英語には，歴史的な大母音推移 (Great Vowel Shift; gvs) とは別に，現代英語の共時的な現象としての "Modern English Vowel Shift Rule (alterations)" というものがある．後者は，現代英語の派生語ペアにみられる音韻形態論的な変異に関する共時的な規則である．Chomsky and Halle に端を発し，その後の語彙形態論でも扱われてきたトピックだ．たとえば，次のようなペアの母音に関して交替がみられる．various ? variety, comedy ? comedian, study ? studious, harmony ? harmonious; divine ? divinity, serene ? serenity, sane ? sanity, reduce ? reduction, fool ? folly, profound ? profundity (McMahon 342) .
　McMahon は，この共時的な規則を最適性理論 (Optimality Theory; ot) で分析した先行研究の不備を指摘しながら，同理論は韻律論 (prosody) の問題を扱うのには長けているが，母音交替のような分節音に関する問題には適用しにくいのではないかと論じている．両部門は，一般には同じ「音韻論」にくくられるとはいえ，実際には独立性が高いのではないかという．McMahon (357) は，独立性の根拠をあげながら次のように論じている．

. . . we must recognise prosodic and melodic phonology as two different systems, and in fact there are many different strands of evidence pointing in that direction. For instance, prosodic phonology shows clearer connections with emotion, with gesture, and with aspects of call systems in other primates. Prosodic and segmental phonology behave very differently in cases of language disorder and breakdown, with some children who are seriously language-impaired finding considerable compensatory strength in prosody, for instance. Prosodic phonology is also acquired early, and there is evidence that the brain lateralisation of prosody and melody is different too, with more involvement of the right hemisphere in prosody, and strong left hemisphere localisation for segmentals. If prosody phonology and segmental phonology are actually two very different systems, which have evolved differently, are processed differently, are acquired differently, and interact with different systems (as with the paralinguistic uses of prosody, for instance), then there is no need for us to expect a single theory to deal with both.

　この見解は，OT の理論的な扱いに関するにとどまらず，言語学における音韻論の位置づけについても再考を促すものとなるだろう．

　・ Chomsky, Noam and Morris Halle. The Sound Pattern of English. New York: Harper & Row, 1968.
　・ McMahon, April. "Who's Afraid of the Vowel Shift Rule?" Language Sciences (Issues in English Phonology) 29 (2007): 341--59.

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