1066 THE STRUCTURE OF EVOLUTIONARY THEORY
basis of many homologies, but stated that the attribution of such similarity to
"developmental constraint" would represent what he proposed to christen as the
"Gould-Lewontin fallacy"—for natural selection can unlock any inherited
developmental correlation if adaptation to immediate environments favors such an
alteration.)
Second, homological holds must be limited in taxonomic and structural extent to
close relatives of similar Bauplan and functional design. The basic architectural
building blocks of life—the DNA code, or the biomolecular structure of fundamental
organic compounds, for example—may be widely shared by homology among phyla.
But the particular blueprints of actual designs and the pathways of their
construction—the form of the Gothic cathedral rather than the chemical formula of
calcite in the facing stone (see pp. 1134-1142 for extensive treatment of this point)—
must be limited to clades of closer relationship. The identical topology of bones in
mammalian forearms of markedly different utility (the whale's flipper, the horse's leg,
the bat's wing, and my typing, or literally manipulating, digits) can be homologous,
but we expect no comparable hold of history upon the more generally similar
segmentation of arthropod metameres and vertebrate somites (not to mention the non-
homology of bones in mammalian forearms and teleost forefins).
Any wider hold of homology would have to inspire suspicions that the central
tenet of orthodox Darwinism can no longer be sustained: the control of rates and
directions of evolutionary change by the functional force of natural selection. In a
particularly revealing quote within the greatest summary document of the Modern
Synthesis, for example, Mayr (1963, p. 609) formulated the issue in a forthright
manner (see p. 539 for previous discussion of this statement). After all, he argued,
more than 500 million years of independent evolution must erase any extensive
genetic homology among phyla if natural selection holds such power to generate
favorable change. Adaptive evolution, over these long intervals, must have crafted
and recrafted every genetic locus, indeed every nucleotide position, time and time
again to meet the constantly changing selective requirements of continually varying
environments. At this degree of cladistic separation, any independently evolved
phenotypic similarity in basic adaptive architecture must represent the selective
power of separate shaping by convergence, and cannot record the conserved
influence of retained genetic sequences, or common generation by parallelism: "In
the early days of Mendelism there was much search for homologous genes that would
account for such similarities. Much that has been learned about gene physiology
makes it evident that the search for homologous genes is quite futile except in very
close relatives."
But we now know that extensive genetic homology for fundamental features of
development does hold across the most disparate animal phyla. For an orthodox
Darwinian functionalist, only one fallback position remains viable in this new and
undeniable light (and Ernst Mayr, vigorous as ever at age 95 as I write these words,
would be the first to welcome this illumination wholeheartedly, and to laugh at his
old cloudy crystal ball on this single issue). One can admit the high frequency and
great importance of such genetic