Let me first mention, which I failed to do in my first message, that I admire
the manner in which you and Dick Fischer conducted your debate over two flood
theories. It stands as a model of how two men of faith can dispute an issue
without coming to blows. Good work.
Your message of May 17 left me with several questions. (I have reproduced it
on page 2 with numbered paragraphs for easy reference.) Most of your message
deals with development as the operative process in bringing about large
morphological change. What Gould (1) describes are clearly developmental
processes-embryonic development, with a hierarchical system of master
switches, and cascading effects throughout the whole body. Gilbert (2)
writes in developmental terms about "differentiation of limb mesoderm into
chondrocytes during embryogenesis in the horse lineage." You (3) wrote,
"Morphology is determined by the Hox genes and they are a nested hierarchy.
They determine morphology." Gilbert continued (4) writing about "co-varying
constellations of characters" which Raisa Berg called "correlation pleades"
which I find a confusing term. Ahlberg (5) continued with examples of
covariation of morphological characters. What they are saying is that the
development of the braincase is developmentally linked to the limbs.
In sum, from your examples your point is well taken, that development, not
natural selection, is the critical causal agent in bringing about major,
early, drastic morphological change.
While you do not dismiss natural selection out of hand as an agent of change,
and you do claim that Darwinian selection is still operative, that is all you
affirm for NS. You hold that NS does not produce morphological change by
smooth morphing of forms, that it is a different issue from how morphological
change occurs. If development is the critical causal factor in causing major
morphological change, what then is left for NS to do?
The study of monkey flowers you cited as reported in Nature (6), however, is
a clear example of natural selection at work. The authors hypothesize a
sequence of three mutations by which a hummingbird-pollinated flower evolved
from a bumblebee-pollinated one. One mutation changed the color, a second
the volume of nectar, the third the length of the pistil, all of which
favored the hummingbird pollinator. In the process a second species, the
hummingbird-pollinated flower, emerged from the first. The process is
radically different from the previous examples you gave of development, not
one of which involved the environment or selection as a critical factor, and
all of which are more drastic than the monkeyflower evolution. Would you
agree that the change in monkeyflowers is a case of natural selection at
work?
This all suggests an additional question. If development and natural
selection are such different issues, as you stated, what is the relationship
between the two? There must be some connection between evolution and
development since they are both agents of change. Can you clarify how they
are related?
Thanks for taking time from what I am sure is a busy schedule to address
these questions.
Peace,
Robert DeHaan
Below is your message of May 17, 1996.
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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. But this is not what modern genetics is learning about how change
occurs. Gould writes:
(1) "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
(2) 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
(3) Morphology is determined by the Hox genes and they are a nested
hierarchy. They determine morphology. But they affect a whole sequence of
structures.
(4) "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:
(5) "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.
(6) 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.
Darwinian selection is still operative.
glenn
Foundation, Fall and Flood
http://members.gnn.com/GRMorton/dmd.htm