Thursday, August 18, 2005

Crimes of Pedagogy

To a remarkable but largely unremarked extent, what the public knows about nature and science is simply the pedagogy of textbooks. The common notion that animals fall into distinct groups marked by unambiguous structural characteristics, for example, reflects the examples teachers use to explain taxonomies. Many taxa contain members that have secondarily lost features that were once definitive of their group—the notion that there is a central plan that plays unchanging substance to the superficial accidents of wing color or tooth shape or behavior is deeply misleading since even basic body plans can change drastically over time. We only know the animals belong together because we can trace their common ancestry through transitional forms. And even when common features persist, there is a temptation to assume that they are functionally fundamental simply because they have persisted, even though some diagnostic features—the details of wing venation in insects, for example—are probably as accidental as the ridges on a finger print. They just happened to be the elements that, for some reason or for no reason at all, didn’t change in a given phylogeny.

By fostering an exaggerated belief in the naturalness of kinds, the just-so textbook version creates a largely artificial mystery, the impression that macroevolution differs qualitatively from microevolution. This may be one of those instances in which there is no answer to a question because there really isn’t a question to answer. Granted that genetic changes are always discrete, Natura semper fecit saltus, but the leaps in question only appear to be impossibly great if you imagine a spectacular vault from the picture of the living starfish on page 121 and the picture of the living hummingbird on page 124 instead of the tiny hop involved in the original divergence between one obscure species of deuterostome invertebrate and another in the slimy bottom of some Vendian estuary.

I was reminded of this issue by a report on the BBC on a strange looking invertebrate from the Cambrian that is said to embarrass the taxonomists. Vetustodermis planus is evidently hard to place in one of the recognized phyla, but it is hardly weirder than many a common living organism. The recherch√© bugs pictured in Grimaldi and Engel’s splendid new book Evolution of the Insects are every bit as bizarre as any relic of the Burgess Shale; but since they don’t fall on a crack between one group and another, nobody thinks they challenge evolutionary theory.

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