Species as Individuals in the Hierarchical Theory of Selection 739
For constraints of internal environments (line IVB), I make a distinction
between negative factors that limit amounts and directions of change, and positive
properties that channel change in certain directions, or provide particular
opportunities for evolutionary novelties and breakthroughs. (I also base Chapter
10, this book's major discussion of constraint, on the same distinction.) The
operation of these constraints often differs in interesting ways at the two levels.
For some of the important limits, line IVB1 specifies a major shaping force of life's
structure, a factor not often explicitly acknowledged. Why does the world contain
stable individuals at all, and at any level? Why doesn't evolution work as
continuous flux at all scales, rather than primarily by selection upon individuals
stable enough to persist, at least through one round of differential sorting?
Comparable reasons can be stated at both the organismic and species levels, thus
giving evolution its primary shape or structure: Lamarckian inheritance does not
occur at the organismal level, thus stabilizing the ontogeny of heritable variability.
At the species level, punctuated equilibrium suppresses anagenesis by maintaining
species-individuals in stasis.
When we explore the structural brakes that limit amounts of change in most
trends (line IVB2), several factors could be mentioned, but I just list, as an
example, the single property that I consider most important. For organisms, those
paragons of individuality by the criterion of structural and functional integrity,
design limits of the Bauplan (both internally by structural constraint, and
externally by adaptive possibilities) place strong brakes upon almost any
evolutionary trend. Contrary to the themes of several popular films, elephants will
never fly, and insects will not reach elephantine proportions and engulf our cities
as a plague of megalocusts.
At the species level, Stanley (1979) made an important observation that has
not been sufficiently appreciated for its defining force in limiting the possibilities
of species selection. If we consider the two major modes of positive species
selection—enhancing the rate of production for new species, and extending the
geological longevity of existing species—why shouldn't some lineages be able to
maximize both properties simultaneously, thus becoming gigantic megaclades,
dominating the earth's biota? (Perhaps, of course, a few clades have been able to
approach this ideal—thus explaining the great success of beetles and nematodes.)
In other words, why don't clades ratchet themselves towards this pinnacle by
species selection—by working both ends of the game, and evolving species of
extraordinary durability and fantastic rates of branching, superspecies that live for
several geological periods and spawn large numbers of daughters all along the
way?
Stanley (1979) argues, with extensive data in support, that the nature of
speciation as a process, and the general rules of ecology, engenders a strong, and
effectively unbreakable, negative correlation between speciation and extinction
rates. Unfortunately for ambitious species with dreams of mega-cladal domination
(but happily for any ideal of a richly varied biota), the major factors that boost
speciation rates also raise the probability of extinction;