900 THE STRUCTURE OF EVOLUTIONARY THEORY
for expansion. To be able to formulate this alternative view at all, we must
reconceive the history of life as expansion and contraction of a full range of taxa
under constraints of systems and environments, rather than as a flux of central
tendencies, valued extremes, or salient features.
- The right tail as predictable, but passively generated. A critic might
respond that he accepts the reformulation but still wishes to assert a vector of
progress as life's central feature in the following, admittedly downgraded, way:
yes, the vector of progress must be construed as the expanding right tail of a
distribution with a constant mode, not as a general thrust of the whole. But this
expanding tail still arises as a predictable feature of the system, even if we must
interpret its origin and intensification as the drift of a minority away from a
constraining wall, rather than the active trending of a totality. The right tail had to
expand so long as life grew in variety. This tail therefore originated and extended
for a reason; and humans now reside at its present terminus. Such a formulation
may not capture the full glory of Psalm 8 ("Thou hast made him a little lower than
the angels"), but a dedicated anthropocentrist could still live with this version of
human excellence and domination.
But the variational reformulation of life's system suggests a further
implication that may not sit well with this expression of human vanity. Yes, the
right tail arises predictably, but random systems generate predictable consequences
for passive reasons—so the necessity of the right tail does not imply active
construction based on overt Darwinian virtues of complexity. Of course the right
tail might be driven by adaptive evolution, but the same configuration will also
arise in a fully random system with a constraining boundary. The issue of proper
explanations must be resolved empirically.
By "random" in this context, I only mean to assert the hypothesis of no
overall preference for increasing complexity among items added to the
distribution—that is, a system in which each speciation event has an equal
probability of leading either to greater or to lesser complexity from the ancestral
design. I do not deny, of course, that individual lineages in such systems may
develop increasing complexity for conventional adaptive reasons, from the benefits
of sharp claws to the virtues of human cognition. I only hold that the entire system
(all of life, that is) need not display any overall bias—for just as many individual
lineages may become less complex for equally adaptive reasons. In a world where
so many parasitic species usually exhibit less complexity than their free living
ancestors, and where no obvious argument exists for a contrary trend in any
equally large guild, why should we target increasing complexity as a favored
hypothesis for a general pattern in the history of life?
The location of an initial mode next to a constraining wall guarantees a
temporal drift away from the wall in random systems of this kind. This situation
corresponds to the standard paradigm of the "drunkard's walk" (Fig. 9-31), used by
generations of statistics teachers to illustrate the canonical random process of coin
tossing. A drunkard exits from a bar and staggers, entirely at random, along a line
extending from the bar wall to the gutter (where he passes out and ends the
"experiment"). He winds up in the gutter on every