“Problems” with Evo-Devo

Synopsis: In Endless Forms Most Beautiful, Sean B Carroll makes several controversial claims regarding evolutionary biology. Here I briefly discuss several of these claims

In my developmental biology course, we are 1) reading Sean B Carroll’s Endless Forms Most Beautiful and 2) expected to blog any thoughts we have about the course.

I didn’t feel the first few chapters of the book had much substance – a lot of fluff about how animals are cool and why form is important and personal anecdotes. It wasn’t until we discussed the readings in class that the substance was really allowed to be pulled out.

A major theme that I want to focus on is “why might a biologist have a problem with the book?” Evolutionary developmental biology is a controversial subject in regards to how important it is and how it relates to the Modern Synthesis. We pulled out several controversial points Carroll makes:

1) This book (and evo-devo in general, I think) is decidedly zoocentric. Plants, protists and bacteria are not discussed much. Thankfully, we will discuss plant development later on in the course but plant evo-devo is just not as widely studied. Protists and bacteria have arguable roles in the study of evo-devo due to their lack of multicellularity and other requirements for development, but their study does have consequences for development. After all, we and bacteria share many genes and developmental processes most likely originated in the bacteria. It is important to keep in mind, however, that animals are only a single branch of the evolutionary tree and evo-devo’s applicability may be limited.

2) Some evo-devo supports small changes in the genome having larger consequences in the development of patterns and form. This was a divisive debate in the early 1900s with saltationists and their “monsters” losing against the gradualists leaving the latter as the orthodox view in the Modern Synthesis. I personally feel like this is a false dichotomy – I’m sure both “saltationist” mutations and “gradualist” mutations have their roles.

3) Relatedly, Carroll argues that “it turns out that [diversity] is not where we have been spending most of our time for the past forty years” (11), i.e., diversity is produced by changes in regulatory sequences, not protein sequences (including transcription factors). To make an extreme case for one over the other is clearly a false dichotomy, as evolgen noted back in 2006. (This debate is apparently called the cis- vs trans- debate.)

I am not aware of the literature in this topic, but I think more interesting questions could be asked, such as do changes in regulatory sequences have a generalizable predictable effect vs changes in protein sequences having other kinds of effects? Stuff like that. Hopefully I’m just ignorant and they are asking those questions.
Also, Carroll says biologists have been looking at the wrong parts of the genome for 40 years which I’m sure ticks off a few people…

4) Carroll and other developmental biologists don’t like that development was excluded from the Synthesis. I have no idea why that happened. I do have a book on loan from the library at the moment (The Changing Role of the Embryo in Evolutionary Thought by Ron Amundson), that surely discusses the issue but I don’t think I have the time to read it.

5) Due to development’s exclusion from the Synthesis, Carroll envisions an “evo-devo revolution” and “Act III of the Modern Synthesis.” Those are fighting words to be sure, and well, I honestly don’t have much of an opinion on whether evo-devo really counts as a revolution. I don’t care much for the word “revolution” itself and generally ignore its use. I do think that evo-devo is important work and will contribute greatly to our knowledge of how evolution has proceeded in animals (and hopefully plants). The Synthesis won’t be the law of the land forever.

As a side note, I really don’t understand what the fuss is all about from the “anti-” evo-devo crowd. Again, I don’t have much experience in this topic but I do know that Michael Lynch takes a shot at Carroll at the end of Origins of Genome Architecture. From what I remember, Lynch claimed Carroll argued for a re-defining of evolution (of sorts) that included changes in form, largely because that is what people care about. Lynch argues against this point by saying evolution is “change in allele frequencies” and reality disagrees with Carroll. Both authors have good points and I’m not sure why Lynch disagrees with Carroll so vehemently – after all, they both seem to argue for an expansion of the Modern Synthesis.

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Carroll, Sean B. Endless Forms Most Beautiful: The New Science of Evo Devo. 2005.

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