By William J. Cromie

Gazette Staff

An elegant and direct example of evolution in action has been found by Harvard paleontologist Stephen Jay Gould while exploring a beach in the Bahamas.

Walking along the south shore of Great Inagua Island, the Agassiz Professor of Zoology noticed a telltale change in the shells of land snails scattered on a mud flat. He saw large, finger-shaped

shells of Cerion excelsior, an extinct snail once widespread in the Bahamas. Nearby were smaller, rounder, vertically striped shells with prominent whorls, belonging to a species called Cerion rubicundum, a well-known resident of the island.

"Scattered between them were thousands of highly variable shells spanning the full range of form from pure C. excelsior through intermediates of all degrees to C. rubicundum," Gould recalled. "It was difficult to escape the idea that the former had evolved into the later."

The history of life on Earth is written in shells, bones, footprints, and imprints found in mountains, canyons, and at the bottom of the seas. Over millions of years, movements of air, water, and continents destroy these silent signs, creating gaps in the natural record, or the fossils are scattered and compressed in different layers of sedimentary rocks. Gould, who is also curator of invertebrate paleontology at the Museum of Comparative Zoology, was looking down at a complete story of one species evolving into another in less than 20,000 years.

"I was enormously lucky," Gould comments. "It's rare to find such a continuous record unbroken by time and erosion."

He was at the right place at the right time, but only a prepared mind could read such a story. Gould has been studying land snails since he was a Ph.D. student at Columbia University in 1967. Cerion, a diverse group of many sizes and shapes, is a favorite of his. Gould has often written about snails in the half-dozen prize-winning books he has authored.

Examining the remains, Gould theorized that Great Inagua had been invaded by C. rubicundum some 20,000 years ago. "I have no idea where they came from," he admits.

Gould hypothesized that rubicundum replaced excelsior by interbreeding and hybridization. The oldest hybrids closely resemble excelsior. Younger and younger shells became more and more like rubicundum.

"Excelsior-like shells are worn and battered," Gould notes. "As the intermediates come more and more to resemble rubicundum, they look fresher and younger. Today, excelsior is extinct on the island, while rubicundum dominates."

Clocking Evolution

That was just a theory, however; Gould hadn't proved it. To do so, he enlisted the help of chemist Glenn Goodfriend of the Carnegie Institution of Washington, D.C. The two used chemical methods to date the extinct excelsiors, the extant rubicundums, and all the forms in between.

"We relied on a quick and cheap method to obtain the relative ages of the shells," Gould explained.

This method relies on nature's preference for left-handedness. Proteins in animals are made up of amino acids, which take one of two three-dimensional forms: they twist or fold in either a left-handed or a right-handed way. When made in a laboratory, half the amino acids come out left-handed, and half-right handed. But nature makes only left-handed protein building blocks.

When animals die, their amino acids start to turn right-handed, and the process continues until half of them become that way. The rate of change depends on temperature, rainfall, and other natural elements, but in the relatively unchanging geology of the Bahamas, Gould and Goodfriend decided they could use handedness as the slow ticking of a natural clock that would tell the ages of the snail shells.

Those ages would, of course, be relative. To pin them to the calendar of natural history, the researchers used a more expensive and time-consuming clock, the decay of radioactive carbon present in the shells.

The oldest hybrids, those most resembling excelsior, were alive about 17,000 years ago. The youngest fossils, those most resembling the rubicundum invaders, lived on Great Inagua as recently as 3,000 years ago.

The dates provided enough proof for Gould and Goodfriend to go public. They published their findings in the Dec. 13 issue of Science, the journal of the American Association for the Advancement of Science.

Evolution by Leaps

The discovery and dating supports a theory about how evolution occurs that Gould and Nils Eldridge of the University of Chicago put forth 20 years ago. Darwin saw evolution as a slow, gradual process. His successors theorized that an animal acquires genetic mutations that make it more fit for survival than other members of its species. Over millions of years, this "natural selection" creates a new species.

Gould and Eldridge propose a different scenario, specifically that evolution takes place in short, sudden leaps, interspersed with long periods of stability. "Short" is relative in the perspective of geology; it means thousands of years, rather than millions of years.

"Our work was not done to demonstrate punctuated evolution, but it supports the idea," Gould says.

Asked if the research might convince some creationists, who insist that a supreme being created each species separately, Gould answers with a smile. "So much good evidence exists for evolution that it's hard to deny," he insists. "But I'm sure they'll explain away our finding, as they do the other evidence. You can say that one snail evolving into another is like a Chihuahua and a Great Dane hybridizing and not like humans arising from apelike ancestors."

Another surprise involves the long survival of hybrids. Most evolutionists think of them as unstable, transient forms.

"From our dates, we conclude that the morphometric effects of hybridity endured for at least 13,000 years," Gould says. "This provides a striking example for substantial persistence of an evolutionary phenomenon often viewed as far more evanescent."

Gould and Goodfriend's work tells an evolutionary short story unique in completeness and elegant in form. "In favorable circumstances such as these, the fossil record can supply direct evidence for evolutionary change at the scale [of thousands of years], rather than the conventional scale of general trends over millions of years," they write. "The usual broad brush represents a fascinating scale in its own macroevolutionary right, but it cannot resolve the blips and fillips that mark the richness of life's history and constitute the only scale directly observable in our own historic time."