The paradox of the visibly irrelevant

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

An odd principle of human psychology, well known and exploited by the full panoply of prevaricators, from charming barkers like Barnum to evil demagogues like Goebbels, holds that even the silliest of lies can win credibility by constant repetition. In current American parlance, these proclamations of "truth" by xeroxing -- if sufficiently benign to do little harm, yet embraced with all the force of a dictum running "from God's mouth to your ear" -- fall into the fascinating domain of "urban legends."

My favorite bit of nonsense in this category hits me daily, and in very large type, thanks to a current billboard campaign by a company that win remain nameless. The latest version proclaims: "Scientists say we use 10 percent of our brains. That's way too much." Just about everyone regards the "truth" of this proclamation as obvious and incontrovertible -- although you might still start a barroom fight over whether the correct figure should be 10, 15, or 20 percent (I have heard all three asserted with utter confidence). But this particular legend is even worse than false, for the statement is truly meaningless and nonsensical. What do we mean by "90 percent unused"? What is all this superfluous tissue doing? The claim, in any case, can have no meaning until we develop an adequate theory about how the brain works. We don't even have a satisfactory account for the neurological basis of memory and its storage -- surely the sine qua non for formulating any sensible notion about unused percentages of brain matter! (I think that the legend developed because we rightly sense that we ought to be behaving with far more intelligence than we seem willing to muster -- and the pseudoquantification of the urban legend acts as a falsely rigorous version of this legitimate, but vague, feeling.)

In my field of evolutionary biology, the most prominent urban legend -- another "truth" known by "everyone" -- holds that evolution may well be the way of the world, but one has to accept the idea with a dose of faith because the process occurs far too slowly to yield any observable result in a human lifetime. Thus, we can document evolution from the fossil record and infer it from the taxonomic relationships of living species, but we cannot see evolution on human timescales "in the wild."

In fairness, we professionals must shoulder some blame for this utterly false impression about evolution's invisibility in the here and now of everyday human life. Darwin himself -- although he knew and emphasized many cases of substantial change in human time (including the development of breeds in his beloved pigeons) -- tended to wax eloquent about the inexorable and stately slowness of natural evolution. In a famous passage from the Origin of Species, he even devised a striking metaphor about clocks to underscore the usual invisibility:

It may be said that natural selection is daily

and hourly scrutinizing, throughout the

world, every variation, even the slightest;

rejecting that which is bad, preserving and

adding up all that is good, silently and

invisibly working... We see nothing of

these slow changes in progress until the hand

of time has marked the long lapse of ages.

Nonetheless, the claim that evolution must be too slow to see can only rank as an urban legend -- although not a completely harmless tale in this case, for our creationist incubi can then use the fallacy as an argument against evolution at any scale, and many folks take them seriously because they just "know" that evolution can never be seen in the immediate here and now. In fact, a precisely opposite situation prevails: biologists have documented a veritable glut of cases for rapid and eminently measurable evolution on timescales of years and decades.

However, this plethora of documents -- while important for itself, and surely valid as a general confirmation for the proposition that organisms evolve -- teaches us rather little about rates and patterns of evolution at the geological scales that build the history and taxonomic structure of life. The situation is wonderfully ironic -- a point that I have tried to capture in the title of this article. The urban legend holds that evolution is too slow to document in palpable human lifetimes. The opposite truth has been a firmed by innumerable cases of measurable evolution at this minimal scale -- but, to be visible at all over so short a span, evolution must be far too rapid (and transient) to serve as the basis for major transformations in geological time. Hence, the "paradox of the visibly irrelevant" -- or, if you can see it at all, it's too fast to matter in the long run.

Our best and most numerous cases have been documented for the dominant and most evolutionarily active organisms on our planet -- bacteria. In the most impressive of recent examples, Richard E. Lenski and Michael Travisano (Proceedings of the National Academy of Sciences, vol. 91, 1994) monitored evolutionary change for 10,000 generations in twelve laboratory populations of the common human gut bacterium, Escherichia coli. By placing an twelve populations in identical environments, they could study evolution under ideal experimental conditions of replication -- a rarity for the complex and unique events of evolutionary transformation in nature. In a fascinating set of results, they found that each population reacted and changed differently, even within environments made as identical as human observers know how to do. Yet, Lenski and Travisano did observe some important and repeated patterns within the diversity. For example, each population increased rapidly in average cell size for the first 2,000 generations or so, but then remained nearly stable for the last 5,000 generations.