Thursday, February 09, 2006

Book review: Carroll, Endless forms

Being a big fan of Stephen Jay Gould, I jumped at this book when I read about it.

Sean B. Carroll, Endless forms most beautiful. The new science of Evo Devo. New York: W.W. Norton 2005. 350 pp. ISBN 0-393-06016-0.

Sean B. Carroll is a molecular biologist working in the field of evolutionary developmental biology, or Evo Devo for short. Evo Devo is the study of how DNA programming directs the construction of an animal body from a single cell onward through the stages of embryology. Carroll sets out to explain this new field to the interested layperson.

Most of us know that DNA codes for the proteins that form the bodies of animals. Many also know that the vast majority of base pairs in an animal’s genome are just noise: they don’t code for anything at all and seem never to be transcribed during development. This book is about a third kind of information in DNA, also forming a minority of the base pairs: tool kit genes.

A bucket of mixed proteins doth not a wildebeest make. Biologists have long realised that DNA must not only code for the building blocks of animals, but also contain assembly instructions like the ones provided with an IKEA clothes cupboard or a model airplane kit. Everyone assumed that these instructions would be unique to each species so that the study of how mice are built would be irrelevant to the study of fruit fly development.

Let me suggest a computer analogy. Computer programming boils down to processor instructions that perform small tasks such as comparing two register bits and turning a third bit on if the two are identical. But writing a computer program in processor instructions is a huge drag, it takes enormous amounts of time and quickly becomes unwieldy. Instead, you write programs in a higher-level language such as C, Perl or Java, and then a computer program called a compiler breaks down your high-level instructions into tiny steps and translates them into the processor’s native language. Biologists used to believe that every species had its own high-level language: that mice were coded in C and fruit flies in AutoLisp.

Evo Devo has proved this assumption to be false. All animals are coded in slightly varying dialects of a single high-level language. This is because the genetic toolkit evolved very early in the history of multicellular life. And it means, for instance, that a spinneret on a spider’s abdomen that produces cobwebs is actually a homolog of my arms and legs. Biologists can now also make inferences about, for instance, the genome of a trilobite species.

Mutations can modify building block genes, but it seems that the most important road to change in form is mutation in tool kit instructions and the genetic switches they operate on. For instance, the instruction “build twenty body segments, all with legs” may be overridden by a new instruction to skip legs on certain body segments. As can be expected from computer code evolving by chance mutation, the resultant programs are huge bundles of spaghetti, rarely taking the closest road to the current goal and containing a lot of redundancy. But that’s also what makes the code robust: remove or change an instruction in a highly optimised program, and it simply stops.

I learned a lot from this book and found it a fairly pleasurable read. Of course, Carroll is no Gould, neither as to breadth of erudition nor as to prose style. But he knows his Evo Devo, he explains it well, he discusses human evolution, and he takes an articulate stand against science illiteracy and religious interference in public education.

Other reviews of the book: H. Allen Orr, D. Brown, P.Z. Myers.

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