昨日に続いて，Gould and Vrba の外適応 (exaptation) の話題．Gould and Vrba (11--13) は，進化論における外適応という用語と概念の重要性を4点ほど指摘している．
(1) 外適応は，従来の "preadaptation" に付随する目的論 (teleology) 的な含意を払拭してくれる．従来の "preadaptation" は，新しい用語体系においては "preaptation" と呼ぶべきものであり，その事実の前において考えられた "exaptation" の一種，すなわち "potential, but unrealized, exaptations" (11) とみなすことができる．
(2) 第1に外適応，第2に適用という順序．"Feathers, in their basic design, are exaptations for flight, but once this new effect was added to the function of thermoregulation as an important source of fitness, feathers underwent a suite of secondary adaptations (sometimes called post-adaptations) to enhance their utility in flight" (11). 外適応によって生まれた性質が，その後，適用の過程を経てさらに機能的になってゆくということは，ごく普通のことである．
(3) 外適応の資源は無限であり，柔軟性に富む．"[T]he enormous pool of nonaptations must be the wellspring and reservoir of most evolutionary flexibility. We need to recognize the central role of 'cooptability for fitness' as the primary evolutionary significance of ubiquitous nonaptation in organisms" (12). 関連して，"[A]ll exaptations originate randomly with respect to their effects" (12) である．
(4) 外適応をもたらした資源は，nonaptation だったかもしれないし adaptation だったかもしれない．"Exaptations . . . are not fashioned for their current role and reflect no attendant process beyond cooptation . . .; they were built in the past either as nonaptive by-products or as adaptations for different roles."
Gould and Vrba (13) は，結論として次のように述べている．
We suspect . . . that the subjects of nonaptation and cooptability are of paramount importance in evolution. . . . The flexibility of evolution lies in the range of raw material presented to processes of selection.
これは，「#2155. 言語変化と「無為の喜び」」 ([2015-03-22-1]) で引用した Lass の言語変化における "The joys of idleness" に直接通じる考え方である．
・ Gould, Stephen Jay and Elizabeth S. Vrba. "Exaptation --- A Missing Term in the Science of Form." Paleobiology 8.1 (1982): 4--15.
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