Only a Theory by Kenneth R. Miller

Author:Kenneth R. Miller
Language: eng
Format: epub
Publisher: Penguin Publishing Group
Published: 2010-02-28T16:00:00+00:00

REWRITING THE PLAN

If the concept of facilitated variation has begun to revolutionize our understanding of evolution at the cellular level, developments at a somewhat higher level are even more exciting. For centuries biologists who studied development gazed in wonder as the embryos of many animal species went through their intricate choreographies of growth, movement, and change to become the fully formed organisms that emerged from egg or womb or cocoon. Indeed, when I studied development in college, embryology was largely a descriptive science, punctuated by an experiment here or there hinting at the genetic and biochemical controls that surely lay just beyond our understanding.

In the last quarter century all of that has changed. One could attribute this progress to any number of researchers, but my first choice would be Ed Lewis, an unassuming biologist who worked at the California Institute of Technology on the genetics of Drosophila (fruit fly) development. Lewis had been trained in the honored tradition of fruit fly genetics established by the first great American geneticist, Thomas Hunt Morgan, and understood the significant advantages of the fly as an experimental system. Drosophila is easily bred in the lab, has a short generation time, and produces plenty of offspring, all of which make it ideal for genetic analysis. The fly, like many other animals, develops its body through a process of segmentation. Its segments, more than a dozen in number, are most easily seen in the fly’s larvae (commonly called maggots). When the larval stage is finished, a pupa is formed, and after a few days an adult fly emerges from it. Earlier researchers had shown that each one of the segments in the larva formed a particular part of the adult fly’s body, and Lewis’s interest was in identifying the genes that told a particular segment what to become in the adult.

Lewis discovered that a series of genes, now known as the Hox genes, determine the fates of each segment of the developing fly. One gene was turned on in the very front segment, another in the next, and still another in the next. Interfere with these genes, by design or by accident, and what might emerge was a fly with an extra set of wings, or with feet dangling from where its antennae or mouth parts should be. Lewis’s clever experiments demonstrated that while hundreds of genes might determine the detailed nature of each of these body parts, only a few actually call the tune. At once, the Hox genes were elevated to the status of master control genes, guiding the process of development.

But there were two unexpected surprises in this little group of genes. The first was almost too convenient to be true: The genes were found on a single chromosome, a single piece of DNA in the fly’s genome, and they were arranged in the exact order in which they were expressed in the fly itself. This extraordinary lineup took biologists by surprise, but it was nothing compared with the next shock. The Hox

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