Editor’s note: In his new book Evolution: Still a Theory in Crisis, Michael Denton not only updates the argument from his groundbreaking Evolution: A Theory in Crisis (1985) but also presents a powerful new critique of Darwinian evolution. This article is one in a series in which Dr. Denton summarizes some of the most important points of the new book. For the full story, get your copy of Evolution: Still a Theory in Crisis. For a limited time, you’ll enjoy a 30 percent discount at CreateSpace by using the discount code QBDHMYJH.
The revolution associated with the field of evo-devo (evolutionary developmental biology) has brought with it the revelation that underlying the development of all organisms is a set of highly conserved gene circuits and integrated developmental modules that guide and constrain phylogeny without regard for the immediate adaptive needs of species. This new knowledge poses a widely acknowledged challenge (and indeed self-evident challenge) to Darwinian evolution.
And it has also undermined completely the previous neo-Darwinian mechanistic view of organisms as analogous to contingent assemblages of LEGO blocks. According to the Darwinian model, organisms are infinitely plastic “additive functional assemblages” which can be changed — LEGO block by LEGO block — from one shape to any other conceivable novel shape bit by bit without any significant constraints. On the contrary, all the evo-devo evidence suggests that there are deep and profound developmental constraints that work against such infinite pliability.
The new conception of organisms arising from the discoveries of evo-devo might be termed the “Transformer” model. Just as in a child’s Transformer toy action figure, the number of forms that can be reached by combining the basic parts in different ways is severely limited by the shape and properties of the component building bricks. The finite set of forms that can be assembled is prefigured into the basic properties of the constructional units. One might view the constraints imposed by the building units in a Transformer set (unlike LEGO bricks) as being analogous to those imposed in living things by the highly conserved toolkit components and developmental modules.
Consequently, pigs will never fly — not only because of functional constraints (far too heavy!), but because of deep internal structural constraints in the way a pig is put together, which greatly restrict the available paths that adaptive evolution can take. On this view, the major novelties actualized during evolution could only have occurred if they were compatible with the pre-existing inner “developmental logic” of the organism, analogous to prefiguring of the Transformer components for a specific set of forms.
In other words, development rules! Or, more precisely, what rules is the collective constraints of toolkit elements, developmental pathways, and modules that underlie the ontogeny of every class of metazoan organisms.
Although there are some dissenters,1 the existence of highly conserved developmental genetic mechanisms, gene circuits, and so forth restricting the paths of evolution has led to the widespread adoption of what might be termed a new constraints paradigm among many evo-devo researchers, who acknowledge that the deep logic of development is bound to restrict the direction of variation and hence evolutionary change along limited paths.2 But what is really radical about the “constraints paradigm” is that the constraints restrict the paths of evolution without any regard for immediate adaptive function.“3
The notion of deeply shared non-adaptive internal causal factors channeling evolution in such a manner is certainly heretical. It is small wonder that, as Gerd Müller and Massimo Pigliucci concede, many leading biologists feel that evo-devo presents “challenges to the received theory… so substantial that no reconciliation with the classical framework [Darwinian, incremental functionalism] is at all possible.”4
(1) Hopi E. Hoekstra and Jerry A. Coyne, “The Locus of Evolution: Evo Devo and the Genetics of Adaptation,” Evolution; International Journal of Organic Evolution 61, no. 5 (May, 2007): 995-1016; see also Lindsay R. Craig, “Defending Evo-Devo: A Response to Hoekstra and Coyne,” Philosophy of Science 76, no. 3 (2009): 335-344.
(2) Wallace Arthur, Evolution: A Developmental Approach; Jerry Fodor and Massimo Piattelli-Palmarini, What Darwin Got Wrong (London: Profile, 2010), Chapters 2, 3, and 4; Günter P. Wagner, Homology, Genes, and Evolutionary Innovation (Princeton: Princeton University Press, 2014).
(3) Ron Amundson, The Changing Role of the Embryo in Evolutionary Thought (Cambridge: Cambridge University Press, 2007), 8.
(4) Gerd B. Müller and Massimo Pigliucci, “Extended Synthesis: Theory Expansion or Alternative?” Biological Theory 5, no. 4 (2010): 275-276, 275.
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