Re: Evolution Statement

Date: Sat Dec 08 2001 - 15:49:54 EST

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    Bob wrote...
    >I accept the (DWM) as a partial definition of evolution, but with
    >a minor proviso that this definition holds best within recent species
    >and is more problematic the farther one goes back in the history of
    >organic life. The statement fails to mention that, and would be
    >improved if it did.

    Hmm... Let's look at another set of historical events...
    I accept that meteoric bombardment is at least a partial description of
    how the moon came to be so pock-marked, with the minor proviso that
    this applies best to recently acquired craters and becomes more
    problematic the farther one goes back in the history of the lunar
    surface and the record becomes obscured by time and more recent

    Thus we should not rule out the possibility that many of the older,
    more disrupted lunar craters could have formed supernaturally and are
    not the products of natural causes. (Or, pace Moorad: We never
    saw the impact which produced Bessel crater, so why couldn't it have been
    made by elves? Science can't apply to any crater formed before 10000 BC,
    because there weren't modern humans or instruments to record it. Let it
    forever be an enigma!)

    ..end speculations here...

    >The statement fails to address the critical question: What is the mechanism
    >that brought about DWM? That brings us to the heart of the matter, because
    >the answer must be, natural selection. Nicht wahr? (True, genetic drift and
    >neutral mutations possibly direct genetic exchange in very early organisms
    >are sometimes invoked as change agents, but they are relatively minor causal
    >factors.) If natural selection is not the major change agent in the overall
    >course of DWM, please correct me and tell me what is.

    Genetic variation is the major change agent. What locks a particular change
    into a population or affects its frequency in a population includes:
    natural selection, neutral drift, and other factors. It is actually a hot
    debate about which of the two, selection or drift, are the most influential
    agents of change. Certainly neutral mutations accumulate in a genome faster
    than selective ones.

    >But to do so would introduce a problem that the statement probably
    >wanted to avoid, namely, that the only direct evidence we have that
    >natural selection is the causal factor in DMW is bacterial resistance
    >to antibiotics, and what Gould called "short-term evolution" (STE)
    >studies, such as the finch's beak, peppered moths, spotted guppies,
    >and so forth. These studies are the only basis for the claim that
    >natural selection is the causal factor for all changes we see in the
    >fossil record and the tree of life. The operation of natural selection
    >throughout the entire tree of life is an extrapolation from such studies.
    >Or to say it differently, evolutionary biologists hold that socalled
    >macroevolution, or large innovative changes in DWM is microevolution
    >WRIT LARGE. This claim, however, is not a fact. Thus while DWM itself
    >is factual, its mechanism is not.

    Descent with modification can arise from multiple mechanisms, some are
    selective, others are not. I do agree that in most cases, it is a practical
    impossibility to determine what specific factors played a role at specific

    >To continue, such an extrapolation is unwarranted because of the simple
    >fact that changes observed in STE studies and bacterial resistance are
    >_reversible_. While finch's beaks become more robust in times of drought
    >when seeds have tougher shells, they revert to the more slender shape when
    >climate returns to normal and seed shells less hard to crack. Such
    >reversibility disqualifies STE from serving as the mechanism of DWM, or
    >at least raises serious questions about it. The statement would be more
    >forthright and therefore improved if it acknowledged this.

    Reversibility is irrelevant and in fact, many of the changes are not
    reversible at the genetic level. Remember that there is a gaping chasm
    between genotype and phenotype. The mapping between the two domains is
    anything but linear in all but the rarest situations. Point mutations
    can confer antibiotic resistance. However, given the odds of hitting that
    exact site again to exactly reverse the point mutation, it's more likely
    that a mutation that eliminates resistance will occur in another
    portion of the genome. Thus a phenotypic reversal (short-term or otherwise)
    does not equate to a genomic "reversal". And we have a recently described
    case where a bacterium acquired streptomycin resistance in a "long-term"
    manner such that loss of the resistance, even in the absence of the
    antibiotic, was selected against (A secondary mutation occurred which
    effectively "locked in" the first one). So, we can say that ratchet-like
    evolutionary progression has been observed even over short timescales.

    I suspect that true reversibility is a relatively uncommon phenomenon.
    Should we be surprised? I think not. In the mind-bogglingly huge
    morphology- or genetic-space available to organisms, it's not
    likely that one could take more than a couple steps in any direction
    and then return to exactly the same starting place.

    >Finally, DWM has produced the enormous diversity of life forms in the
    >tree of life. This is acknowledged briefly in the statement. Natural
    >selection is ideally suited to produce diversity.

    Actually, neutral drift also contributes to the production of diversity.

    >But what is more difficult to explain is _disparity_, or the highly
    >organized nature of the organic world, with its deep discontinuities
    >between the major families of organism, roughly designated as phyla,
    >using natural selection as the operating change agent. Dobzhansky stated,
    >"fundamental characteristics of organic diversity-- [are] its
    >discontinuity and hierarchical organization." If such a giant as
    >Dobzhansky made such a statement, shouldn't the statement under
    >consideration at least acknowledge that existence of the hierarchical
    >organization and discontinuities in nature, instead of silently passing
    >by on the other side of the road?

    Nobody is failing to acknowledge the hierarchical pattern of life and
    discontinuities. These are what led to the conclusion of descent with
    modification in the first place.

    Discontinuity is a product of speciation (a mechanism of evolution) and
    extinction. Such discontinuities tend to manifest more in the
    morphological arena than the biochemical one. Also, hierachical
    organization is a manifestation of the rates of change and the nature
    of speciation and organismal genetics. Understanding the timing and details
    of specific steps behind evolution is real challenge, IMHO.

    Tim Ikeda

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