Re: limits of variation.

Brian D. Harper (
Tue, 1 Aug 1995 22:10:32 -0400

Stephen wrote:

>It has not been demonstrated that "nonlinear dynamics" explains "major
>morphological change." It might be a factor, but that could be as much
>support for PC than NE. The real point is that it is not a *Darwinian*
>mechanism. Not yet anyway! :-)

I am really excited about Goodwin's book and I hope people will read it
if they get a chance. There are many things I like about the book, perhaps
the two most important being:

a) There is a real attempt to actually explain something without appeal to
just so stories or historical accidents.
b) The view that emerges from this approach is much more appealing to
a theist, in my humble opinion of course, than the blind watchmaker
or the genocentric "we're just disposable containers for genes"

That historical explanations are not very satisfying is a recurring theme in
Goodwin's book. At one point he uses the following analogy to illustrate

Imagine a time when you didn't know why Earth goes around the sun in
an elliptical orbit. You ask someone to give you an explanation and
they say: "Earth goes around the sun in an elliptical orbit this
year because that is what it did last year, and the year before that,
and so on back to the origin of the planetary system, and nothing has
happened to change it." Are you satisfied? ...
-- Brian Goodwin, _How the Leopard Changed its Spots_, p. 85

More in keeping with the title of this thread, Goodwin argues for the existence
of "generic forms" which are present due to organizational principles and the
laws of physics and chemistry. This he says opposes the traditional view that
there is a continuum of possible forms and the ones we observe are due to
historical accidents. I would take this to be a "limit of variation", but
not in the sense that this phrase has been used here. In other words this
is a type of barrier that would tend to keep organisms within certain
boundaries (potentially an explanation for stasis), however, the barrier
is not insurmountable.

My favorite section of the book comes in chapter 4. Goodwin asks the question
"why does _Acetabularia_ make whorls?". [To keep this brief, I won't go into
minor details such as what _Acetabularia_ and whorls actually are :-)].
Goodwin then gives the traditional "just so" historical explanation for
his question, explains why it is insufficient and then gives a remarkable
numerical simulation in which the morphological features of the _Acetabularia_
emerge as a natural consequence of the nonlinearity in the model. I suppose
one reason I like this is that this is perhaps the only case I have found
so far where there is a real overlap with my own area of expertise. This is
not a numerical just so story, Goodwin uses an established numerical method,
the Finite Element Method, real physics and continuum mechanics and real
(empirically determined) nonlinear constitutive properties. In view of this,
I have real problems with the recent SciAm article characterizing complexity
as a "fact free" science.


Brian Harper:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=:=
"I believe there are 15,747,724,136,275,002,577,605,653,961,181,555,468,
044,717,914,527,116,709,366,231,425,076,185,631,031,296 protons in the
Universe and the same number of electrons." Arthur Stanley Eddington