1274 THE STRUCTURE OF EVOLUTIONARY THEORY
both in external environments and internal states. But a capacity to adjust to chaotic
situations also confers evolvability. Adaptation at the edge of chaos balances both
desiderata of current functionality and potential for future change, or evolvability.
Yet, however much biologists can document and articulate these components of
evolvability, the general concept itself has remained uncomfortable, even
paradoxical, because the clear existence of "flexibility for future benefit," and the
equally obvious importance of evolvability to the long-term persistence and success
of lineages, cannot be rendered as a directly causal and explicit outcome of the
Darwinian mechanisms that we have viewed as fully sufficient for understanding the
causes of evolutionary change. Organismal selection for traits that confer differential
reproductive success in the ecological moment simply cannot generate, in any active
or direct manner, a set of features that achieves evolutionary significance only by
imparting flexibility for change in distant futures. We cannot deny that these features
of evolvability deeply "matter" in the history of lineages; but how can benefits for
futures arise by any causal process in the here and now? Moreover, and adding insult
to anomaly, the major components of evolvability apparently reside in "superfluous"
genetic elements, and in supernumerary items of anatomy, that almost seem to mock
our usual concepts of the stark efficiency of selection as a natural arbiter between the
immediately useful and the discardable junk. As an example of this discomfort and
puzzlement, just consider the language of Kazazian's excellent review of mammalian
retrotransposons (2000, pp. 1152-1153), as he struggles to grasp and communicate
the apparent discordance between current irrelevancy (at the level of necessary
generation) and future benefits: "Although retrotransposons have been viewed as
selfish DNAs that provide no benefit to their host cell, we now know that over
evolutionary time they have increased the diversity of the genome through a variety
of mechanisms providing it with considerable 'added value.' ... It is clear that LI
elements are the master mammalian retrotransposons. Although L1s may be selfish,
they are clearly not junk, for they have played a major part in our evolution and the
evolution of our genomes." But the genuine junk of today can be exapted for the
triumphs of tomorrow. The spores of penicillin didn't do us much good (and even
imposed substantial harm in spoiling our foods) until Dr. Fleming made his fortuitous
discovery of a previously unrecognized, and now eminently lifesaving, property.
As its central theme and purpose, this book proposes a set of expansions and
revisions to Darwinian theory that, among other salutary features, can resolve the
paradox of evolvability by exposing the issue as a Scheinproblem, or problem of
appearance—that is, a spurious appearance within an overly restricted theory
presently lacking the language and concepts for granting causal intelligibility to this
evidently vital theme in evolutionary studies. The key revisions proposed on each leg
of the logical structure that I have called the tripod of essential components in
Darwinian theory provide, in their ensemble, an explanation of evolvability in
hierarchical and structuralist terms.
- The expansion of selection to a hierarchical theory of simultaneous operation