Flocking Behaviour: The Regularity of Sound Change

Sound change is amazingly regular. If different words contain the same sound (especially in the same context), the sound will be similarly affected by historical changes. As an example, consider Proto-Germanic /ai/, which regularly yields Old English /ɑː/, Middle English /ɔː/, and Modern English /oʊ/ (with the usual proviso concerning slightly different dialectal developments).

PGmc. /ai/	OE /ɑː/	ME /ɔː/	Mod.E /oʊ/
*aik-	āc	ǭk	oak
*baina-	bān	bǭn	bone
*faima-	fām	fǭm	foam
*gait-	gāt	gǭt	goat
*xlaiba-	hlāf	lǭf	loaf
*raidō	rād	rǭd	road
*staina-	stān	stǭn	stone

You could say that words containing the same sound (or phoneme, to use a technical term) prefer to evolve together in the same way, rather than independently. They behave like a flock of sheep moving in the same direction. The full picture is actually more complicated. Lexical sets may have subsets in which a more specific phonetic environment affects the pronunciation, giving rise to a subregularity.

• For example, PGmc. *klaini- developed into pre-Old English *klāni-, but the *i in the second syllable caused an assimilatory change known as i-umlaut: the back vowel /ɑː/ changed into front /æː/. OE clǣne eventually developed into Modern English clean /kliːn/; and that is the regular development whenever i-umlaut applies.
• If OE /ɑː/ (ME /ɔː/) was followed by /r/, the vowel failed to develop an u-like glide in mainstream Modern English, ending up as /ɔː/ instead, as in boar from OE bār.

In such cases, a smaller group becomes isolated from its mother flock and follows its own course. They often join another flock. Thus, clǣne and other umlauted forms joined the OE words containing /æː/ of other origin. In Middle English times they merged with still other lexical sets: words containing the Old English diphthong ēa (pronouced /æːɑ/), and words with an OE short /e/ that underwent lengthening in Middle English before a single consonant followed by a weak vowel (as in OE mete ʻfoodʼ). They formed one mighty herd of words with Middle English /ɛː/ > Modern English /iː/. Can you guess which Modern English word is the descendant of OE mete? [see bottom right].

Flocking behaviour (induced by a stimulus)

The regularity of sound change (at least in macroevolutionary time-scales) is extremely important in historical linguistics. It allows us to demonstrate, for a pair of languages, that they derive from a comon ancestor: if they do, regular sound changes leave a statistically significant pattern of correspondences between their vocabularies. The expectation of regularity also helps us to distinguish evidence of common ancestry from the effects of linguistic borrowing, and from chance similarities. This remains true even if we realise that the regularity of sound change is not always perfect. In further posts I shall try to show when and why we may expect words to stray from their flock and travel on their own.

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