From: keithd@well.sf.ca.us (Keith Doyle)
Subject: Re: Atheistic creationism -- a tutorial
Organization: Whole Earth 'Lectronic Link
Date: Sat, 10 Oct 1992 04:48:45 GMT



rh@smds.com (Richard Harter) writes:
>But that's not all folks, there's more bad news to come.  Let us
>suppose that the AB combination can propagate just as though it were
>a single favorable allele even though, as we have seen, it can't.
>It takes many generations for a favorable trait to fix itself in a
>population.  The number of generations depends on the selection
>coefficient, k.  This number is almost always quite low, e.g. .01
>or less, because fitness depends on many factors.  In turn this means
>that it takes thousands of generations to fix a trait.  When we take
>the fact that it takes on the order of a thousand generations to
>produce a single individual, we see that it takes millions of
>generations to fix the trait.

At best, this is only an argument with regards to recessive traits,
and doesn't appear to take into account that the size of the population
has a great deal to do with the rate of propagation through the
population.

>Now we are talking about incremental improvements, i.e. the accumulation
>of traits.  Even allowing for overlap we end up with tens of millions
>of generations to produce species changes.  For animals with a generation
>of >=10 years, e.g. the hominid line of descent, we end up with
>hundreds of millions of years.  If we assume that the evolution of
>homo sapiens sapiens took 100,000 years from neanderthal forbears,
>we seem to have lost three orders of magnitude of time.

Based on the faulty assumption that all mutations are recessive it
seems.

Ray Ingles responds to you in another message:
> So, what I gather you're arguing is, the more complex the lifeform, the
>exponentially harder it is for a mutation to be beneficial? Something
>like that?

I would say that this may be a partially accurate statement.  It has
been shown that sex can perform both the function of limiting variation,
and encouraging it, depending on the size of the population and the
selection pressures:

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As the microbiologist W.F. Bodmer has succinctly put it, sex
"facilitates the accumulation in a single individual of advantageous
mutations which arose separately in different individuals."

In terms of the individual, sex appears paradoxical because it would seem
an inefficient way to propagate his genes, each child has only 50 percent
of his genes, the other 50 percent supplied by the sexual partner.
Sexual reproduction apparently has more group level advantages than
individual level advantages.  Efficiency from the whole individuals
point of view is then seen to be irrelevant.

Current studies of the origin of sexual reproduction have been focusing
on the numerous variations of reproduction that exist in nature.  While
most organisms reproduce sexually, there are quite a few examples of
organisms which will resort to asexual reproduction if environmental
conditions are unfavorable to sexual reproduction.

When in sexual reproduction two cells fuse, their product has twice
as much DNA, twice as many genes and chromosomes than the original
cells.  Such a cell is called "diploid".  In a reduction step, the
diploid cells divide (such as in the production of a sperm or egg)
and these are referred to as "gametes" or "haploid" cells.

In animals and most plants, the body is formed by the diploid cells,
the gametes are haploid.  Some lower plants like mosses are exceptional,
haploid cells form the main plant on which diploid cells grow in small
appendages.  Mushrooms are prevalently haploid, when a diploid cell
arises by fusion of two haploid cells, it segregates immediately
into two haploid cells.

What is fundamental in sexual reproduction is not a building up of
a population of diploid cells, but the scrambling of the genes.

In some cases, this gene scrambling can compensate for the presence
of altered (mutated) genes.  If in a diploid organism one of the genes
of a pair is altered, the consequences are not necessarily bad because
the other gene of the same pair can supply the needed product.  In an
organism that is particularly sensitive to gene alterations, such as
an organism that has either a more limited total number of genes
or has not developed other mechanisms for insulating from detrimental
alterations, sexual reproduction provides a stabilizing force.

In other cases, such as in wheat for example, the organism can be
regularly propagated by self-fertilization.

The number of altered genes can be affected in a variety of ways.
There are even genes, called "mutators" which manipulate the
rates of copying errors in other genes.

Even bacteria and viruses exhibit forms of sexual reproduction in
various situations where recombination of different individuals
genes form progeny with mixed genes.

Diploid yeast, the product of sexual reproduction, is more vigorous than
haploid yeast.  Also important for yeast and similar creatures is the
ability to switch reversibly from the haploid to the diploid state,
a property that is intimately connected to sexuality.  In the haploid
state yeast can more readily adapt to changes in the environment by
expressing useful changes of the genes immediately after they occur
(because there is only a single copy of each gene).  In the diploid
state, this type of adaptation is not so easy, because a change in a
gene may be masked by the unaltered gene of the same pair.  But the
diploid state has the advantage that an unfavorable gene is masked by
a normal homologous gene.  So yeast can overcome both the problem of
environmental change and that of the accumulation of unfavorable genes
by dwelling in one or the other form.

There is even a species of fish where each male swims around near the
bottom surveying a small harem of females, which live in individual
burrows where they raise their young.  The male fertilizes each
female in turn.  This routine continues undisturbed until the male
dies or disappears.  When that happens, one of the females emerges from
her burrow, becomes a male, and performs the role of fertilizing the
remaining females.

Higher (sic) creatures are usually diploid with pairs of genes that may
be different, this common trait can be attributed to the slow growth of
these creatures which do not need to adapt rapidly to fast changes in
the environment.  The ability of diploidy to cover up unfavorable genes
becomes the most desirable characteristic.

The basic attribute of sexuality, the ability of DNA molecules to
exchange parts, exists in all forms of life.  To some extent, sexual
reproduction promotes the multiplication of creatures more effectively
than does asexual reproduction.

Sexual reproduction, plays a fundamental role in the formation of
new species.  The formation of complex organisms during evolution would
have been impossible, or would have occurred in a very different way
if the primitive short DNA molecules had not been able to fuse by
recombination.  Without fusion the big genomes with large gene
repertoires present in the world today would not exist.  These, of
course, are hypotheses.  But they do account in a logical way for the
observations, on the basis of events that are experimentally observable.

The question is why did DNA relinquish the autonomy of the single genome.
The answer is: because mixing genomes gave DNA a tremendous boost.  But
as this goal was pursued, sex achieved a development perhaps far beyond
the needs of DNA.  Once the process was started it became irresistible;
it proceeded in every possible direction, reaching the limits of what
is feasible.  In biology, once a door is opened, the space behind it
is quickly filled.

Recently, a series of evolutionary simulations on computers have resulted
in the spontaneous generation of sexual reproduction.  (Ref: Tom Ray,
University of Delaware, School of Life & Health Sciences, Newark, Delaware)

References:
Richard Dawkins, "The Selfish Gene", Oxford University Press, 1976
Renato Dulbecco, "The Design of Life", Yale University Press, 1987.
_____________________________________________________________________


More Richard Harter:
>Single point changes are reversible; some have phenotypic consequences
>that selection can operate on.  However morphological changes that are
>not allometric require multiple modifications of development sequences.
>These are the modifications of consequence for phenotypic change with
>morphological components.  These are essentially irreversible.  What I am
>arguing is that it is this class of change that is insufficiently likely
>to account for morphological evolution.

Could you explain the definition of "allometric" here, and how you have
determined that non-allometric features require multiple modifications?

And, how does your theory take into account the other types of mutational
processes, such as transposition, etc.?

Keith

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