From Natural History, April, 2002, page 79
The Nature of Change
Eugenie C. Scott
Evolutionary mechanisms give rise to basic structural differences.
(reply to Elusive Icons of Evolution by Jonathan Wells)
Without defining "design," Wells asserts that "many features of living things appear to be designed." Then he contrasts natural selection (undirected) with design (directed), apparently attempting to return to the pre-Darwinian notion that a Designer is directly responsible for the fit of organisms to their environments. Darwin proposed a scientific rather than a religious explanation: the fit between organisms and environments is the result of natural selection. Like all scientific explanations, his relies on natural causation.
Wells contends that "Darwin's theory cannot account for all features of living things," but then, it doesn't have to. Today scientists explain featuresof living things by invoking not only natural selection but also additional biological processes that Darwin didn't know about, including gene transfer, symbiosis, chromosomal rearrangement, and the action of regulator genes. Contrary to what Wells maintains, evolutionary theory is not inadequate. It fits the evidence just fine.
Reading Wells, one might not realize the importance of the Grants' careful studies, which demonstrated natural selection in real time. That the drought conditions abated before biologists witnessed the emergence of new species is hardly relevant; beak size does oscillate in the short term, but given a long-term trend in climate change, a major change in average size can be expected. Wells also overstates the importance offinch hybridization: it is extremely rare, and it might even be contributing to new speciation. The Galapagos finches remain a marvelous example of the principle of adaptive radiation. The various species, which differ morphologically, occupy different adaptive niches. Darwin's explanation was that they all evolved from a common ancestral species, and modern genetic analysis provides confirming evidence.
Wells admits that natural selection can operate on a population and correctly looks to genetics to account for the kind of variation that can lead to "new features in new species." But he contends that mutations such as those that yield four-winged fruit flies do not produce the sorts of anatomical changes needed for major evolutionary change. Can't he see past the example to the principle? That the first demonstration of a powerful genetic mechanism happened to be a nonflying fly is irrelevant. EdwardLewis shared a Nobel Prize for the discovery of the role of these genes, known as the Ubx complex. They are of extraordinary importance because genes of this type help explain body plans—the basic structural differences between a moUusk and a mosquito, a sponge and a spider.
Ubx genes are among the HOX genes, found in animals as different as sponges, fruit flies, and mammals. They turn on or off the genes involved in— among other things—body segmentation and the production of appendages such as antennae, legs, and wings. What specifically gets built depends on other, downstream genes. The diverse body plans ofarthropods (insects, crustaceans, arachnids) are variations on segmentation and appendage themes, variations that appear to be the result of changes in HOX genes. Recent research shows that fly Ubx genes suppress leg formation in abdominal segments but that crustacean Ubx genes don't; a very small Ubx change results in a big difference in body plan.
Mutations in these primary on/off switches are involved in such phenomena as the loss of legs in snakes, the change from lobe fins to hands, and the origin ofjaws in vertebrates. HOX-initiated segment duplication allows for anatomical experimentation, and natural selection winnows the result. "EvoDevo"—the study of evolution and development— is a hot new biological research area, but Wells implies that all it has produced is crippled fruit flies.
Wells argues that natural explanations are inadequate and, thus, that "students should also be taught that design remains a possibility." Because in his logic, design implies a Designer, he is in effect recommending that science allow for nonnatural causation. We actually do have solid natural explanations to work with, but even if we didn't, science only has tools for explaining things in terms of natural causation. That's what Darwin did, and that's what we're trying to do today.
Eugenie C. Scott holds a Ph.D. in physical anthropology. In 1987, after teaching physical anthropology at the university level for fifteen years, she became executive director of the National Center for Science Education. She is currently also the president of the American Association of Physical Anthropologists.
Special Report: Intelligent Design?
The Challenge of Irreducible Complexity by Michael J. Behe
The Flaw in the Mousetrap by Kenneth R. Miller
Detecting Design in the Natural Sciences by William A. Dembski
Mystery Science Theatre by Robert T. Pennock
Elusive Icons of Evolution by Jonathan Wells
The Nature of Change by Eugenie C. Scott
The Newest Evolution of Creationism by Barbara Forrest