Endless Forms Most Beautiful by Sean B Carroll is an excellent book. Both easy to read and understand in addition to containing some gorgeous pictures, Carroll makes his case for evo-devo clear and to the point. However, the book is not without its flaws, and I would like to discuss them here.
Adaptationism. Natural selection plays a major role in the book, and little mention is made of how genetic drift, neutral evolution, and constraints may factor into evo-devo. These may be difficult to determine as of now, but Carroll automatically attributes morphological changes to natural selection as the trait must be an adaptation. This is clearly premature and as Michael Lynch notes in the final chapter of Origins of Genome Architecture, evo-devo has to work within our current population genetic framework to be integrated into evolutionary theory.
Nebulousness of switches. Although switches make sense logically, they seem like a deus ex machina that can explain almost any developmental change. I want to see some more actual evidence for such switches. What do these switches do? Why do they affect gene regulation in such a way? How do they change the topology of DNA? How does a switch evolve to perform a particular function, spatially and temporally? Believing they exist is one thing, but I want to know how and why those switches exist and what they do. For such a key piece of his argument, I believe it is just too simple to say a switch must have evolved. I am surprised Carroll does not bother to further evaluate this aspect of gene regulation.
Zoocentrism. The book is clearly centered on animals and other groups are clearly missing. Not only that, but non-vertebrates and non-insects are barely mentioned – how does evo-devo affect cnidaria, for example? Where do the Hox genes come from? What about plants? Does evo-devo change our perspective of bacteria and protists in any way? These are obvious questions, in my opinion, and Carroll does not spend any time answering them.
Bench-centrism. Carroll’s work (and much of evo-devo) has primarily focused on model organisms and benchwork. This is extraordinarily useful, of course, but more work must be done on understanding how these developmental changes work in an ecological context. Eco-devo, if you will. This is an important and prominent area in the field and that Carroll fails to discuss it is a major flaw in the book. In the spirit of my following comments, to not include ecology severely limits the importance of evo-devo.
Revolution. Carroll also makes the claim that evo-devo is a third “revolution” in biology following Darwin and the Modern Synthesis. I do not buy this claim. My previous comment mentions how Carroll’s evo-devo is limited to animals, a very small branch of the evolutionary tree, and the same idea applies here: Darwin and the Modern Synthesis affected how we perceived ALL of life, not just a single group. For evo-devo to count as a revolution in biology, in my opinion, it has to affect more than just the animals.
Instead, evo-devo seems to fit in nicely with the Modern Synthesis as is – it just adds embryology and development to the system (after they were left out in the 1940s). In fact, Carroll makes strong cases against “hopeful monsters” and saltationism in favor of gradualism, an important tenet of the synthesis. Adding switches does not change population genetics either as far as I can tell – it just makes things a lot more complicated for molecular ecologists!
Evo-devo is clearly an important development – it helps explain how different animal forms can evolve with little change to structural gene sequences. Instead of revolutionizing biology, it fills in a ton of gaps. We can now know more about why and how animal diversity arose.
Despite my negative comments, Endless Forms Most Beautiful really is a good book. Chapter 5, particularly, which is about genetic switches and combinatorial logic, illustrates the potential power of evo-devo – more evidence is just needed to fulfill that theory. Hopefully my criticisms are already being worked out as the field has progressed since 2005 as evo-devo has some major potential in elucidating how evolution has worked and hopefully in more than just animals.