Thursday, July 13, 2006

D'Arcy Thompson and "Front-Loaded" Intelligent Design



AUTHOR: Salvador Cordova

SOURCE: Marsupials and placentals: A case of front-loaded, pre-programmed, designed evolution?

COMMENTARY: Allen MacNeill

The concept of "front-loading" as described in Salvador Cordova's post at Telic Thoughts bears a remarkable resemblance to the ideas of the Scottish biomathematician D'Arcy Thompson (1860-1948). In his magnum opus, Growth and Form, Thompson proposed that biologists had over-emphasized evolution (and especially natural selection) and under-emphasized the constraints and parameters within which organisms develop, constraints that "channel" animal forms into particular patterns that are repeated over and over again across the phyla.

However, while Thompson's ideas strongly imply that there is a kind of teleology operating at several levels in biology (especially developmental biology), Thompson himself did not present hypotheses that were empirically testable (sound familiar?):

Thompson did not articulate his insights in the form of experimental hypotheses that can be tested. Thompson was aware of this, saying that 'This book of mine has little need of preface, for indeed it is 'all preface' from beginning to end.'

Thompson's huge book (over 1,000 heavily illustrated pages) is a veritable gold mine of ideas along the lines articulated in Sal's post. However, Thompson's underlying thesis is just as inimical to ID as is the explanation from evolutionary biology. His argument is essentially that biological form is constrained by the kind of mathematical relationships that characterize classical physics. That is, there are "built-in" laws of form that constrain the forms that biological organisms can take. And therefore, physical law provides the “front-loading”, not a supernatural “intelligent designer.”

For example, Thompson pointed out that the shape that droplets of viscous liquid take when dropped into water are virtually identical to the medusa forms of jellyfish, and that this "convergence of form" is therefore not accidental. Rather, it is fundamentally constrained by the physics of moving fluids, as described in the equations of fluid mechanics. Thompson's book is filled with similar examples, all pointing to the same conclusion: that biological form is constrained by the laws of physics (especially classical mechanics).

Evolutionary convergence, far from departing from Thompson's ideas, is based on essentially the same kinds of constraints. Sharks, dolphins (the fish, not the mammals), tunas, ichthyosaurs, and porpoises all appear superficially similar (despite significant anatomical differences) because their external shapes are constrained by the fluid medium through which they swim. In the language of natural selection, any ancestor of a shark, dolphin, tuna, ichthyosaur, or porpoise that (through its developmental biology) could take the shape of a torpedo could move more efficiently through the water than one that had a different (i.e. less efficient) shape, and therefore would have a selective advantage that would, over time, result in similar shapes among its proliferating ancestors. The same concept is applied to the parallel evolution of marsupial and placental mammals: similar environments and subsistence patterns place similar selective constraints on marsupial and placental mammals in different locations, resulting in strikingly similar anatomical and physiological adaptations, despite relatively non-homologous ancestry.

This evolutionary argument is now being strongly supported by findings in the field of evolutionary development ("evo-devo"), in which arguments based on "deep homology" are providing explanations for at least some of the seemingly amazing convergences we see in widely separated groups of organisms. Recent discoveries about gene regulation via hierarchical sets of regulatory genes indicate that these genes have been conserved through deep evolutionary time, from the first bilaterally symmetric metazoans to the latest placental mammals, as shown by their relative positions in the genome and relatively invariant nucleotide sequences. These genes channel the arrangement of overall anatomy and body form throughout the course of development, producing the overall shapes of organisms and the relationships between body parts that we refer to when discussing evolutionary convergence.

However, as should be obvious by now, this in no way provides evidence for the currently popular ID hypothesis of “front-loading”, except insofar that it states that the hierarchical control of overall development evolved very early among the metazoa. It provides no empirically testable way to distinguish between an evolutionary explanation and a “design” explanation. Indeed, all of the evidence to date could be explained using either theory.

And so, by the rules of empirical science, since the evolutionary explanation is both sufficient to explain the phenomena and does not require causes that are outside of nature (i.e. a supernatural designer, that is neither itself natural nor works through natural – i.e. material and efficient – causes), evolutionary biologists are fully justified in accepting the evolutionary explanation (and disregarding the “front-loaded ID” explanation.

