[2010/6/26] Scott F. Gilbert, Developmental Biology, & Michael Behe
Posted by Jonathan McLatchie on June 26
Scott F. Gilbert's "Developmental Biology" (eighth edition) provides a stunning overview of the elegant biochemical mechanisms controlling the development of organismal form during ontogeny. The final section of the book, chapter 23 ("Developmental Mechanisms Of Evolutionary Change") is devoted to a discussion of the new evolutionary synthesis, encompassing the new science of 'evo devo' (short hand for 'evolutionary developmental biology'). The book even contains a short rebuttal directed at proponents of intelligent design and, in particular, Michael Behe.
Gilbert writes on page 749:
"Leaving developmental biology out of evolutionary biology has left evolutionary biology open to attacks from promoters of 'intelligent design.' According to one of them, Michael Behe (1996), population genetics cannot explain the origin of structures such as the eye, so Darwinism must be false.* How could such a complicated structure have emerged by a collection of chance mutations? Mutations, claims Behe, would serve only to destroy complex organs, not create them."
The asterisk (*) denotes a footnote at the bottom of page 749, which reads:
"Behe (1996) makes this point explicitly, using the example of the eye. Although he attempts to disprove the theory of evolution by using the eye as an example, he never mentions the myriad studies on Pax6 or reciprocal induction. Rather, Behe mentions theories from the 1980s (based solely on population genetics) and puts them forth as contemporary science."
Where is this attempt at a rebuttal in error? In Darwin's Black Box, Behe did not use the eye as an example of an irreducibly complex system. Rather, Behe argued that the biochemistry of vision is irreducibly complex. These systems are not equivocal. On page 22 of Darwin's Black Box, Behe stated:
"Now that the black box of vision has been opened, it is no longer enough for an evolutionary explanation of that power to consider only the anatomical structures of whole eyes, as Darwin did in the nineteenth century (and as popularizers of evolution continue to do today). Each of the anatomical steps and structures that Darwin thought were so simple actually involves staggeringly complicated biochemical processes that cannot be papered over with rhetoric...Anatomy is, quite simply, irrelevant to the question of whether evolution could take place on the molecular level."
After having misrepresented Behe's work in this manner, Gilbert proceeds to explain how evolutionary novelty can be accounted for by developmental mechanisms:
"But once development is added to the evolutionary synthesis, it is straightforward to see how the eye can develop through induction, and that the concepts of modularity and correlated progression can readily explain such a phenomenon (Waddington 1940; Gehring 1998). Moreover, when one sees that the formation of eyes in all known phylogenetic lineages is based on the same signal transduction pathway and uses the Pax6 gene, it is not difficult to see descent with modification forming the various types of eyes."
Behe discusses this concept fairly extensively in his second book, The Edge of Evolution, written the same year as Gilbert's Developmental Biology.
In a 2005 Nature review, evolutionary geneticist, Jerry Coyne, expressed skepticism of the proposed mechanism of evo devo:
"The evidence for the adaptive divergence of gene switches is still thin. The best case involves the loss of protective armor and spines in sticklebacks, both due to changes in regulatory elements. But these elements represent the loss of traits, rather than the origin of evolutionary novelties...We now know that Hox genes and other transcription factors have many roles besides inducing body pattern, and their overall function in development - let alone in evolution - remains murky."
Moreover, mechanisms involving evo devo are not necessarily inconsistent with a broadly-defined ID theoretic. It is possible for evolutionary mechanisms involving such processes as 'facilitated variation' to bias evolutionary trajectories in a fashion that some (such as Mike Gene) have described as 'front loading'. While much work remains to be done, it is conceivable that principals of evo devo and 'facilitated variation' could serve to feed research from the standpoint of ID.
At the time of the Cambrian, when the major distinctive body plans (phyla) first emerge, we find that the organisms become endowed with their set of toolbox components such as Hox genes, which play a fundamental role in conventional evo devo mechanisms. But that does nothing to explain the origin of the developmental pathways controlling the morphogenesis of an organism. Can such systems be built up one step at a time, whilst maintaining at least some degree of functional utility before the emergence of a viable embryo? At present, there is no known materialistic or mindless process which is able to account for this type of system.