A Theory Evolves
How evolution really works, and why it matters more than ever
This might seem like little more than a cruel parlor trick, and the resulting monstrosities would never survive in nature. But small changes in these master-switch genes may help explain some major changes in evolutionary history. This past winter, evo-devo biologists showed that an important animal transition 400 million years ago, when many-legged arthropods (think lobsters) gave rise to six-legged insects, was due to just a few mutations in a Hox gene. In the past few months, researchers have found that a change in the regulation of a growth-factor gene could have resulted in the first vertebrate jaw. And, incredibly, researchers reported in the journal Science last week that a single mutation in a regulatory gene was enough to produce mice with brains that had an unusually large, wrinkled cerebral cortex resembling our own. (No word, though, on whether the mutant mice gained smarts.)
Some critics of evolution argue that animals are so complex and their parts so interconnected that any change big enough to produce a new species would cause fatal failures. Call it the Microsoft conundrum. But just as Judge Thomas Penfield Jackson managed to delete that company's Web browser on his own computer without crashing the operating system, evo-devo biologists are learning how evolution can tweak one part of an animal while leaving everything else alone. The key to modifying the machine of life while it's running, says biologist Sean Carroll of the University of Wisconsin-Madison, is mutations in the stretches of DNA that homeotic proteins bind to.
"If you change a Hox protein, you might mess up the whole body," says Carroll. "But if you change a control element, you can change a part as small as a bristle or a fingernail." He explains that genetic accidents can set the stage by duplicating segments, creating spares that are free to evolve while the other segments carry on with their original function. Biologists now believe that appendages like insect wings and the proboscis a mosquito jabs you with evolved from spare leg segments.
Making do. This process may be rapid, but it's not elegant. Instead of inventing new features from scratch, evolution works with what it has, modifying existing structures by trial and error. The result is a messy legacy of complicated biochemical pathways and body parts that are more serviceable than sleekly designed. Although proponents of intelligent design (story, Page 52) hold that organisms are too "perfect" to have arisen by chance, science shows that organisms don't work perfectly at all; they just work.
While many scientists busy themselves figuring out the history and mechanics of evolution, others are already putting it to use. Jonathan Eisen of the Institute for Genomic Research in Rockville, Md., deciphers the information stored in organisms' genomes for clues to their ancestry and how they function. For him, evolution is as critical a tool as DNA-sequencing machines and supercomputers. "If I didn't approach everything with an evolutionary perspective," says Eisen, "I'd miss out on most of the information."
That's because genomes are the handiwork of evolution, and their origin can be key to making sense of them. Researchers analyzing the human genome, for example, reported finding a series of human genes that were also common in bacteria but absent from invertebrates like fruit flies. They concluded that bacterial genes had infiltrated vertebrate animals, helping to shape our genetic identity. But the explanation turned out to be more mundane. Knowing how evolution often prunes away unneeded genes, Eisen and several others showed most of the suspect genes had simply been dropped during the evolutionary history of flies. The moral of the story: "I'm begging people to treat evolution as a science and not just tack it on as an explanation afterwards," says Eisen.
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