Structural Constraints, Spandrels, and Exaptation 1273
1997). But the importance of constraint and preexisting opportunity in channeling the
pathways of change should not be underestimated—for the message of evo-devo does
not proclaim that "all is possible under such flexible rules." One might, for example,
denigrate the importance of constraint in noting that a rod-like element of an ancestral
agnathan gill arch exhibited sufficient malleability in form and function to become
the tiny, disparately shaped and divergently functioning malleus of the mammalian
middle ear. The gill-arch elements may therefore work as general building blocks of
unconstraining Pharaonic bricks in the metaphor of my treatment of this subject (pp.
1134 - 1142). But we must also remember that, absent some skeletal element of
appropriate form and position (whatever its capacity for future modification to almost
any other shape or function), vertebrates would probably never have evolved jaw
bones that could transmute to ear bones—and our lineage (if it had survived at all)
might have remained an insignificant component of bottom-dwelling mud-sucking
marine faunas, thus precluding, at least for this planet at this time, the evolution of
any species with enough cognitive capacity to fret about such issues.
- From studies of genetic structures and sequences, the astonishingly high
relative frequency of multiply repeated elements (with respect to previous
assumptions about the nature of genomes), and the multiplicity of ways, from gene
duplication to retrotransposition, for generating them, have documented enormous
redundancy and combinatorial flexibility within genomes —even if we have
designated the objects of this discovery with the disrespectful name of "junk DNA"
(but see p. 1269 for my favorable reevaluation of this term). Retrotransposons, for
example, constitute about 40 percent of mammalian genomes (Kazazian, 2000). The
human genome includes about 500,000 truncated versions and some 3000 to 5000
full copies of the LINE-1 long terminal repeat. Chromosome 2 of Arabidopsis
includes 239 tandem duplications involving 593 genes. A larger duplication of almost
2.5 million bases appears on two chromosomes in four large blocks. A long stretch of
chromosome 4, including 37 genes, has been duplicated on chromosome 5.
Chromosome 2 also contains a region with 75 percent of the mitochondrial genome,
reflecting a recent transfer of a substantial block of DNA from an organelle to the
nucleus (Meyerowitz, 1999)—quite a "gift" of "play" from the genie to the
organismal level! - From studies of the properties of populations, communities and interactions
among evolving entities by mathematical modeling and computer simulation. Several
researchers have used these methods in attempts to identify the abstract and general
conditions that might confer flexibility, persistence, and capacity for change upon an
evolving population or group of entities. Most notably, as discussed previously (p.
1210), Kauffman's (1993) claim that successful systems move towards "adaptation at
the edge of chaos" rests upon attention to evolvability as a key ingredient of longterm
success. Such systems must be adaptive, but too much (and too precise) a local fitting
may freeze a system in transient optimality with insufficient capacity for future
change. Too much chaos may prove fatal by excessive and unpredictable fluctuation,