The Modern Synthesis as a Limited Consensus 579
in a valley is divided in two by a river that cuts a channel through their
valley. The two segments of the population are now effectively separated
from one another, and any environmental differences that exist between the
two regions of the valley will result in adaptations restricted to one side or
the other of the river.... Different selective pressures now will be operating
on opposite sides of the river. Given sufficient time, the two gopher
populations may diverge quite extensively.With speciation thus explained as an extended consequence of adaptation
under certain environmental circumstances, the same argument can then be
smoothly extended to life's full pattern in geological time. Alexander (1962, p.
826) tells students that all phylogeny flows from "the fact of adaptation." I can
hardly imagine a more gradualistic and meliorist account of evolution, with all
death for improved existence, and all life in continual motion towards more and
better:
We need only accept the fact of adaptation—the idea that organisms are
fitted for the particular environments in which they live—to see the
necessity for a process of organic evolution. The environment in which
organisms live has not been constant... Organisms, of course, do not exist
under conditions for which they are not adapted. They have, therefore, met
these various conditions at different times and places; in order to persist
under a changing environment they themselves have had to change. We
may think of organic evolution, therefore, as the progressive change of
plants and animals in harmony with the changes in their environments. The
unadapted die out and disappear. Those organisms whose descendants can
fit into the new conditions survive, expand in numbers and kinds, and take
over the changing habitat.Reduction and trivialization of macroevolution
The hypothesis of selection's Allmacht, and adaptation's ubiquity, rests upon the
validity of extrapolation to the full range of geological time, for what power (or
generality) can a well-formulated theory of local adaptation assert if the same
process, by uniformitarian extension, cannot explain the origin of multicellularity,
the rise of mammals, and the eventual emergence of human intelligence?
Paradoxically perhaps, this extrapolationist assertion becomes, at the same time,
the most vulnerable and the most essential of all synthetic propositions—
vulnerable in necessary reliance upon a "consistency argument" in the absence of
empirical proof, and essential because the theory becomes such a paltry and limited
device if its explanatory range cannot extend beyond the compass of its directly
observable effects.
No evolutionary assertion has been more commonly advanced in textbooks,
or more superficially (and almost nonchalantly) proclaimed by fiat, than the claim
that adaptation by natural selection must be fully sufficient to render life's entire
history. In the last section, I documented the "promotion" of arguments about
pervasiveness of adaptation in local circumstances, to speciation, to the entire tree
of life. Capping this sequence, Howells (1959,