528 THE STRUCTURE OF EVOLUTIONARY THEORY
adaptive system of mammals, which are ecologically and biologically
segregated, as a group, from the adaptive systems of birds, reptiles, etc. The
hierarchic nature of the biological classification reflects the objectively
ascertainable discontinuity of adaptive niches, in other words the
discontinuity of ways and means by which organisms that inhabit the world
derive their livelihood from the environment (pp. 9-10).Thus, Dobzhansky renders the hierarchical structure of taxonomy as a fitting
of clades into preexisting ecological spaces. Discontinuity emerges not so much as
a function of history, but as a reflection of adaptive topography. But this
interpretation cannot hold; surely, the cluster of cats exists primarily as a
consequence of homology and historical constraint. All felines share a basic
morphology because they arose from the common ancestor of this clade alone. We
doubt neither the excellent adaptation of this common ancestor nor the claim that
all descendants may fit equally well into their current environments. But the feline
group and the gaps that separate this cluster from other families of carnivores
reflect history above all, not the current organization of ecological topography. All
feline species have inherited the unique Bauplan of cats, and cannot deviate far
from this commonality as they adapt, each in its own particular way. Genealogy,
not current adaptation, provides the primary source for clumped distribution in
morphological space.
THE SHIFT IN G. G. SIMPSON'S EXPLANATION OF "QUANTUM
EVOLUTION" FROM DRIFT AND NONADAPTATION (1944) TO
THE EMBODIMENT OF STRICT ADAPTATION (1953)
Although Simpson, probably more than Dobzhansky, personally favored
selectionist arguments in the initial version of his seminal work (1944), he also
adopted a pluralistic stance at first. In fact, at the crux of his book, Simpson
proposed an explicitly nonadaptationist theory to resolve the greatest anomaly in
the fossil record; he also considered this theory of "quantum evolution" as the
crowning achievement of his book.
Like Dobzhansky in his first edition (1937), Simpson (1944) espoused
consistency of all evolutionary change with principles of modern genetics as his
primary assertion for a general and synthetic theory. The major challenge to unity
and consistency arose from the infamous "gaps" or discontinuities of the fossil
record—particularly at the largest scale of appearances for new Bauplan without
fossil intermediates. Simpson wrote:
The most important difference of opinion, at present, is between those who
believe that discontinuity arises by intensification or combination of the
differentiating processes already effective within a potentially or really
continuous population and those who maintain that some essentially
different factors are involved. This is related to the old but still vital
problem of micro-evolution as opposed to macro-evolution ... If the two
proved to be basically different, the innumerable studies of microevolution