The Modern Synthesis as a Limited Consensus 557
there was at that time among the evolutionists. The change since then has
been startling. Symposia and conferences were held all over the world in
1959 in honor of the Darwin centennial, and were attended by all the
leading students of evolution. If we read the volumes resulting from these
meetings ... we are almost startled at the complete unanimity in the
interpretation of evolution presented by the participants. Nothing could
show more clearly how internally consistent and firmly established the
synthetic theory is. The few dissenters, the few who still operate with
Lamarckian and finalistic concepts display such colossal ignorance of the
principles of genetics and of the entire modern literature that it would be a
waste of time to refute them. The essentials of the modern theory are to
such an extent consistent with the facts of genetics, systematics, and
paleontology that one can hardly question their correctness (1963, p. 8).Later on, Mayr proposes a succinct definition of the Synthesis, emphasizing
all three legs (or branches) of the essential tripod (or tree) of Darwinian logic "The
proponents of the synthetic theory maintain that all evolution is due to the
accumulation of small genetic changes, guided by natural selection, and that
transpecific evolution is nothing but an extrapolation and magnification of the
events that take place within populations and species" (1963, p. 586).
I regard Mayr's balance of emphases as particularly revealing. His definition
includes two phrases. The first statement buttresses the two legs of Darwin's tripod
that receive most attention in this work—control of direction by external selection
rather than internal constraint (with attendant gradualism of change), and operation
of the process through differential reproductive success of organisms (implicit in
the term "natural," as opposed to some other form or level of selection). But the
second, and longer, statement affirms Mayr's appreciation for the importance of the
third leg—the complete sufficiency of microevolutionary theory to explain the
entire history of life—so long as the earth's geological behavior sets a proper stage,
and does not derail a full extrapolation of microevolutionary mechanics into all
geological time and to the entire extent of phylogenetic change. For how can we
celebrate the power and generality of a beautiful, sufficient and completely
validated mechanism of change, established for the immediacy of an ecological
here and now, if the same processes cannot render the broad pattern of life's history
as well?
This theme of extrapolation becomes, in many ways, the most comprehensive
issue of all. In Chapter 2, I distinguished two separate aspects of Darwin's
radicalism in proposing the theory of natural selection—a methodological pole to
grant operational status to the study of evolution by asserting that observable
events, however apparently trivial and inconsequential, can explain change at all
scales by extension; and an ideological pole to present a radical mechanism of
evolutionary change for rendering all "higher" attributes of good design and
organic harmony as side-consequences of a process