As the worm turns

Natural History, Feb, 1997 by Stephen Jay Gould

Examples of this primary reversal of standard theory have been accumulating for the past fifteen years. I have written three previous essays on earlier cases. In 1995, for only the second time in history, the Nobel Prize honored an evolutionary study--as my colleagues Edward Lewis, Christiane Nusslein-Volhard, and Eric Wieschaus won the award for their work on unraveling the developmental genetics of the fruit fly Drosophila and discovering homologs of the same genes in vertebrates.

In the first, path breaking case, the homeotic genes of insects, responsible for specifying the separate identities of segments along the main body axis (by orchestrating the growth of antennae, mouthparts, legs, and so on in their proper places), were also discovered, in minimally altered form, in vertebrates. (The homeotic genes were first recognized in oddball mutants with body parts in the wrong places--legs growing out of the head where antennae should be, for example. In Drosophila, the homeotic genes occur in two arrays on a single chromosome. Interestingly, in vertebrates, these same arrays exist in multiple copies as four sequences on four separate chromosomes.) These vertebrate homologs do not control the basic segmentation of the vertebral column (so insect segments are not simple homologs of vertebrae, as Geoffroy had originally proposed). But the homeotic genes of vertebrae do regulate the embryonic segmentation of the mid- and hind-brain, and they do strongly influence other important repetitive structures, including the positioning of cranial nerves along the body axis.

A second case then seriously compromised the classic textbook example of convergence--the paired eyes of three great phyla: vertebrates, arthropods (with the multiple-faceted eye of flies as a primary example), and mollusks (particularly the complex lens eye of squids, so similar in function to our own, but built of different tissues). We had always assumed that eyes in the three phyla must have completely independent evolutionary origins because they differ substantially in basic anatomy. And we viewed this supposed convergence as a premier example of natural selection's power to produce organs of similar and optimal function, but built from different materials and evolved from entirely separate starting points.

But we now know that eyes in all three phyla share an inherited embryological pathway largely orchestrated by a gene (called Pax-6 in its vertebrate form) retained in all these phyla from a common ancestor--and remaining similar enough to work interchangeably (for the fly version will induce the formation of eyes in vertebrates, and vice versa). The end results vary substantially (the multifaceted fly eye is not homologous with our single-lens eye), but the embryological blueprints share a common ancestry, and the eyes of different phyla can no longer be viewed as a case of pure convergence.

The reversal of opinion during the past decade has been astonishing. Mayr argued that we shouldn't even bother to look for genetic homology and shared embryological pathways between distinct phyla. We have now moved to the opposite pole of being surprised when we identify a basic gene of developmental architecture in Drosophila and then do not find a homolog in vertebrates. Charles B. Kimmel began a recent paper on this subject (Trends in Genetics, September 1996) by writing: "We have come to find it more remarkable to learn that a homolog of our favorite regulatory gene in a mouse is not, in fact, present in Drosophila than if it is, given the large degree of evolutionary conservation in developmentally acting genes."