The Structure of Evolutionary Theory

by Stephen Jay Gould

This book asserts, as its key premise and one long argument, that such an understanding of modern evolutionary theory places the subject in a particularly “happy” intellectual status—with the central core of Darwinian logic sufficiently intact to maintain continuity as the centerpiece of the entire field, but with enough important changes (to all major branches extending from this core) to alter the structure of evolutionary theory into something truly different by expansion, addition, and redefinition.

“A proposition can evolve into its contradictory,” Hull allows (1988, p. 18). Thus,

Darwin’s brave and single-minded insistence on the exclusivity of the organismic level, although rarely appreciated by his contemporaries, ranks as the most radical and most distinctive feature of his theory.

Darwinism, in this sense, is a functionalist theory, leading to local adaptation as the environment proposes and natural selection disposes.

We may revere Newton and Lavoisier as men of equal impact, but do modern physicists and chemists actively engage the ideas of these founders, as they pursue their daily work? Darwin, on the other hand, continues to bestride our world like a colossus—so much so that I can only begin this book on the structure of evolutionary theory by laying out Darwin’s detailed vision as a modern starting point, a current orthodoxy only lightly modified by more than a century of work.

That theory, when fully formulated in the 1850’s, emerged as an intricately devised amalgam of logically connected parts, each with a necessary function—and not as a simple message from nature. We must treat this theory, as Kellogg does, with respect for its integrity.

“Protagoras asserted that there were two sides to every question, exactly opposite to each other.”

“It is generally acknowledged that all organic beings have been formed on two great laws—Unity of Type, and the Conditions of Existence.”

Natural selection is stern, but she has her tolerant moods”

the dichotomy between structuralist and functionalist thought, the conceptual basis and primary theme of this chapter, cannot be regarded as an idiosyncratic or artificial device of rhetoric or textual organization, but rather denotes a widely perceived antithesis

CONTINUITY. An individual cannot fade in and out of existence during its lifetime, but must maintain material continuity throughout. Members of classes, on the other hand, are not so constrained, for classes are defined by common properties, not by historical continuity. As Hull (1980) argues, the class of gold atoms does not require continuity or filiation. If all gold disappeared, its position on the periodic table would remain—and an element later reconstituted with the right atomic particles and requisite properties would still, and legitimately, be called gold. But if all peacocks die, the species-individual Pavo cristatus disappears forever. Even if some human engineer retained an electronic record of the entire Pavo cristatus genome, and future technology permitted chemical reconstitution from nucleotides, we couldn’t call the resurrected creature a member of Pavo cristatus, even if the reconstituted object looked and acted like an extinct peacock of old.

individuality. Species, in an argument dating to both Lamarck and Darwin, have often been construed as mere names of convenience attached to segments of evolving continua without clear borders. Under this gradualistic and anagenetic view (see Fig. 8-1), a species near the end of its arbitrary existence must be phenotypically more similar to a forthcoming descendant than to the initial ancestor. (Indeed, under strict gradualism, we even face the definitional absurdity that the last generation of an ancestor should be reproductively isolated from its own offspring—that is, the first generation of a new descendant. Some creatures may eschew such incest on moral or adaptive grounds, but no one would gainsay the biological possibility.) Thus, on this traditional view, species cannot maintain sufficient temporal stability to be called individuals. In addition, species do not have discrete birth points if they branch from their ancestors at rates no different from characteristic tempos of transformation during their subsequent anagenetic lifetimes (see Fig. 8-1). At most, some species display clear termination in extinction (but others evolve gradually to descendants.) Thus, species do not function as good vernacular individuals if gradualism and anagenesis pervade the history of life.

replicators simply aren’t units of selection or, for that matter, causal agents at all under our usual notions of mechanism in science. The misidentification of replicators as causal agents of selection—the foundation of the gene-centered approach—rests upon a logical error best characterized as a confusion of bookkeeping with causality.

When Williams (1966) famously, if a bit facetiously, remarked that we shouldn’t consider a flying fish’s capacity to fall back into the water as an adaptation because their descent represents a necessary consequence of physical mass—even though this capacity may be vital to the continued life of the fish, and therefore strongly aptive—he invoked a direct physical property of matter (not subject to alteration by selection at all in this case!).

But archetypal claims for homology across distantly related phyla raise far more serious theoretical problems. No shockwaves attended the discovery of common genetic and developmental pathways for the serial array of arthropod appendages, despite their functional differentiation as antennae, mouth parts, legs, genital claspers, etc. But the discovery that homological pathways also persist among animal phyla that have evolved independently since the Cambrian explosion has reversed previous certainties and brought Geoffroy’s despised archetypal theories into renewed respectability.

(The Nobel awards include no category for evolutionary studies. Only twice has a prize been given for work in evolutionary biology, each time by nuancing the definition of medicine to include work with legitimate consequences for health, but scarcely in the mainstream of medical research—first to Lorenz, Tinbergen, and von Frisch, for foundational studies in ethology, and second to my dear colleagues Ed Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus for unlocking the genetic basis of fundamental architectures in animal development.)

all major arthropod Hox genes had already appeared before the separation of arthropod classes and, for that matter, of protostome phyla as well.

phyletic analysis indicates a minimum of 7 Hox genes for the bilaterian ancestor, and at least 8 for the common ancestor of protostome phyla.

Drosophila’s first specification even begins in a prior generation, for protein products of maternal genes like bicoid and nanos appear in the egg cytoplasm to designate the anterior and posterior embryonic poles.

Despite these caveats, we can only conclude that the first fruits of evo-devo have revealed some remarkable, and extensive, homolgy in both genetic structure and action among animal phyla (particularly between arthropods and chordates, the former prototypes of separation in our traditions and literature), and that these data have confirmed some important aspects of the most ridiculed formalist theory of constraint in the history of morphological and evolutionary thought: Geoffroy’s claim for homology between vertebrate and arthropod segments, with the idealized segment itself regarded as the basic unit of generation.