October 19, 2005

Light Switches, Eh?

At long last, the readable layman's primer on evolutionary development (or as the cool kids apparently say, "evo devo") I've been waiting for, courtesy of H. Allen Orr in the New Yorker:
Why, then, do different creatures look so different? How do penguins and people emerge from the same genes? Evo devo's answer to this question represents its second big finding. Different animal designs reflect the use of the same old genes, but expressed at different times and in different places in the organism. As embryos, penguins might express one combination of genes in their limbs (and the result is wings), while people might express another (and the result is arms).

The basis of this selective expression involves that part of the DNA which is noncoding. Most genes, like most light fixtures, have "switches" near them. These switches, which are made of DNA, affect only whether a gene is on in a particular cell at a particular time; they do not change the actual protein coded by a gene. One switch might specify whether a gene should be on in the pancreas and another whether it should be on in adults. What’s special about many of those tool-kit genes is that they make proteins that toggle these switches. If a tool-kit protein finds and binds to a switch, it insures, through a complex molecular choreography, that a certain gene is expressed (or, in some cases, not expressed). In effect, tool-kit proteins act like molecular fingers, reaching out and physically turning on or off the switches that sit next to genes. … If all goes well, each of the possibly trillions of cells in an animal’s body will express just the right genes: insulin in your pancreas, not in your eye.

The real excitement about evo devo, however, has to do with its third claim. Carroll and others have taken the next, and by far the most radical, step and argue that evolution is mostly a matter of throwing these switches.
Undeniably cool. Though when I first saw the article's subheading ("A revolution in the field of evolution?") it looked like, egad, we were in for yet another full-length treatment of intelligent design. Not so, happily. Which brings me back to the most infuriating part of the entire ID "debate": namely, that there are so many genuine controversies in the field of evolution out there, most of them actually interesting, that it seems like a shameful waste of paper to spend time dithering over a bunch of creationists who pretend not to understand how light-sensitive receptors could evolve into eyes. In not-really-related news, except insofar as I lump all things vaguely related to biology together, it's not a good idea to stop worrying and embrace the world's invasive species.
-- Brad Plumer 12:58 PM || ||