The explanation of deep conservation presented above (Section 3.3) repeats Cuvier’s three-step argument point-for-point, albeit in a quite different argumentative context. In
23 Though distinct, the two are compatible, as is exemplified by St. George Jackson Mivart’s On the Genesis of Species (Mivart 2009). In that work, he both reiterated Jenkin’s arguments based on natural state thinking (Mivart 2009, 118) and challenged the ability of Darwin’s theory to account for “the infinitesimal commencement of structures” on Cu-
particular, where Cuvier applied his functionalist argument globally, considering the functional integration of the entire organism, Davidson and Erwin apply the argument in a locally restricted setting: they apply it to particular GRN subcircuits.
One of the key features of GRN kernels is their recursive wiring: kernels are charac- terized by extensive positive feedback. This allows them to play a crucial developmental role: they can convert transient inputs into a stably maintained regulatory state (Wagner 2014, chap. 3; Peter and Davidson 2015, chap. 6). Because they are functionally integrated to such a degree, loss of a single gene in the kernel can destroy kernel function entirely, resulting in failure to develop the body parts under their developmental control (Davidson and Erwin 2006). Combined with the importance of these parts to the organ- ism, this makes gradual evolution of kernels extremely difficult.
In this argument, we can see each step of Cuvier’s three-part argument. Davidson and Erwin begin with an emphasis on the recursive wiring of special GRN subcircuits (step 1: functional integration). Note that functional integration here is inward-looking. If the wiring of kernels is disrupted, then they fail to serve their developmental function and so fail to produce a viable offspring. This non-viability is largely environment-independ- ent. A fly without a heart will not survive in any naturally occurring environment. Nor will a sea urchin that fails to properly develop its endomesodermal layer in early devel- opment. This leads to the conclusion that small changes to the linkages in these subcir- cuits will lead to developmental catastrophe, not in some particular environment, but in any environment the organism is likely to encounter (step 2: the non-viability of
intermediates). These subcircuits therefore cannot evolve in a gradual manner (step 3: anti-gradualism).
A word is required about what it means for kernels to evolve gradually. Gradual change is most readily understood at the phenotypic level, where it implies that large phenotypic changes are produced by the continuous accumulation of smaller such changes. As all genetic changes are discontinuous, the notion of gradual evolution is ef- fectively meaningless at the genetic level. Though changes to GRN structure are techni- cally phenotypic changes, as they are the consequences of (rather than identical to) changes in DNA sequences, the best available models of GRN structure are nonetheless discrete. Either a gene activates/represses another, or it does not, and so a linkage is ei- ther present in a GRN, or it is absent (Peter and Davidson 2015, chap. 1). In this regard, GRN evolution, like genome evolution, is essentially discontinuous, and the idea of grad- ual evolution does not straightforwardly apply. Provisionally, however, we might un- derstand gradualism at the GRN level as involving the change of a single regulatory link- age at a time, in contrast to large-scale rewiring in a single step.
Understanding GRN-level gradualism in this way, we can expand on the sense in which Davidson and Erwin argue for the non-viability of intermediates as applied to ker- nels. The recursive wiring of kernels makes it difficult for them to evolve gradually, i.e. one linkage at a time, once they are established. Any one change would require compen- satory changes to preserve the kernel’s function. This is a GRN-level analog of Cuvier’s view that single parts cannot change independently: change to any one part requires mul- tiple coordinated changes. As acknowledged above (footnote 2), some changes to kernel
structure are possible, but they are of a special type: they convert a direct linkage to an indirect linkage.24 Thus they preserve kernel structure, considered at the appropriate level of abstraction, and are therefore the exceptions that prove the rule. Since no mech- anism is known that could simultaneously produce coordinated changes to multiple link- ages, saltatory evolution of kernels is off the table. The only remaining option is non- evolution: once formed, kernels cease evolving.
As my aim in this chapter is ultimately to show how evo-devo fits into the broader landscape of evolutionary theorizing, I have focused on a developmental genetic expla- nation of deep conservation. Now that the Cuvieran functionalist argument pattern is on the table, however, we can see that it is in fact applied rather widely. In particular, it is the argument pattern that underlies Wimsatt’s notion of generative entrenchment (Schank and Wimsatt 1986; Wimsatt 2015). Generative entrenchment is a relational prop- erty of entities: the more the activity or structure of a complex system depends on the presence of a given entity, the greater that entity’s generative entrenchment, and the stronger the constraints it imposes on the evolutionary process. The emphasis is on dia- chronic dependencies: traits are generatively entrenched “to the degree that they have a number of later developing traits depending on them” (Wimsatt 1986, 198). Cuvieran functionalism, by contrast, also covers synchronic dependencies (cf. Wagner and Schwenk 2000; Schwenk and Wagner 2001). Models of generative entrenchment thus pro- vide one way of locally applying Cuvieran functionalist reasoning: they show that
gradual evolution is difficult in highly entrenched parts of an organism, and thus explain why evolutionary change is displaced onto other parts of an organism.
Mayr’s notion of typological thinking was meant to capture a pattern of thought in- herently at odds with Darwinian evolutionary theory—a pattern of thought he believed was pervasive enough to explain centuries’ worth of opposition to gradual evolution. We’ve seen that at least two distinct patterns of thought can generate anti-gradualism: natural state thinking and Cuvieran functionalism. We know that the former is incom- patible with Darwinian evolutionary theory (Sober 1980). The question is whether the latter fares any better. If it does not, the problems are more extensive than the loss of a particular explanation of deep conservation.