David Tyler wrote:
>In my comment, I was concerned about the reductionistic tendency
>within science today which has led numerous people to think that
>if we can only decode the genome, we will have solved the
>problems of development.
>
>Glenn wrote:
>I would suggest Scott Gilbert's _Developmental Biology_ He
>writes: "Nuclear control of cell morphogenesis and the
>interaction of the nucleus and cytoplasm are beautifully
>demonstrated by studies of Acetabularia. ...
>[text omitted]
>This would seem to imply that the nucleus, which is mostly DNA
>controls development."
>
>There are numerous questions here, including: What is the nature
>of this control? Is it the only control?
>
You are correct that mere knowledge of the genome would not solve development.
And no DNA is not the only control for development. Each egg cell has
chemicals which start the process of development. So if you took the genes of
a dinosaur and put them into the egg of a crocodile, you might not get a
dinosaur. These chemicals act as initial conditions for the DNA-cell system
(as I understand it).
A case in point occurs with snails. Gilbert writes:
"Thus, the genetic factors involved in snail coiling are brought to the
embryo in the oocyte cytoplasm. It is the genotype of the ovary in which the
ooctye develops that determines which orientation the cleavage will take.
When Freeman and Lundelius injected a small amount of cytoplasm from dextrally
coiling snails into the eggs of dd mothers, the resulting, the resulting
embryos coiled to the right. Cytoplasm from sinestrally coiling snails did
not affect the righ-coiling embryos. This confirmed the view that the wild-
type mothers were placing a factor into their eggs that was absent or
defective in the dd mothers."~Scott F. Gilbert, Developmental Biology
(Sunderland: Sinauer Assoc. Inc., 1991), p. 86
Even some fertilized egg itself differentiates chemicals in it two halves and
these chemicals tell the egg which half becomes the head and which the tail.
(See Gilbert p. 259)
>Gilbert distances himself from the "solely genetic model of
>evolution and development" in a recent paper. I'll quote one
>relevant passage:
>"Just as the cell is seen to be the unit of structure and
>function in the body - not the genes that act through it - so the
>morphogenetic field can be seen as a major unit of ontogenetic
>and phylogenetic change. In declaring the morphogenetic field
>to be a major module of developmental and evolutionary change,
>we are, of course, setting it up as an alternative to the solely
>genetic model of evolution and development. This, however, is
>not to be seen as antagonistic to the principle that genes are
>important in evolution or development. This is not in any way
>denied. But just as the genes make the cells and the cells form
>the body, so the gene products first need to interact to create
>morphogenetic fields in order to have their effects. Changes in
>these fields then change the ways that animals develop." (p.368).
>Gilbert, S.F., Opitz, J.M. and Raff, R.A. 1996, Resynthesizing
>evolutionary and developmental biology, Developmental Biology,
>173, 357-372.
>
That was an excellent response, and I agree that Man is more than mere genes.
Here is where Terry comes in. Terry wrote:
> I'm a bit surprised of Glenn's consistent
>opposition to this questioning of the hegemony of the DNA only school of
>development and evolution since he is well-versed in these complex and
>dynamic system issues.
As they say in China, I am "ee ge ben dan" (a stupid egg). In the last post, I
misunderstood David and was not per se, questioning the DNA reductionism. But
since it has been brought up, I will pursue it and try to learn something.
Terry and I had this discussion about a year and a half ago. Maybe I can be
educated this time or at least express my problem better. My problem in seeing
how people escape reductionism lies in the origin of DNA reductionism lies in
the source of the morphogens found in the egg. If they are ultimately derived
from the genes, then they reduce ultimately to DNA. If these morphogens come
from cytoplasm alone and are passed from mother to child from the cytoplasm
alone, generation after generation, then DNA reductionism is not possible.
Although this does not escape from a more general form of reductionism - pure
naturalism. Assuming that the initial morphogens come from the DNA, I will go
on to make the argument (subject to my assumption being correct.
I am not sure that animal is more than mere genes. And I am not sure that
Gilbert et al distance themselves from a reductionist scenario because their
next to last sentence. Gene products interact to create the morphogenic
field. Ultimately the morphogens come from the genes themselves.
Is it correct or incorrect that the morphogens ultimately are coded somewhere
in the genome?
>I'm citing Gilbert because Glenn appealed to him as witness
>against the point I made. I consider that Gilbert is more of an
>ally than an opponent on this issue.
>
>I'm not endorsing everything quoted here - for example, Gilbert's
>constant linking of developmental change with evolutionary
>change. I consider this a linkage resulting from presupposition
>rather than the results of science. If Gilbert were to address
>the distinctions to be made between empirical science and
>historical science, he might conclude that there are two
>separate, but potentially related, issues to be considered. The
>nature of that relationship needs to be carefully probed before
>anything meaningful can be said.
>
>Is this an adequate response, Glenn?
It is an adequate response. And I now understand what you are saying.
Consider though what Gilbert is saying about development and evolution:
"How, then, can one modify one Bauplan to create another Bauplan? The
first way would be to modify the earliest stages of development. According to
von Baer, animals of different species but of the same genus diverge very late
in development. The more divergent the species are from one another, the
earlier one can distinguish their embryos. Thus, embryos of the snow goose
are indistinguishable from those of the blue goose until the very last stages.
However, snow goose development diverges from chick embryos a bit earlier, and
goose embryos can be distinguished from lizard embryos at even earlier stages.
It appears then, that mutations that create new Bauplane could do so by
altering the earliest stages of development."~Scott F. Gilbert,Developmental
Biology (Sunderland, Mass.: Sinauer Associates, Inc., 1991),p. 831-832
Since the body plan and ultimately the Linnean classification of an animal is
due to the developmental pattern, a change in the developmental pattern
changes the body plan, and thus the classification.
glenn
Foundation,Fall and Flood
http://members.gnn.com/GRMorton/dmd.htm