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Internal Constraints vs. External Pressures: The Revelations of Evo-Devo

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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.

One of the major advances since the publication of Evolution: A Theory in Crisis has been the revolutionary increase in our understanding of development and especially the genetics of development, about which little was known in 1985. These advances have led to the emergence of a whole new field, nicknamed “evo-devo” (evolutionary developmental biology). At the heart of the new field has been the discovery that a limited set of highly conserved genes, gene circuits, and developmental mechanisms — for example, Hox genes, signaling proteins such as “sonic hedgehog,” chemical gradients, and gene regulatory networks — are involved in the construction of the bodies of all bilaterally symmetric animals,1 all the morphological homologs, and indeed all higher organismic form. Sean Carroll has termed this set of conserved genes and developmental mechanisms the “toolkit.”2

One now well-established finding of evo-devo was totally unexpected from Darwinian first principles. It is that what are referred to by evo-devo researchers as “constraints,” or what might also be termed “internal causal factors,” have played a far more important role in the origin of many of the homologs and Bauplans than was previously envisaged. Before evo-devo it was widely thought that internal factors played little or no role in shaping the course of evolution. But it is now increasingly obvious that internal causal factors have played a far more prominent role in the actualization of evolutionary innovations than cumulative selection.

One of the most fundamental of all evolutionary innovations occurred when some ancestral chordate switched its body plan from the design previously shared by all other animal groups (in which the nerve chord is in a ventral position and the heart and main blood vessel are placed dorsally) to a design that was the exact reverse (in which the nerve chord is placed dorsally and the heart placed ventrally), and that has remained invariant in all chordates ever since. The evidence for this dorsal-ventral inversion (D-V) was revealed when evo-devo studies showed that the signaling molecules that specify the back of an insect also specify the ventral side (the belly) of a vertebrate.3 Before evo-devo revealed that the same genes were involved in specifying the dorsal-ventral axis of chordates and non-chordates, no one took seriously Goeffroy’s suggestion, made early in the 19th century, that all animals shared the same basic body plan.

Self-evidently, Darwinian scenarios must confront the obvious question as to whether this transition was gradual or sudden. On the one hand, it is very hard to imagine how gradual cumulative selection could carry out such a radical re-engineering of the basic body plan. What mystifying adaptive path can be proposed along which such a gradual transition might have occurred? On the other hand, if the change occurred suddenly in one massive macro-mutational saltation, then Darwinian causation is ruled out of court altogether.

In the case of this dramatic innovation, the revelations of evo-devo provide no support at all for the Darwinian notion that cumulative selection is the major causal engine of evolutionary transitions.

In taxa-defining homologs, it is self-evident that the emerging evo-devo picture provides no support whatsoever for the Darwinian claim that novelties came about during the course of evolution to serve a succession of functional ends. In this case, the structuralist inference that internal constraints and not cumulative selection played the key role in their actualization is difficult to refuse.

This example is not atypical. Evo-devo advances have revealed that in many, if not the great majority, of innovations in the history of life, internal causal factors have played a predominant role. Against all traditional expectations, Darwinian selection to serve adaptive ends could only have played, in most cases, a relatively peripheral causalrole. Before evo-devo, no one would have imagined that a vast amount of organic order arises from internal constraints and causal factors within organisms themselves and is not imposed by selection. What was heresy only three decades ago is now accepted doctrine.


(1) Lewis I. Held, How the Snake Lost Its Legs: Curious Tales from the Frontier of Evo-Devo (New York: Cambridge University Press, 2014), Chapter 1.

(2) Sean Carroll, Endless Forms Most Beautiful, Chapter 3, see fig. 3.7.

(3) Held, How the Snake Lost Its Legs, Chapter 1; for alternate scenarios, see “Inversion (evolutionary biology),” Wikipedia, Wikipedia Foundation Inc., March 22, 2015 (accessed on August 19, 2015).

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