Saturday, March 30, 2013

Learning About Evo-Devo

We talked about evolutionary developmental biology (evo-devo) in my class last week. The main issue is whether the proponents of evo-devo are making a substantive contribution to evolutionary theory. Is evo-devo going to be part of an extended modern synthesis, and, if so, how? My own view, which I express to the class, is that the discoveries of developmental biology pretty much confirm what Stephen J. Gould wrote in Ontogeny and Phylogeny back in 1977.

What, then, is at the root of our profound separation? King and Wilson argue convincingly that the decisive differences must involve the evolution of regulation: small changes in the timing of development can have manifold effects upon a final product "Small differences in the timing of activation or in the level of activity of a single gene could in principle influence considerably the systems controlling embryonic development. The organismal differences between chimpanzees and humans would then result chiefly from genetic changes in a few regulatory systems, while amino acid substitutions in general would rarely be a key factor in major adaptive shifts." Differences in regulation may evolve by point mutations of regulatory genes or by rearrangement of gene order caused by such familiar chromosomal events such as inversion, translocation, fusion, and fission. Studies of banding indicate that at least one fusion and ten large inversions and translocations separate chimps and humans.

Stephen J. Gould (1977) Ontogeny and Phylogeny, Harvard University Press, Cambridge Massachusetts, USA pp. 405-406
This helps us understand the history of life, especially the evolution of animals, but it doesn't contribute to evolutionary theory.

PZ Myers is teaching a developmental biology course and his students are dealing with three take-home questions this weekend [What I taught today: O Cruel Taskmaster!]. I'd like to reproduce two of them here since they're very relevant to the debate over the importance of evo-devo.
Question 1: One of the claims of evo devo is that mutations in the regulatory regions of genes are more important in the evolution of form in multicellular organisms than mutations in the coding regions of genes. We’ve discussed examples of both kinds of mutations, but that’s a quantitative claim that won’t be settled by dueling anecdotes. Pretend you’ve been given a huge budget by NSF to test the idea, and design an evodevo research program that would resolve the issue for some specific set of species.
I'd like my students to keep in mind Richard Lenski's ongoing evolution experiment in E. coli. Recall that evolution of the ability to grow on citrate depended mostly on mutations that changed the regulation of citrate utilization genes.

Since we have many examples of mutations that affect regulation of gene expression in bacteria, yeast, and other single-cell organisms, why do the proponents of evo-devo think they're on to something special when they look at development in animals? What is there about the evolution of "form" that changes our views on evolution?
Question 2: Every generation seems to describe the role of genes with a metaphor comparing it to some other technology: it’s a factory for making proteins, or it’s a blueprint, or it’s a recipe. Carroll’s book, Endless Forms Most Beautiful, describes the toolbox genes in terms of “genetic circuitry”, “boolean logic”, “switches and logic gates” — he’s clearly using modern computer technology as his metaphor of choice. Summarize how the genome works using this metaphor, as he does. However, also be aware that it is a metaphor, and no metaphor is perfect: tell me how it might mislead us, too.
Before answering PZ's question about Sean Carroll and metaphors, I'd like my students to remember the quotation I gave them in class. Discuss the use of hyperbole and metaphor in this context.

The key to understanding form is development, the process through which a single-celled egg gives rise to a complex, multi-billion-celled animal. This amazing spectacle stood as one of the great unsolved mysteries of biology for nearly two centuries. And development is intimately connected to evolution because it is through changes in embryos that changes in form arise. Over the past two decades, a new revolution has unfolded in biology. Advances in developmental biology and evolutionary developmental biology (dubbed “Evo Devo”) have revealed a great deal about the invisible genes and some simple rules that shape animal form and function. Much of what we have learned has been so stunning and unexpected that it has profoundly reshaped our picture of how evolution works. Not a single biologist, for example, ever anticipated that the same genes that control the making of an insect’s body and organs also control the making of our bodies.

This book tells the story of this new revolution and its insights into how the animal kingdom has evolved. My goal is to reveal a vivid picture of the process of making animals and how various kinds of changes in that process have molded the different kinds of animals we know today and those from the fossil record.

Sean B. Carroll Endless Forms Most Beautiful: The New Science of Evo Devo, W.W. Norton & Co., New York (2005) p. x
I'd also like Sandwalk readers to keep in mind the recent ENCODE publications. They talked extensively about genetic circuits and regulation. In fact, their major "finding" was the idea that our genome is full of regulatory elements; so many, in fact, that most of what we thought was junk DNA is actually part of a a vast control circuit. Has this emphasis on a multitude of switches and controls been misleading or is it turning out to be correct?

I would ask a third question. The evolution of toolkit genes (i.e. transcription factors) make it possible to evolve many different body plans with only a small number of mutations. It helps explain the Cambrian explosion. Given our current understanding of evolution, is it possible to select for a evolution of a toolkit that has this potential for future evolution? Explain your answer.