,
    filled with hormones, signalling molecules and other cells. I suspect that
    electromagnetic fields are also involved but that's unproven.

    So imagine trillions of little computing machines putting a living thing
    together out of their collective actions. Kind of like the pixels in an
    image of a complex fractal - simple global and local rules governing what
    state it will be in. Or the complexity that can be created by cellular
    automata [an aptly named analogy] using the same rules for every cell and
    yet mimicking living things. However no CA has ever unfolded from "simple"
    beginnings like cells proper. But the idea of complexity out of simplicity
    is there.

    That's why there is no easy route from genetic mutation to a full blown
    organism. One base change can produce dwarfism in humans - a simple change
    affecting trillions of cells. Yet a full chromosome duplicated in some
    people produces Downs Syndrome which is not the horrific disaster that so
    much random change might suggest, since it is often quite mild in outcome.
    And dwarfism is not a horrific mutation either, so don't get me wrong - many
    see it as a normal variation.

    Mutations can affect single proteins or whole developmental sequences and so
    their effects vary widely. Most are just plain neutral, many are harmful and
    a few just make people subtly different. That's where the variations we call
    "alleles" come from. A very few produce a difference that might mean life
    or death and so natural selection comes into play... subtle or unsubtle.
    Subtle selection takes a few generations to be significant, unsubtle usually
    kills in childhood or child-bearing years.

    Since currently we know so little about development - though more all the
    time - there's a lot that can't be answered about how organisms went from
    one form to another. But the fact remains that seemingly related organisms
    exist and others that neatly bridge the differences between groups keep
    getting dug up as fossils... so something is definitely going on that is
    suspiciously like Darwin's "descent with modification".

    Adam

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