You wrote:
>Let me respond to these statements:
>
>1. It is not just Christian apologists who hold to an infinite
> sequence of infinitesimal transitions. Dawkins and Dennett, two
>prominent eovlutionists, hold that any complex design can occur only if
>the steps to the change are small enough, if there is enough time, and
>it each one enhances survival of the organism in some way. They both go
>back to Darwin and embrace his basic concept of natural selection as the
>universal agent of change. They are hardly straw men. Do you depart
>from them?
>
No one is dismissing natural selection as the agent of change. Natural
selection is a different issue from how the morphological change occurs.
It is true that the evolutionists and Darwin, in particular, held to a
view that morphological change was carried out by a smooth morphing of
form. This is similar to what political ads do to a picture of their
opponent. The frame by frame transform the man into a donkey or something
else. But this is not what modern genetics is learning about how change
occurs. Gould writes:
"If embryology is a hierarchical system with surprisingly few master
switches at high levels, then we might draw an evolutionary message after
all. If genetic programs were beanbags of independent genes,each
responsible for building a single part of the body, then evolution would
have to occur slowly and sequentially as thousands of parts achieved their
independent modifications. But genetic programs are hierarchies of master
switches, and small genetic changes that happen to affect the switches,
might engender cascading effects throughout the body. Homeotic mutants
teach us that small genetic changes can affect the switches and produce
remarkable changes in an adult fly. Major evolutionary transitions may be
instigated (although not finished all at once as hopeful monster
enthusiasts argue) by small genetic changes that translate into
fundamentally altered bodies. If classical Darwinian gradualism is now
under attack in evolutionary circles, the hierarchical structure of
genetic programs forms a powerful argument for the critics."Stephen J.
Gould, Helpful Monsters," _Hen's Teeth and Horse's Toes_, 1984,
Penguin,p196
Gilbert writes:
"Thus when we say that the contemporary one-toed horse evolved from a
five-toed ancestor, we are saying that hereditable changes occurred in
the differentiation of limb mesoderm into chondrocytes during
embryogenesis in the horse lineage. In this perspective, evolution is the
result of hereditary changes affecting development. This is the case
whether the mutation is one that changes the reptilian embryo into a bird
or one that changes the color of Drosophila eyes."~Scott F. Gilbert,
Developmental Biology (Sunderland: Sinauer Assoc. Inc., 1991), p. 841
Morphology is determined by the Hox genes and they are a nested hierarchy.
They determine morphology. But they affect a whole sequence of
structures.
"When a drastic remodeling of morphology takes place during
evolution, one has to account for the developmental changes that have
caused it. As early as 1871, St. George Mivart pointed out that large
evolutionary changes were not due to the simple alteration of one
structure of an organism. Rather, an entire group of structures changed.
Raisa Berg has called these co-varying constellations of characters
'correlation pleades."~Scott F. Gilbert, Developmental Biology
(Sunderland: Sinauer Assoc. Inc., 1991), p. 844
Thus when the tetrapod brain case evolved, Ahlberg et al wrote:
"Panderichthys shows that the tetrapod braincase structure evolved more
abruptly than the external skull morphology. The transformation seems to
have coincided with the origin of digits and tetrapod pelvis, at the
internode traditionally taken to mark the 'origin of tetrapods. Although
the hypothesis will need to be tested through the investigation of other
Devonian stem tetrapods, such as the enigmatic Ichthyostega, we infer
tentatively that this latter correlation is a real phenomenon reflecting
functional and/or developmental linkage, rather than merely being an
artefact of poor sampling." "Rapid Brain case evolution between
Panderichthys and the earliest tetrapods," Nature May 2, 1996, p. 63
What they are saying is that the development of the braincase is
developmentally linked to the limbs.
Very few mutations are required to radically change morphology.
A recent study in Nature showed that most of the change between a flower
that looked designed to attract hummingbirds and one that looked to be
designed for bumblebees was accomplished by 8 mutations and that is all.
(~H.D. Bradshaw Jr., S. M. Wilbert, K. G. Otto and D. W.
Schemske, "Genetic mapping of Floral Traits Associated with Reproductive
isolation in monkeyflowers (Mimulus)," Nature, 376 Aug. 31, 1995, p. 762)
We are on the verge of an age of experimental evolution to retrace which
mutations separate species. In the case of monkey flowers, it was only 8
mutations.
>2. I'm not familiar with chimeras except in broad outline. Calling the
>transitional sequence chimerical, however, hardly explains how it
> originated,does it?. A chimera, as I understand it, is a plant or
>animal whose tis sues are of two different genotypes, produced either by
>mutation or by grafting >(Martin. DICTIONARY OF LIFE SCIENCES).
>Grafting, or mixing together cells of very early embryos, as is done
>experimentally to make chimeric mice, is of course ruled out. That
>leaves mutations, doesn't it, to explain how the genomes of
>one/three-toed horses became chimeric? If so, are the mutations small,
>occurring unpredictably, over long periods of time, the phenotypes of
>which are selected by the environment because of their superior
>adaptibility and reproductive success? Or are they saltations?
>
They are saltations only in that one feature at a time saltated. I used
the word chimerical loosely, not in the dictionary definition. But the
fact that the mesonychids were the only mammals which had a tiny hoof on
each finger and that the last whale with legs had tiny hooves on their
finger (a mesonychid trait) can be called chimerical in some sense.
>I would appreciate your views on these matters. I am genuinely
> interested in what is left of evolution if Darwinian natural selection
>is abandoned, as you seem to be doing.
>
Darwinian selection is still operative.
glenn
Foundation,Fall and Flood
http://members.gnn.com/GRMorton/dmd.htm