The paradox of the visibly irrelevant

Natural History, Dec-Jan, 1997 by Stephen Jay Gould

This study lacks a crucial piece of documentation that the Trinidadian guppies did provide -- an absence immediately noted by friendly critics and fully acknowledged by the authors. Losos and colleagues have not studied the heritability of leg length in Anolis sagrei and therefore cannot be certain that their results record a genetic process of evolutionary change. Perhaps leg length is development mentally plastic, so that the same genes yield longer legs if lizards grow up on trees and shorter legs if they always cavort in the bushes (just as the same genes can lead to a thin or fat human being depending upon a personal history of nutrition and exercise). In any case, however, a sensible and apparently adaptive change in average leg length has occurred within twenty years on several islands, whatever the cause of modification.

3. Snails from Great Inagua, Bahama Islands. Most of Great Inagua, the second largest Bahamian island (Andros wins first prize), houses a large and ribby Cerion species named C. rubicundum. But fossil deposits of no great age lack this species entirely and feature instead an extinct form named C. excelsior, the largest of all Cerion species. Several years ago, on a mud flat in the southeastern comer of Great Inagua, David Woodruff (of the University of California, San Diego) and I corrected a remarkable series of shells that seemed to span (and quite smoothly) the entire range of form from extinct C. excelsior to modern C. rubicundum. Moreover, and in general, the more eroded and "older looking" the shell, the closer it seemed to lie to the anatomy of extinct C. excelsior.

This situation suggested a local evolutionary transition by hybridization, as C. rubicundum, arriving on the island from an outside source, interbred with indigenous C. excelsior. Then, as C. excelsior declined toward extinction while C. rubicundum thrived and increased, the average anatomy of the population transformed slowly and steadily in the direction of the modern form. This hypothesis sounded good and sensible, but we could devise no way to test our idea -- for all the shells had been collected from a single mud flat (analogous to a single bedding plane of a geological stratum), and we could not determine their relative ages. The pure C. excelsior shells "looked" older, but such personal impressions (subject as they are to a researcher's bias) count for less than nothing in science. So we were stymied and put the specimens in a drawer.

Several years later, I teamed up with paleontologist and geochemist Glenn A. Goodfriend, from the Carnegie Institution of Washington. He had refined a dating technique based on changes in the composition of amino acids in the shell over time. By keying these amino acid changes to radiocarbon dates for some of the shells, we were able to estimate the age of each shell. A plot of shell age versus position on an anatomical spectrum from extinct C. excelsior to modern C. rubicundum produced a beautiful correlation between age and anatomy: the younger the specimen, the closer to the modern anatomy.