716 THE STRUCTURE OF EVOLUTIONARY THEORY
fruitful and fully justified, but I would also defend such an effort as the basis for an
independent macroevolutionary theory that can harmoniously expand our
conventional and exclusive focus on organisms to yield a more satisfactory general
account of life's workings and history.
I also continue to regard the individuality of species as the central proposition
of such an expanded theory. If organisms are the traditional units of selection in
classical Darwinian microevolution within populations, then species operate in the
same manner as basic units of macroevolutionary change. This perspective
establishes an irreducible hierarchical structure in nature, precluding the smooth
upward extrapolation of microevolutionary change within populations to explain
evolution at all scales, particularly phenotypic trends and patterns of diversity
displayed in geological time—the proposition that true devotees of
microevolutionary exclusivism rightly feared. If species, as stable units and
genuine evolutionary individuals, interpose themselves between populational
anagenesis and trends within clades, then the lower-level process cannot smoothly
encompass the higher-level phenomenon. For this fundamental (and excellent)
reason—and not because any "new" genetics or anti-Darwinian forces reign in a
threatening world of macroevolution—Stanley introduced his key notion of
"decoupling."
The levels become decoupled because macroevolution must employ species
as "atoms," or stable and basic units of change. Decoupling then becomes in-
tensified because higher levels exhibit allometric properties that distinguish their
phenomenology from the workings of lower levels. Thus, macroevolution with
species as individuals must differ, in deep and interesting ways, from
microevolution with organisms as individuals. These differences, and not any
fatuous claims about "new genetics," express the uniqueness of macroevolution,
and the validity of our argument for decoupling.
An extensive analogy—"the grand analogy," if you will (see Gould and
Eldredge, 1977, p. 142)—between organismal microevolution and speciational
macroevolution provides a good tool for assessing the differences imposed by
scaling among the levels. Stanley (1975, p. 649) and Gould and Eldredge (1977,
pp. 142-145) proposed some partial and preliminary schemes, and several others
have added components along the way (Stanley, 1979; Vrba, 1980; Grantham,
1995, for example). I present this grand analogy below, largely in the form of a
chart contrasting the key features of organic structure and evolution in their
organismal and speciational manifestations. For each major category, I list the
most important differences between the levels. A fuller explication of all items on
the chart follows.
THE PARTICULARS OF MACROEVOLUTIONARY EXPLANATIONThe Structural Basis
The first category of structural differences seems straightforward enough. In order
to construct the analogy, we ratchet the focal level of individuality up from the
organism to the species, thus redefining both lower components and higher
contexts in the structural triad of part-individual-collectivity (see page