Structural Constraints, Spandrels, and Exaptation 1233
Table 11-1. A Taxonomy of Fitness.
consequence of quirky functional shift, do not reveal their original evolutionary
context in their current utility. If all phenotypic traits are adaptive, and built by
adaptation, why bother to make a formal distinction between features crafted, and
features merely coopted, for their current utility?
But our renewed respect and attention to structuralist themes now makes such a
formal distinction essential. Thus, Vrba and I recommended that features crafted for
current use continue to be called adaptations (adopting the restriction advocated by
Darwin and Williams), and that features coopted for current use, following an origin
for some other reason, be called exaptations. We would also prefer that biologists
embrace "aptation" rather than "adaptation" as the general descriptive term for a
character now contributing to fitness, with exaptation and adaptation defined as the
two sub-categories of aptation, thus designated to recognize the crucial distinction
between cooptation and direct shaping in the historical construction of characters.
This simple terminological strategy addresses the fair criticism that we can often
only know the current basis of fitness—when we do not have enough evidence to
determine whether a character developed as an exaptation or adaptation. In such
cases, under our scheme, we refer to the character as an "aptation" and leave the
further specification of its origin unaddressed. (Our current terminological
conventions operate in this manner after all, for I may call a character an adaptation
whether I accept Paley's belief in divine creation or Darwin's mechanism of origin by
natural selection. Both authors did, indeed, call useful structures "adaptations.")
In an ideal world (and if I held the powers of a czar, which would, of course,
then make such a world unideal ipso facto) I would fight for the full scheme, and
campaign to replace "adaptation" with "aptation" as a base-level description (with no
implication about mode of origin) for features now contributing to fitness. But I know
the odds against unseating centuries of usage for a word that not only serves as a
staple of vernacular speech, but also enjoys unparalleled professional salience as a
standard-bearer for our preferred evolutionary theory. I don't play the lottery, and I
don't understand the recreational appeal of skydiving or bungee jumping. So I will
mortify my desires and learn to live with the traditionally broad use, thus facine with
stoicism