This chapter addresses the concern that evo-devo is perniciously typological. This project runs into an immediate difficulty: the notion of typological thinking was developed by Ernst Mayr (1959), but Mayr’s understanding of it was notoriously unprincipled (Witteveen 2015, 2016). As a result, when one surveys the range of discussions of evo- devo that attribute to it typological thinking, one finds a striking diversity of ways of
understanding the notion (Amundson 1998; Jenner 2006; Brigandt 2007; Lewens 2009a, 2009b; Love 2009a; Riegner 2013).
Accordingly, the first aim of this chapter is to understand in a principled way what it might mean for evo-devo to be typological. I argue that one of the functions of Mayr’s notion of typological thinking was to understand what was common to all critics of grad- ualistic evolutionary theories. Typology, for Mayr, was closely connected to anti-gradu- alism: a typologist could only be either a saltationist or against evolutionary theory alto- gether. Moreover, one of the reasons that evo-devo is often considered typological is its central concern with the deep conservation of animal body plans. But the problem of ex- plaining deep conservation is precisely that of explaining why something has failed to evolve. It’s thus a promising place to look for “typological” thinking in evo-devo.
Mayr’s project, however, was a failure: there is no one strand of thought common to all anti-gradualists. In the chapter, I distinguish two distinct threads: natural state think- ing (given a precise account by Sober 1980) and Cuvieran functionalism. Natural state thinking involves modeling systems as having a natural state from which they can be deflected by an interfering force, but to which they will tend to revert when the force is removed. Many early critics of Darwin opposed his theory on such grounds (Jenkin 1867; Mivart 2009).
Georges Cuvier, by contrast, opposed gradualistic evolutionary theories on other grounds (Coleman 1964; Burkhardt 1977; Russell 1982). His argument had three steps: (1) show that organisms are highly functionally integrated, (2) argue on that basis that inter-
and (3) conclude that gradual evolution was therefore impossible. Though functionalist (it emphasizes the functional integration of organisms), this argument is not functionalist in the same sense that Darwin’s reasoning was functionalist (Russell 1982). Specifically, where Darwin focused on organism-environment relations, Cuvier emphasized relations between parts internal to the organism. Cuvieran functionalism marks a second path to anti-gradualism, distinct from natural state thinking.
The second aim of this chapter is to show that Cuvieran functionalism plays an im- portant role in evo-devo’s explanation of the deep conservation of animal body plans. The task of explaining deep conservation is challenging: it requires explaining why parts that have (a) varied and (b) been exposed to numerous distinct environments have none- theless failed to undergo evolutionary change. Evo-devo does this by applying a localized form of Cuvieran functionalism. According to this localized Cuvieran functionalism, there are certain regions of the genome, called GRN “kernels”, that (a) underlie the de- velopment of deeply conserved elements of body plans, (b) are themselves deeply con- served (their topology is identical or nearly so in widely diverged taxa), and (c) are char- acterized by extensive positive feedback (Davidson and Erwin 2006; Peter and Davidson 2015, chap. 7). They are integrated in the sense that any disruption of the linkages in the kernel destroys overall kernel function, leading to the production of organisms that lack, e.g., a heart. This combination of functional integration and essential developmental role explains why they (and their associated body plan elements) have failed to evolve for hundreds of millions of years.
The third and final aim of this chapter is to show that Cuvieran functionalism is com- patible with exclusive population thinking. The basic idea behind exclusive population thinking is that evolutionary causes are fundamentally causes that occur within inter- breeding populations (there are exceptions to this, e.g., Erwin 2000, 2011; Doolittle 2017, but they do not help to vindicate Cuvieran functionalism). Once populations diverge (once gene flow ceases), they pursue independent evolutionary trajectories. Exclusive population thinking, on this basis, forbids invoking evolutionary causes that transcend populations. I argue that exclusive population thinking has a strong and a weak interpre- tation, based on how one understands “transcends”. On the strong interpretation, exclu- sive population thinking forbids any explanation that unifies what is going on in inde- pendently evolving lineages. Each population requires its own, unique explanation. But this, I argue, rules out explanations well beyond evo-devo, such as explanations of eco- logical trends (which by definition transcend the single species). Such explanations unify by showing how independently evolving populations are responding similarly to similar causes. A weaker, more defensible understanding of exclusive population thinking should permit such explanations, which respect that populations without gene flow are following independent (but similar) evolutionary trajectories.
This weaker understanding of exclusive population thinking allows for Cuvieran functionalist explanations. Precisely because GRN kernels are preserved, and because disruption of them leads to developmental catastrophes that are fatal in all naturally oc- curring environments, they will give rise to similar selection pressures in all members of
lineages, but only because the same causes is repeated within each individual lineage. Exclusive population thinking does not forbid Cuvieran functionalist explanations. Re- casting the role of typological thinking in evo-devo as the role of Cuvieran functionalism in evo-devo thus allows us to see how evo-devo can complement, rather than conflict with, synthesis-derived evolutionary theorizing.