hellog〜英語史ブログ / 2015-11-02

■ #2380. Gould and Vrba の外適応 (1) [exaptation][terminology][evolution][diachrony][teleology]

　昨日の記事「#2379. 再帰代名詞の外適応」 ([2015-11-01-1]) でも取り上げた外適応 (exaptation) は，本来，生物進化の用語である．「#2152. Lass による外適応」 ([2015-03-19-1]) で触れたように，Gould and Vrba による生物進化論の外適応を，生物学にもよく通じた言語学者 Lass が，言語変化に当てはめたのだった．
　Gould and Vrba の論文は，それほど長い論文ではないが，啓発的である．生物学の門外漢でも読めるほどの一般性を備えており，だからこそ言語学へも適用する余地があったのだろう．進化論における術語の整理を通じて，対応する概念の相互関係を明らかにしようとした論文であり，それらの術語や概念は，確かに工夫すれば言語変化にも当てはめることができるように思われる．論文冒頭の要旨 (4) が，素晴らしく的を射ているので，そのまま引用したい．

Adaptation has been defined and recognized by two different criteria: historical genesis (features built by natural selection for their present role) and current utility (features now enhancing fitness no matter how they arose). Biologists have often failed to recognize the potential confusion between these different definitions because we have tended to view natural selection as so dominant among evolutionary mechanisms that historical process and current product become one. Yet if many features of organisms are non-adapted, but available for useful cooptation in descendants, then an important concept has no name in our lexicon (and unnamed ideas generally remain unconsidered): features that now enhance fitness but were not built by natural selection for their current role. We propose that such features be called exaptations and that adaptation be restricted, as Darwin suggested, to features built by selection for their current role. We present several examples of exaptation, indicating where a failure to conceptualize such an idea limited the range of hypotheses previously available. We explore several consequences of exaptation and propose a terminological solution to the problem of preadaptation.

　通時的な発生と共時的な有用性の混同という問題，そしてそれを是正するための新用語としての exaptation は，言語論においても役立つはずである．コミュニケーションに役に立つべく通時的に発生してきた言語項 (adaptation) と，偶然に発達して結果的に役立つ機能を得た言語項 (exaptation) を区別しておくことは，確かに必要だろう．また，exaptation は，目的論 (teleology) 的な言語変化観に対抗する用語と概念を与えてくれるようにも思える．
　Gould and Vrba (5) は，"A taxonomy of fitness" と題する表で，以下のように関連する術語を整理している．

Process	Character		Usage
Natural selection shapes the character for a current use --- adaptation	adaptation	aptation	function
A character, previously shaped by natural selection for a particular function (an adaptation), is coopted for a new use --- cooptation	exaptation		effect
A character whose origin cannot be ascribed to the direct action of natural selection (a nonaptation), is coopted for a current use --- cooptation

　これらの術語群を言語変化に（部分的にであれ）適用するには試行錯誤が必要なことはいうまでもないが，この方向には多くの可能性が開かれているように直感される．

　・ 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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