Macro- and microevolution [was: Re: Icons of Evolution]

Date: Mon Jun 18 2001 - 11:11:11 EDT

  • Next message: "Re: Macro- and microevolution [was: Re: Icons of Evolution]"

    Hi George, David, Todd, Allan, Paul, Tim

    In my discussion with George Murphy, I (again) brought up the topic of
    macro- vs. microevolution (12 Jun 2001 21:38:34 +0200). There were
    several comments, some questioning my doubts about microevolution being
    sufficient to account for macroevolution. Let me try and answer the
    following posts together:

    George Murphy <gmurphy> (14 Jun 2001 07:28:07 -0400)
    David Campbell <bivalve> (14 Jun 2001 14:39:35, 15 Jun 2001 15:17:56
    Todd Greene <tgreene> (14 Jun 2001 23:39:22 -0400)
    Allan Harvey <SteamDoc> (14 Jun 2001 23:42:07 EDT)
    Paul Nelson <pnelson2> (15 Jun 2001 09:32:09 -0500)
    Tim Ikeda <tikeda> (15 Jun 2001 15:14:25 -0400)

    In case some of the points raised will not be adequately dealt with,
    please let me know, and I shall try to say something to these in

    My definition of macroevolution, together with an argument for the
    inability of microevolution to account for it, may be found in my
    article, "How has life and its diversity been produced?" PSCF 44
    (2/1992), 80-94. For your convenience I quote:

    "... For a realistic evaluation of the adequacy of proposed mechanisms,
    a clear distinction has to be made between _microevolution_ and
    _macroevolution_. I define a macroevolutionary step or development as
    any transition producing a fundamentally novel structure and function,
    based upon a sequence of deoxyribonucleic acid (DNA) which is not
    derivable from a previous one by means of a series of individually
    selected mutational steps, but only through a random-walk process
    involving a series of _nonselected_ intermediates. This definition may
    not be conventional, but it points out a crucial distinction. The
    assumption that any macroevolutionary event consists of a series of
    microevolutionary ones is usually treated as axiomatic. If it holds,
    any distinction between the two modes of evolution is basically
    irrelevant. An argument that it does _not_ hold [from P. Rüst, "The
    unbelievable belief that almost any DNA sequence will specify life",
    Conference on "Sources of Information Content in DNA", Tacoma, WA
    (1988)] will be summarized below... [p.82]

    "Apart from point mutations, there are other mechanisms producing
    variants, but they usually do not create any new _functional
    information_. A definition of functional (constructive, or semantic)
    biological information will be given below. Deletions and most
    insertions destroy such information, sequence shufflings by genetic
    recombination, transposition, duplication and other mechanisms move
    preexisting information. These other genome modifications may, of
    course, have profound functional consequences, often on a regulatory
    level, but possible constructive effects they might have on their target
    genes or larger contexts are likely to occur very much less frequently
    than constructive effects of point mutations.

    "One has to distinguish between new features produced by shuffling or
    recombining preexisting functionalities, on the one hand, and new
    functional features which _never existed_ before, but arose in sequences
    having no function, or a different one, on the other hand. Although it
    might in some cases be difficult to distinguish between these two kinds
    of novelty, it is clear that many fundamentally new features must have
    been produced in the biosphere as a whole. Unfortunately, the term
    "evolutionary novelty" is sometimes indiscriminately applied to both of
    these possibilities. The first kind is certainly relevant for the
    origin of biological information. A recent investigation led to the
    (still disputed) estimate of 1000 to 7000 basically different protein
    exon or domain subunit families... [p.83]

    "Natural selection of a new function presupposes a _minimal
    functionality_: where nothing is selectable, nothing can be selected.
    This minimal functionality, therefore, must be produced by random
    processes. The probability of its spontaneous emergence depends on its
    semantic information content, or the size of the minimal specification
    required to define it, but not on the possible pathways leading to it.
    It is, however, difficult to estimate the size of such minimal

    "One approach might be to consider the invariant configuration of a set
    of known sequences performing a given function in different organisms.
    Certain sequence positions are observed to be occupied by the same amino
    acids in all known versions of a protein of a given specificity. It is
    then assumed that functionality requires these specific occupations. An
    anologous argument applies to positions permitting a certain restricted
    variability. For good measure, all amino acids chemically similar to
    the ones actually observed at a given position might be added to the set
    of permissible ones [H.P. Yockey]. The totality of these restricted
    occupations found for a given protein type constitutes its invariant
    configuration. This is a lower-bound estimate for minimal
    functionality, since positional interdependencies and species-specific
    requirements are ignored. It may be compared with an upper-bound
    estimate of the longest feasible non-selected path.

