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'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