Is there a biological reason to vowel shift?

Question

Trying to explain vowel shift in French I wrote on FL&U that "Vowel sounds are more prone to evolution than consonants. I suppose it is because phonatory organs are not as easily controlled when sounding vowels than when sounding consonants". Could someone tell me if my supposition is supported by scientific evidence?
My question is not on the historical evolution of pronunciation in languages (as the English Great Vowel Shift or the French vowel shift) but on the actual morphology of the phonatory organs and the way we use them.

Answer 1

The answer has an air of reasonableness, but the only problem is that it is not exactly clear what it means for the phonatory organs to be "loose." I think, however, a reasonable construal is that it is more difficult to achieve precision in vocalic gestures than in consonantal gestures. There are a couple of reasons why this might be a reasonable explanation: first, the main articulator responsible for vowel sounds, the tongue body, moves more slowly than does the tongue tip, which is more associated with consonant sounds. Second, proprioception is not as good a guide for vowel sounds as it is for consonant sounds; stop and obstruent articulations provide tactile (from direct contact or pressure-sensing) feedback which is not present in vowel articulations.

Another perspective which I think may be just as valid is the auditory/acoustic perspective. According to this perspective, the issue is not that there is a difficulty in controlling gestures, but that there is a difficulty in hearing the difference between similar-sounding vowels. In other words, we are able to reproduce vowel gestures which are very faithful to an acoustic image, but it is the acoustic image which is difficult to recall with accuracy.

An excellent discussion on these two competing explanations can be found in
LINDBLOM, B.- ENGSTRAND, O. (1989) "In what sense speech is quantal?", Journal of Phonetics 17: 107-121. (see also other contributions to that issue)

A third possibility, which I don't think can be ruled out right away, is the sociolinguistic explanation. It might be that both auditory and articulatory precision are within our control, but that vowel sounds carry sociolinguistic information which needs to be adjusted depending on social forces (e.g., a cognate vowel in two dialects might diverge as the two groups try to emphasize their linguistic distinctness).

See work by William Labov for this tradition of research.

Answer 2

Your question is very interesting and challenging.

I dare to say that vowels are not more prone to change, generally.
Although it is the case in Germanic languages, this is not true in general.

What triggers languages to change

Schleicher is usually considered the first linguist who suggested that languages evolve in a similar manner to biological processes.
Therefore, it would be accurate to assume that language changes are similar to biological mutations.
As all mutations, these changes are random by their nature. Randomness imply that there is no natural preference over vowels or consonants.

Why vowels, not consonants

English, likewise many other Germanic languages, have a very developed vowel set. Look at these maps. Note: only monophthongs are listed, no tones, no vowel length.

English (16 key points)

Thai (9 key points);

Farsi (6 key points)

Arabic (3 key points)

The more places for possible "mutation", the more likely a mutation will occur.

Why changes can be massive

This article has an interesting insight:

To use a silly metaphor, imagine that Spanish is a train car with only five riders, while English is a car packed with thirteen people. The five people in the “Spanish” car are likely to remain put for the entire journey (there’s so much room!) The thirteen people in the “English” car, however, tend to jostle around, move to less crowded parts of the train, make room for people as the enter, etc. Simply put, they’re more likely to shift.

But how do particular vowel shifts begin in the first place? What gets the ball rolling?

There are two ways a vowel shift can be described. The first is as a “pull chain.” Extending the above metaphor, imagine that a passenger on our crowded car train notices an open space a few feet down, so he moves. A second rider moves into the empty space that the first passenger left behind, then another person moves into passenger #3′s space. And so on and so forth.

A “push chain,” on the other hand, means the opposite: turning again to our “train,” this means an obnoxious train rider pushes another rider out of the way, that rider stumbles and pushes a third person out of the way as well. In real terms, this means that “pot” moves toward the vowel in “pat,” pushing it toward the vowel in “pet.”

Counter-examples

Moreover, there are many evidences when consonants evolved to a larger extent than vowels. Sanskrit had rather a poor vowels set ([a/i/e/o] plus phonated [r/l/m]), but it has developed a huge consonants list, including such exotic ones like ব্ধ্ব [bdhva] or ন্ধ্ব [ndhva]. No surprise that modern languages evolved from Sanskrit (e.g., Thai, Lao, Hindi) dramatically simplified the consonants, while the vowels actually retained.

So, the answer is:

• Language change processes are similar to biological mutations;
• The more "placeholders" for mutation, the more likely a mutation will occur;
• Germanic languages often have large vowel sets, so the changes in vowels are more likely to happen;
• Many languages have dramatically changed their consonants contrary to their vowel sets that remained more or less intact for hundreds of years;