Only in the case that the kinds of natural causes described above (especially the ability of evo-devo processes to constrain the development of overall form via purely natural means via the known biochemistry of development) can NOT explain the patterns we observe in convergent evolution should we entertain other hypotheses (especially if those other hypotheses are not empirically testable). Only then, and not before…and therefore certainly not now.

FOR FURTHER READING:

For more on Thompson and his work, see:
http://www.google.com/search?hl=en&q=D%27Arcy+Thompson&btnG=Google+Search
and especially:
http://www-history.mcs.st-andrews.ac.uk/Mathematicians/Thompson_D'Arcy.html
and follow the links at:
http://en.wikipedia.org/wiki/D'Arcy_Thompson

Also, a thread that included a discussion of Thompson's work has already appeared at Telic Thoughts http://telicthoughts.com/?p=763

--Allen

Labels: , , , , , , ,

5 Comments:

At 7/13/2006 11:03:00 PM, Anonymous Anonymous said...

Hi Allen,

Since this is not the course blog I can be a bit more blunt about the thoughts I had while reading Sal's post and your comments:

Sal's post was basically incoherant word salad, involving wild misunderstandings of several standard topics like convergence, non-coding DNA, etc.

Sal's postulation of "front-loading" is basically just Behe's discredited supercell idea, perhaps turned into a "super-proto-mammal" idea, and perhaps more specifically turned into an idea of special creation where placentals and marsupials, for some completely inscrutable reason, were given similar external "designs" based on two different mammalian plans.

Either way, Sal is saying that the organism's DNA hides, somehow without mutation decay, the information for vastly different organisms in its non-coding DNA. Behe has already given this up as unworkable and Sal should also.

While I'm complaining, "magical frontloading with magical mutation-proof noncoding DNA" has nothing whatsoever to do with D'Arcy Thompson view of developmental or physical law constraints on evolution, and suggesting that it does accomplishes nothing except giving Sal a warm fuzzy feeling and giving him the delusion that he has said something that makes sense.

We can't sacrifice critical thinking just to be polite or gracious with the creationists, I think.

Nick

 
At 7/17/2006 01:38:00 PM, Blogger Allen MacNeill said...

It's funny what you see when you begin to pay attention. This appeared this week:

http://scienceweek.com/2006/sw060721-1.htm

 
At 7/19/2006 05:15:00 PM, Anonymous ivy privy said...

It's funny what you see when you begin to pay attention. This appeared this week:

http://scienceweek.com/2006/sw060721-1.htm


------------
Funny indeed. I'm not sure though if you're referring to the "DEVELOPMENTAL BIOLOGY: ON THE SHAPES OF ORGANISMS" news item, or the ad for a book I see at the top of the page: JUNK SCIENCE: How Politicians, Corporations, and Other Hucksters Betray Us by Dan Agin.

The news item mention of Ultrabithorax reminds me that I read Wells' Icons of Evolution just recently.

Matzke accuses Cordova of tossing word salad? Careful, or Sal will toss a quantum Schrodinger eigenstate at you.

Your "word verification" spam blocking is sure hard to read the way the letters are all run together. I do not wonder that non-humans have a hard time dealing with it.

 
At 7/19/2006 05:35:00 PM, Anonymous ivy privy said...

I read Salvador's essay. The images of animals seem very selective; no bipedal kangaroos, no aquatic placentals (Do aquatic marsupials even exist? The platypus would be the closest one I'm aware of.)

It is difficult to explain so many similarities as the result of coincidence.

Well duh. Perhaps similarity of habitat might have something to do with it?


Given the administrative practices over at Uncommon Descent, are you disappointed at all that they considered your post so nonthreatening that they didn't bother to delete it?

 
At 7/26/2006 12:55:00 PM, Anonymous ivy privy said...

And bats! Where are the marsupial bats?

 

Post a Comment

Links to this post:

Create a Link

<< Home