1068 THE STRUCTURE OF EVOLUTIONARY THEORY
Darwin's famous words, so often quoted, haunt the background of this discussion
(1859, p. 84): "It may be said that natural selection is daily and hourly scrutinising,
throughout the world, every variation, even the slightest; rejecting that which is bad,
preserving and adding up all that is good; silently and insensibly working, whenever
and wherever opportunity offers, at the improvement of each organic being in relation
to its organic and inorganic conditions of life."
Therefore, any uncannily detailed phenotypic similarity evolved between
distantly related groups must arise by convergence from substrates of
nonhomologous genotypes—thus affirming our usual view of selection's overarching
power, especially if common function for the two similar forms can validate the
hypothesis of generation within a comparable adaptational matrix. (Note the logical
danger of circularity that intrudes upon the argument at this point, for this extent of
detailed similarity—the very datum that, in an unbiased approach, would lead one to
entertain parallelism based upon common internal constraint as a viable alternative to
convergence based on similar adaptive needs—now becomes an a priori affirmation
of selection's power, the hypothesis supposedly under test.)
For this reason, such detailed functional and structural similarities, evolved
independently in distantly related lineages, have become "poster boy" examples of
convergence—itself the "poster boy" phenomenon and general concept for
showcasing selection's dominant sway—precisely because similarities evolved in this
mode cannot, by Mayr's argument, be ascribed to parallelism based on positive
constraint imposed by homologous genetic and developmental pathways. With
internal channeling thus theoretically barred as a potential source of impressive
similarity, convergence becomes the favored explanation by default. The argument,
surely "tight" in logic and principle, seems incontrovertible.
Since I do not wish to dwell on the previous errors that we all committed on this
issue, let me simply illustrate the older view, and the magnitude of current reversal,
with one of my own mistakes—from a 1976 paper extolling convergence as
evolutionary biology's closest natural analog to replication in the experimental
sciences (Gould, 1976, p. 177):
The convergent evolution of similar structures fulfills, at least imperfectly, the
criterion of independent replication that any experiment requires. An adequate
theory of functional morphology must explain adaptive design by studying
how different organisms react to the same selective regime. If we want to
know whether plate tectonics is a true, universal physics of large bodies or
only a descriptive account of this planet's history, other planets must be
studied. If we want to know whether the biochemical unities of all life on
earth have general import as optimal designs (given the nature of universal
chemistry) or merely reflect the monophyletic origin of life on earth (and the
homologous status of ATP and left-handed amino acids), then we shall have
to hope for life on Mars. If, to retreat to something more immediate, one
wishes to assess