    "The result is that reaching a given invariant by a mutational random
    walk within 300 million years is already too improbable for three
    specific amino acids. This estimate, presupposing 3.05 codons per amino
    acid, 2.16 mutations per specific amino acid change (geometric average),
    and a mutation rate of 10^-8 per nucleotide replicated, is based on very
    optimistic assumptions: 10^16 moles C per year metabolized in the
    earth's biosphere (today's total biomass production) consisting entirely
    of bacteria (5x10^6 nucleotide pairs and 10^-14 moles C per bacterium),
    and all of this DNA continuously participating in this particular random
    walk. Yet _known invariants_ comprise not 3, but about _30 amino acids_
    for basic enzyme functions, such as cytochrome c or ribonuclease, or at
    least 5 amino acids for additional adaptations differing between groups
    of organisms. These requirements are even below the real lower bounds
    for functionality, as they reflect unique occupations only. At present,
    it is unknown whether any smaller invariants might provide some minimal
    functions. The restrictions on functional structures, such as enzymes,
    are such that all mutations we observe today are detrimental or at best
    neutral. To suppose otherwise for earlier organisms is speculative..."
    [pp.84-85] (End of quote. For 3 specific amino acids, 40 billion years
    would be required, on the average!)

    All this, of course, deals with one of the lowest levels of biological
    structures, that of specific functional protein domains. The reason I
    chose this level is that it is understood in sufficient molecular detail
    to permit the estimation of rough probabilities for the occurrence of
    particular evolutionary paths (as long as there is no selection). All
    hierarchically higher structural levels, like complex proteins, protein
    complexes, organelles, cells, tissues, organs, physiology, limbs,
    organisms, are as yet beyond such possibilities. I realize that the
    different uses of the term "macroevolution" found in the literature
    usually deal with the higher levels. I have not found any suitable term
    specifically denoting the distinction I wanted to make, so I adapted the
    term "macroevolution" for this (is this a coaptation? ;-)). Does anyone
    have a better suggestion?

    As the two main factors contributing to evolution are the generation of
    variation followed by natural selection, the initial event in each
    evolutionary step is a change in the genome, i.e. an event on the level
    of molecular biology (apart from the random physical or biochemical
    event causing it). All other changes, on higher levels, resulting from
    this initial event, as well as selection and fixation of a change, are
    much more complex and more difficult to analyze, as far as mechanism
    (rather than mere phenomenology) is concerned. An attempt at evaluating
    the probability and plausibility of an evolutionary step happening must
    therefore begin on the level of molecular biology. Evaluations of
    similarities on the morphological or molecular level start at the
    opposite end, that of observed phenotype differences between different
    species. Comparing the relevant plausibilities of different hypothetical
    paths (or evolutionary models) is always based on a series of
    assumptions, including those pertaining to the unknown selection
    coefficients for the intermediates. The basic mechanism cannot be traced
    in this way, and I have never found any data allowing probability
    estimates for specific mutational paths on these higher levels.

    I think the assumptions made in my model probability calculation are
    sufficiently over-optimistic for evolution to allow the conclusion that
    we are in trouble if we hope for the accidental emergence of all minimal
    functionalities required for life. What would be needed now are
    experiments demonstrating ways in which the initial random-path
    evolution of a molecular function can indeed occur. This problem has to
    be solved for peptide evolution, even if there was an intermediate RNA
    world displaying more easily reached minimal functionalities, because
    the take-over of a ribozyme function by a protein can only occur once
    the new protein sequence has the required minimal functionality, which
    must be generated by a random-walk mutational process. And this problem
    is the same for many of the perhaps thousands of fundamentally different
    protein domain "folds" or super-families (for the origin of life, the
    combination of all minimally required functions has to be taken into
    consideration). I think it is clear that my definition of macroevolution
    doesn't produce a specification for classifying any given evolutionary
    transition on the morphological level, as there probably is no case in
    which we know what exactly happened on the DNA sequence level.

    Now who should have the burden of adducing more evidence relevant for
    the issue of whether the microevolutionary mechanism is or is not
    adequate to explain macroevolution - the "unificationists" or the

    To again set the record straight: I am _not_ postulating that different
    species (or higher taxa) must have been created ex nihilo (I believe in
    common descent, including humans) or that divine interventions should
    replace macroevolutionary steps. For _theological_ reasons I _do_
    believe God used evolution and other natural processes throughout, with
    the probable exceptions of the origins of life, of the psychological
    domain (not the bodies of these animals, Genesis 1:21) and of the
    spiritual domain (not the psychosomatic dimension of these humans,
    Genesis 1:27), cf. A. Held & P. Rüst, "Genesis Reconsidered", PSCF 51
    (4/1999), 231-243. But an adequate mechanism for macroevolution on the
    molecular level just has not yet been defined, let alone made plausible
    experimentally. So, I think it would be unfair to charge me with
    defining microevolution as "evolution I believe in" and macroevolution
    as "evolution I do not believe in". For the time being, I propose "God's
    hidden options" as a worldview (not scientific) solution to the riddle
    of macroevolution: P. Rüst, "Creative Providence in Biology", PSCF,
    accepted for publication.

    Peter Ruest

    Dr Peter Ruest			Biochemistry
    Wagerten			Creation and evolution
    CH-3148 Lanzenhaeusern		Tel.:	++41 31 731 1055
    Switzerland			E-mail:	<
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