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By Root 631 0

we humans can share so many genes with other creatures quite different from us is that, although the genes might be the same, the sequences making up switches have often evolved to be different. Small changes in switches can produce very different patterns of genes turning on and off during development. Thus, according to Evo-Devo, the diversity of organisms is largely due to evolutionary modifications of switches, rather than genes. This is also the reason that large changes in morphology—possibly including speciation—can happen swiftly in evolutionary time: the master genes remain the same, but switches are modified. According to Evo-Devo, such modifications—in the parts of DNA long thought of as “junk”—are the major force in evolution, rather than the appearance of new genes. Biologist John Mattick goes so far as to say, “The irony … is that what was dismissed as junk [DNA] because it wasn’t understood will turn out to hold the secret of human complexity.”

One striking instance of Evo-Devo in action is the famous example of the evolution of finches’ beaks. As I described in chapter 5, Darwin observed large variations in beak size and shape among finches native to the Galápagos Islands. Until recently, most evolutionary biologists would have assumed that such variations resulted from a gradual process in which chance mutations of several different genes accumulated. But recently, a gene called BMP4 was discovered that helps control beak size and shape by regulating other genes that produce bones. The more strongly BMP4 is expressed during the birds’ development, the larger and stronger their beaks. A second gene, called calmodulin, was discovered to be associated with long, thin beaks. As Carol Kaesuk Yoon reported in the New York Times, “To verify that the BMP4 gene itself could indeed trigger the growth of grander, bigger, nut-crushing beaks, researchers artificially cranked up the production of BMP4 in the developing beaks of chicken embryos. The chicks began growing wider, taller, more robust beaks similar to those of a nut-cracking finch …. As with BMP4, the more that calmodulin was expressed, the longer the beak became. When scientists artificially increased calmodulin in chicken embryos, the chicks began growing extended beaks, just like a cactus driller …. So, with just these two genes, not tens or hundreds, the scientists found the potential to re-create beaks, massive or stubby or elongated.” The conclusion is that large changes in the morphology of beaks (and other traits) can take place rapidly without the necessity of waiting for many chance mutations over a long period of time.

Another example where Evo-Devo is challenging long-held views about evolution concerns the notion of convergent evolution. In my high school biology class, we learned that the octopus eye and the human eye—greatly different in morphology—were examples of convergent evolution: eyes in these two species evolved completely independently of one another as a consequence of natural selection acting in two different environments in which eyes were a useful adaptation.

However, recent evidence has indicated that the evolution of these two eyes was not as independent as previously thought. Humans, octopi, flies, and many other species have a common gene called PAX6, which helps direct the development of eyes. In a strange but revealing experiment, the Swiss biologist Walter Gehring took PAX6 genes from mice and inserted them into the genomes of fruit flies. In particular, in different studies, PAX6 was inserted in three different parts of the genome: those that direct the development of legs, wings, and antennae, respectively. The researchers got eerie results: eye-like structures formed on flies’ legs, wings, and antennae. Moreover, the structures were like fly eyes, not mouse eyes. Gehring’s conclusion: the eye evolved not many times independently, but only once, in a common ancestor with the PAX6 gene. This conclusion is still quite controversial among evolutionary biologists.

Although genetic regulatory networks directed by master genes