Historical Constraints and the Evolution of Development 1175
of variants prevails. The alternative is the "developmental constraint" view
that many of the gaps we observe between different morphologies do not arise
from the non-adaptiveness of the absent forms but rather from the difficulty of
making them through an ontogenetic process.I began this "symphony" of evo-devo with a quotation from one of the great
architects of the Modern Synthesis—Mayr's statement, based on adaptationist
premises then both reasonable and conventional, that any search for genetic
homology between distantly-related animal phyla would be doomed a priori and in
theory by selection's controlling power, a mechanism that would surely recycle every
nucleotide position (often several times) during so long a period of independent
evolution between two lines. The new data of evo-devo have falsified this claim and
revised our basic theory to admit a great, and often controlling, power for historical
constraints based on conserved developmental patterns coded by the very genetic
homologies that Mayr had deemed impossible.
For the sake of both symmetry and logic, it seems fitting to end this section by
recalling another quotation by another great architect of the Synthesis, based on the
same panadaptationist assumptions about natural selection's controlling power—and
also falsified, since then, by new information on historical constraints, impelling
renewed respect for formalist themes in revising and expanding our theories of
evolutionary mechanisms. But Dobzhansky's closing statement (1951) differs from
Mayr's opener (1963) in one crucial way: Mayr's denial of genetic homology
represented a sensible consensus for his time; whereas Dobzhansky's assertion of
purely adaptational mapping upon ecological places to explain the clumpy population
of morphospace made little sense, even at the height of enthusiasm for natural
selection's exclusive power—and I can only conclude (as discussed more fully in
Chapter 7, pp. 526-528) that Dobzhansky, in his enthusiasm for strict Darwinian
theory, had temporarily undervalued a cardinal fact of natural history that his initial
training as a systematist had certainly infused into the marrow of his understanding.
In a brilliant opening move, Dobzhansky began the third (1951) edition of his
founding document for the Synthesis, Genetics and the Origin of Species, by
recognizing the diversity of modern organisms, and the striking discontinuities within
this plethora of form, as the central problem of evolutionary biology—at a time when
most colleagues would surely have cited modes of continuous transformation, or
mechanisms for changes in gene frequencies, within single populations instead.
(Despite this unconventionality in subject and level of focus, Dobzhansky opted for a
traditional selectionist explanation by titling the first subsection of his book:
"diversity and adaptedness.")
As a wise and wonderful human being, and as a humanist at heart, Dobzhansky
began his book with a generous perspective on the meaning and importance of
organic diversity. The opening paragraph (1951, p. 3) reads: "Man has always been
fascinated by the great diversity of organisms which live in the world around him.
Many attempts have been made to understand