1240 THE STRUCTURE OF EVOLUTIONARY THEORY
Exaptation has already passed through the three major stages of this sequence.
In a first stage, exaptation, as a novel term, became an explicit focus for studies to
test or illustrate its utility—as in Arnold's title (to his 1994 paper, discussed above):
"Investigating the origins of performance advantage: adaptation, exaptation and
lineage effects"; or in Almada and Santos (1995): "Parental care in the rocky
intertidal: a case study of adaptation and exaptation in Mediterranean and Atlantic
blennies."
In a second stage, the concept serves, inter alia, as part of an ordinary analysis,
and not as an explicit focus of study—but the term still requires a citation to its
source of origin, or at least must be defined and presented within quotation marks.
For example, Chatterjee (1997), advocating the currently less popular "trees down"
arboreal (rather than the "ground up" terrestrial) theory for the origin of birds, argued
that many climbing adaptations of tree-dwelling ancestors "were exapted for gliding"
in the transitional stages towards full flight. The language of Chatterjee's full sentence
records an interesting, and undoubtedly unconscious, intermediary stage in the
acceptance of functional shift as a principle in evolutionary analysis: "Surprisingly,
many of these arboreal innovations were exapted for gliding" (p. 311). But such
cooptations and functional shifts can only be deemed "surprising" when contrasted
with expectations of continuous improvement within a single "adaptive zone" (to use
Simpson's classical terminology of 1944). Once we recognize functional shift and
cooptation as important components in almost any extensive evolutionary sequence,
we will no longer label exaptations as surprising.
In an example from the most fecund realm of exaptation in molecular evolution,
Weiner and Maizels (1999) explained to their biochemical colleagues who may not
be au courant with the literature of evolutionary theory: "Those with an evolutionary
bent sometimes use the word 'exaptation' to describe the appropriation of a molecule
with one job for a completely different purpose. Exaptation contrasts with
'adaptation,' a seemingly natural extension of preexisting functions" (1999, p. 64).
Their article, entitled "a deadly double life," documents the fascinating "remarkable
discovery" (1999, p. 63) that the carboxyl-terminal domain of human tyrosyl-transfer
RNA synthetase (the enzyme that catalyzes the attachment of the amino acid tyrosine
to the appropriate tRNA molecule prior to protein synthesis) shows clear homology
(49 percent sequence similarity) with a cytokine performing the quite different—one
might say conceptually opposite—function of attracting phagocytic cells to sites of
apoptosis, suggesting in a broader sense that "secretion of tyrosyl-tRNA synthetase
may help to shut down residual protein synthesis in the dying cell" (p. 63).
In this case, the synthetase activity seems primary (for a set of reasons
elaborated in Weiner and Maizels, 1999), and the "opposite" role in cell death
secondary, following gene duplication, and leading to the molecule's "deadly double
life." Weiner and Maizels (1999) argue that the utility in apoptosis originates as an
exaptation recruited from an "accidental" effect of the gene's primary activity: "The
recruitment of tryrosyl-tRNA synthetase as an extracellular