Asktutor Query:
Name: Jan
Country and Region: Trinidad and Tobago, UWI, St. Augustine
Native Language: English
Student or Teacher: Student
Age or Grade: undergraduate 1st. year

Subject of Question: Morphology

Give a well-illustrated explanation of why variants of a morpheme may
sometimes be described as phonologically conditioned.  How does
phonologically conditioned variation differ from other forms of variation?

Please note that I only wish some direction as to what/where to start.

Thank you


Hi Jan,
 
I am writing on behalf of Ask a Linguistics Tutor in response to your question on Morphology.  To gain an insight into morphophonemics, first consider the following set of data:
 
List 1: intolerant, intractable, indefeasible, indefinite
List 2: impossible, impoverished, imbalance
 
In both lists, we unilaterally find a prefix meaning 'not', but we find it with two different phonetic realizations, namely, [In-] and [Im-].  We might ask, then, are we dealing with two separate, but synonymous, prefixes?  Most likely not.  For one, the two prefixes occur in a highly restricted distribution.  We cannot say 'imdefinite' or, conversely, 'inpoverished'.  This situation contrasts with lexical synonymy, in which the relevant lexical items are distributed much more fluidly.  Consider the (near) synonyms 'pretty,' 'beautiful,' and 'attractive'.  Now take the sentence 'The ______ girl dropped her ice-cream cone'.   We can insert any of the three words in the blank without a large shift in meaning, or, for that matter, semantic well-formedness.
 
Second, notice that the two prefixes sound remarkably similar, differing only by one feature of articulation, namely, place of articulation.  [Im-] is articulated at the lips, while [In-] is pronounced slightly farther back at the alveolar ridge.  Notably, this articulation schema holds for the segments immediately following the prefix.  Every instance of an [In] prefix is followed by a sound that is articulated at the alveolar ridge, while every instance of an [Im] prefix in our list is immediately followed by a sound produced at the lips.  This scenario represents a textbook case of regressive ASSIMILATION, which is a co-articulatory process whereby features of one sound can 'bleed' onto features of another near-by sound.  In this case, the place-of-articulation feature on the stop is anticipated by and adapted onto the nasal in the prefix.
 
Now that we've seen that [Im-] and [In-] are contextually related, the burning question now becomes: which of the two forms is the lexical realization of 'not' and which of the two forms is the surface/conditioned realization?  To find out, we simply adopt an Ockhamist stance and assume that the form whose distribution is more difficult to motivate (i.e. whose derivation would require all sorts of disjoint and/or crazy phonetic rules) is the underlying one.  So, hunting more diverse contexts in which to place the 'not' prefix, we get the following:
 
List 3: ineffable, inanimate, inoperable, invalid, infidelity
 
Crucially, after front and back vowels, high and low vowels, and after labiodental fricatives, we find the alveolar prefix [In-].  Thus, it's a good bet that [In-] is the lexical/underlying form of the prefix meaning 'not', while [Im-] is derived solely from a lexical item's contextual factors.
 
Now, to answer the second part of your query, that is, why phonetic variation need not be conditioned, take the word 'aunt,' meaning the sister of one's mother or father.  I pronounce the word as (approximately rendered) [awnt], that is, rhyming with 'daunt' or the vowel in 'launch'.  Most other native speakers of American English, however, pronounce the word as [aent], that is, identically to the word 'ant' and rhyming with 'can't' and 'plant'.  The pronunciation difference isn't due to contextual factors, given that I can say 'I like my [awnt]' just as much as my neighbor can say 'I like my [aent]'.  Rather, it is simply due to dialect differences.  Many people in the state of Virginia say [awnt], and my Mom, born and raised as a Virginian, is one of them.  Hence, I picked up the trait from her speech, even though I was raised in Tennessee and even though I attend school in Michigan, both of which are places where people typically say [aent].  
 
This variation is aptly called 'FREE VARIATION'.  As you can tell, the origins of any one variant can be rather difficult to trace and can often be ascribed only to a matter of personal history or preference. 
 
I hope this helps!
 
Sincerely,
 
sarah goodman
Ask a Linguistics Tutor
University of Michigan