Reflectorites
On Thu, 21 Sep 2000 11:07:04 EDT, Bertvan@aol.com wrote:
[...]
BV>The only answerable question I found in your post is "What is a Darwinist".
>I define a Darwinist as those believing evolution can be explained by
>"chance variation and natural selection". Many of them want to add, "plus
>drift". I've never figured out what "drift" was other than more "chance
>variation", so it's ok with me.
Genetic drift is not so much "chance variation" but a way of bypassing
RM&NS altogether. It is the fixing of genes in a small population by purely
stochastic (~statistical) factors. For example, a favourable gene in a small
population might be eliminated by a chance event like the animal bearing it
being killed by lightning:
"Genetic drift is any change in gene frequency that results from
chance and not from selection pressures. It is a statistical
phenomenon that depends on the size of the gene pool and is only
important in small, isolated populations ... You can simulate this
phenomenon by flipping coins. After many trials, one expects heads
to have turned up as often as tails. .... after four trials one would
not be surprised to have flipped all heads ... In a population, let
allele A represent heads and allele a represent tails. If many trials
correspond to many individuals (a large gene pool), we can see at
once that the variance in such a large population is negligible and
that the allele frequencies of A or a will not be changed by chance
alone. However, in a small population, the variance may be large
and an allele may even be lost by chance (as when all heads were
tossed-the tails were lost). Hence, in small populations that are not
subject to frequent immigration, genetic drift can be an effective
agent of evolutionary change. This type of evolutionary change is
random, so there is no way to predict what direction it might take.
Harmful mutations may even be retained and spread in the
population in this way." (Boolootian R.A. & Stiles K.A. "College
Zoology," 1981, pp.669-670).
I mentioned how in one of my labs we simulated RM&NS and then Genetic
Drift in reducing populations. It was quite amazing and counter-intuitive
how RM&NS was overridden by Drift as the population got smaller.
The discovery of Genetic Drift effectively rendered Darwinism untestable,
and hence unscientific, as Patterson pointed out:
"Darwinian evolution, by natural selection, predicts that organisms
are as they are because all their genes have been and are being
subjected to selection, those that reduce the organism's success
being eliminated, and those that enhance it being favoured. This is a
scientific theory, for these predictions can be tested. 'Non-
Darwinian' or random evolution predicts that some features of
organisms are non-adaptive, having neutral or slightly negative
survival value, and that the genes controlling such features are
fluctuating randomly in the population, or have been fixed because
at some time in the past the population went through a bottleneck,
when it was greatly reduced. When these two theories are
combined, as a general explanation of evolutionary change, that
general theory is no longer testable. Take natural selection: no
matter how many cases fail to yield to a natural selection analysis,
the theory is not threatened, for it can always be said that these
failures of selection theory are explained by genetic drift. And no
matter how many supposed examples of genetic drift are shown to
be due, after all, to natural selection, the neutral theory is not
threatened, for it never pretended to explain all evolution."
(Patterson C., "Evolution", 1978, p.70)
Of course just as undirected genetic changes like Genetic Drift can be
decisive in small populations, so could *directed* genetic changes
implemented by an Intelligent Designer in small populations!
Steve
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"The most obvious contrasts between the darwinian view of the patterns
and the rates of evolution, and the evidence that has since been
documented by the fossil record.... Darwin used the only illustration in the
first edition of The Origin of Species to explain his hypothesis that the
patterns of evolution over hundreds of millions of generations were the
same as those at the level of populations and species. In fact, they are
clearly distinct in all taxonomic groups. Evolution at the level of
populations and species might, in some cases, appear as nearly continuous
change accompanied by divergence to occupy much of the available
morphospace. However, this is certainly not true for long-term, largescale
evolution, such as that of the metazoan phyla, which include most of the
taxa that formed the basis for the evolutionary synthesis. The most striking
features of large-scale evolution are the extremely rapid divergence of
lineages near the time of their origin, followed by long periods in which
basic body plans and ways of life are retained. What is missing are the
many intermediate forms hypothesized by Darwin, and the continual
divergence of major lineages into the morphospace between distinct
adaptive types." (Carroll R.L., "Towards a new evolutionary synthesis,"
Trends in Ecology and Evolution, 2000, Vol. 15, pp.27-32)
Stephen E. Jones | sejones@iinet.net.au | http://www.iinet.net.au/~sejones
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