THERE'S NO SUCH THING AS SPECIES:

Why so cross?: a review of Unweaving the Rainbow by Richard Dawkins and The Pattern of Evolution by Niles Eldredge (Thomas Nagel, London Review of Books)

The biological problem that is the focus of the dispute is set out most clearly in Eldredge's book: what modification of the conception of the evolutionary process is required by the fact that the fossil record does not support Darwin's belief that evolution proceeded gradually, and at a more or less constant rate? The fossil record is of course very patchy, but what it seems to reveal are species that come into existence, persist largely unchanged, often for millions of years, and then become extinct. It does not reveal long sequences of gradually changing ancestors of new species, linking them by minute intergenerational variation to predecessor species. Nor does it reveal the kind of gradual, cumulative intraspecies evolution that finally results in a difference great enough to constitute a new species.

Rather, the time required for the appearance of a new species is apparently very short by comparison with the time during which it then persists largely unchanged. A species that appears in the geological blink of an eye - say, ten thousand years, too short a time for any transitional stages to show up as fossils - may stay the same for five to ten million years after that. The process by which a new species is formed is apparently too fast to show up in the fossil record, but too slow to be observed in human experience. 'No utterly convincing case of true speciation (that is, involving sexually reproducing organisms) has as yet emanated from a genetics lab,' Eldredge writes.

He believes that these facts are incompatible with an unmodified version of Darwin's theory, because Darwin believed in gradualism: that evolution proceeds at a constant rate, with gradual variations within species (of the kind commonly produced by animal breeders) leading to differences and branching that eventually become so great that they turn into separate species. In a sense, Eldredge says, Darwin didn't believe in the reality of species, as discrete entities. What was real were individuals - and the gradually developing differences between them. Species, however, do seem in some sense real: their distinctness and internal uniformity both at a time and over time are striking. Eldredge and Gould coined the term 'punctuated equilibria' to describe their theory of this non-gradual evolutionary process whereby short bursts of rapid change are followed by long periods of stasis.

Now this might seem like the locus of a profound disagreement within evolutionary theory. What gives rise to new species, suddenly, if not the slow process of incremental change that Darwin envisioned? Does it mean that creative forces of some kind are at work, generating radically new forms of life through a process internal to the genetic material? That would certainly be incompatible with the reductionist outlook of the traditional theory of natural selection.

Eldredge thinks nothing of the kind, however. He is as committed to the mechanism of natural selection as Dawkins is.

And, after all, isn't the true test of any scientific theory just how committed its adherents are to it in the face of the evidence?

Posted by Orrin Judd at October 16, 2006 12:00 AM