Stephen Jay Gould's new take on Charles Darwin

Saturday, August 3, 2002

Stephen Jay Gould's distinguished dual careers as a paleontologist and as a writer about science both, in their different ways, paid homage to Charles Darwin and the theory of evolution. Not long before his death in May, Gould published The Structure of Evolutionary Theory, a massive reorganization of the subject that incorporates the findings of 20th-century science.

When even people who get their science news from Saturday Night Live have heard about the sequencing of the human genome, and the genetic engineering of plants and animals, it's startling to be reminded that Darwin didn't know what genes were, let alone what they looked like or how to build them to order.

Of course he know that there was variation among the members of the same species, and that those variations could be inherited. It was central to his theory of natural selection. But he had no clue what the mechanism might be. We do, and we also know that important stretches of DNA are shared by species whose ancestors separated hundreds of millions of years ago. It's stunning confirmation of the fundamental truth of biology: all living creatures, past and present, are related by descent over geological time.

Gould characterizes the central argument of Darwin's 1859 Origin of Species as an attempt to develop a science of historical inference. We weren't around when dinosaurs roamed the earth, nor when the impact of a 10-km asteroid wiped them out 65 million years ago. We can't do experiments on the past. But we can fashion theories about what happened in the past, predict what the consequences of those happenings would have been, and look for present evidence of the consequences.

That is science, because it makes testable predictions that would disprove the theories if the predictions were false. The asteroid-impact theory predicted that a layer of iridium would be found worldwide in rocks dating from the last great extinction, with sharp discontinuities in the fossil record below and above that layer.

Gould's remodeling of Darwinism has three main branches, which he illustrates with a lovely 1670 drawing of a branched coral.

The first branch is Darwin's belief that natural selection operates primarily through competition between individuals in a species. Now it appears that it acts on multiple levels -- genes, cell lineages, organisms, demes (subpopulations of a species), species, and clades (a clade is a node on the evolutionary tree along with all its descendants). Among the radical features of Gould's theory of punctuated equilibrium (long periods of stability in a species "punctuated" by brief periods of rapid change) is its thesis that in macroevolution entire species act as Darwinian individuals, analogous with organisms within a species in microevolution.

(To anyone inclined to write and complain that macroevolution can't have happened because we have never seen it, please refer to asteroid-impact theory, above.)

The idea of species selection was not as foreign to Darwin's thought as scientists long thought. He discussed it at some length in a longer version of Origin that was not published until 1975. Most biologists have never read it, Gould notes, while the historians of science who did failed to recognize its importance.

The second branch of Darwinism was that natural selection as he described it had sufficient power to explain the origin of novel features and novel creatures, as well as eliminating the unfit and unsuccessful. That's true enough, but other mechanisms act as well.

Since different mechanisms operate at the various levels of selection, a feature selected at one level may be incidental at another, and available therefore for adaptation for another purpose. Early fish had both gills and lungs. Since the gills were sufficient for respiration, the lungs evolved into swim bladders. (Darwin discussed that example, but thought the evolution went in the opposite direction.)

The third branch was that change was gradual and mostly uniform -- a part of the theory not well supported by the jumpiness of the fossil record, as Darwin knew. But unexpected events such as the asteroid impact periodically rewrite the rules for survival. The dinosaurs thrived as long as size was an advantage; when it wasn't, they died. Some small mammalian species survived -- perhaps because they could scavenge the carcasses of dead dinosaurs until the sunlight came back? We don't know.

Without the asteroid, the dinosaurs might still dominate the world, "and mammals would still be rat-sized creatures living in the ecological interstices of their world," Gould conjectures.

And in that case, it's very unlikely indeed that anybody would be writing books about how it all came about.