1) It's generally considered that allophonic modification is caused by the economy of effort, which means that the speaker avoids articulatory movements which are not absolutely necessary for intelligibility of speech. That's why English lenis consonants /b,d,g/ in final positions can be voiceless, but they are not replaced by the fortis /p,t,k/. /kæb/ - /kæp/.
2) Grammont M. claims that allophonic variations are regulated by the law of the stronger. According to it the stronger phoneme influences the weaker, adapting it to itself, because its articulation is stronger and more stable or because it has a particular position in the syllable. The phonetician supposes that the analysis of combinatory phenomena among phonemes can reveal the phonemes that resist modifications. /əfkɔːs/.
3) Some scholars consider that the factor allophonic modifications is a frequency of occurance of phonemes in phonemic clusters. Frequent phonemes resist modifications and modify rare phonemes. Fletcher: /t,n,s,θ,l,d/ - the most frequent resisting modifications.
In English as well as in the other languages there are cases when different speech sounds occur in different derivatives from the same root. E.g. speak – speech, speak – spoke. Such changes are known in linguistics as sound interchange (=alternation of sounds). The substitutions of one sound by another is not accidental, but a regular phenomenon, possessing definite functions in a language. The sounds that can replace each other in definite cases form a so-called alternation series. The causes of sound interchange may be synchronic and diachronic.
Synchronic causes are those, which are connected with the influence of phonetic laws operating in the language at a given stage of its development. E.g. contextual assimilation, reduction of vowels due to the laws of stress, peculiarities of pronunciation of the sounds in different positions, etc. Such sound interchanges are called phonetic alternations or positional alternations of sounds. In most cases the most important phonetic alternations accompany grammatical phenomena: 1) /t||d||ɪd/ - alternation series in the suffix -ed of the past tense of regular verbs; 2) /s||z||ɪz/ -occurs in the plural forms of nouns and in the 3rd person singular in verbs; 3) /s||z||ɪz/ - in the 3rd person singular of nouns.
Diachronic causes of sound interchange date back to some previous periods in the historical development of the language. E.g. gradation and mutation of sounds. Sound interchange caused by diachronic causes is called historical alternation of sounds. Historical alternation play an important part in present day form and word building. Vowel alternations are used: 1) to form the plural of some nouns, e.g. man – men /æ - e/; 2) to build the basic forms of the irregular verbs, /ɪ – æ – ʌ/ (begin – began - begun), /iː – e – e/(meet – met – met) and so on; 3) to distinguish different parts of speech, e.g. /ɔ – iː/ (hot -heet); 4) to distinguish causative verbs from other verbs, e.g. to rise – to raise /aɪ – eɪ/; 5) to distinguish words which are etymologically related, e.g. mead (used in elevated style) – meadow (neutral), shade – shadow.
Consonant alternations are used: 1) to distinguish forms of words, e.g. send – sent – sent; 2) to distinguish parts of speech, e.g. to speak – speech, n house /s/ - v house /z/, to applaud – applause; 3) in word building when a suffix is added, e.g to correct – correction, to express – expression, to decide – decision; 4) to form the plural of some nouns, e.g. leaf – leaves, house – houses.