Why is this?
There are two different L allophones: a “light L” [l] is pronounced more at the front of the mouth while a “dark L” [ɫ] is pronounced more at the back of the mouth. For example:
In some English dialects, the words light [lʌɪt] and lit [lɪt] have “light L’s” at the start, while the words occult [əˈkʰʌɫt] and full [fʊɫ] have “dark L’s” at the end of the syllable.
In some Portuguese dialects, the words ler [leɾ] ‘read’ and libra [ˈliβɾɐ] ‘pound’ both start with light L’s, but the words fácil [ˈfasiɫ] ‘easy’ and mal [maɫ] ‘bad’ both end with a dark L in those Portuguese dialects that do this, although those same words in Spanish always have only a light L.
In a number of languages, including most varieties of English, the phoneme /l/ becomes velarized in certain contexts, a sound often called "dark l". Some languages, like many North American dialects of English, may not have a "clear" /l/ at all, or use it only before front vowels (especially [i]).
The velarized alveolar lateral approximant (dark l) is a type of consonantal sound used in some languages. It is an alveolar, denti-alveolar, or dental lateral approximant, with a secondary articulation of velarization or pharyngealization. The regular symbols in the International Phonetic Alphabet that represent this sound are ⟨lˠ⟩ (for a velarized lateral) and ⟨lˤ⟩ (for a pharyngealized lateral), though the dedicated letter ⟨ɫ⟩, which covers both velarization and pharyngealization, is perhaps more common.
How does this phenomenon arise phonologically, and why do native speakers of languages in which this occurs not consider those distinct phonemes? Is this a property that the physical and acoustic characteristics of the buccal cavity lend themselves to naturally, or is it instead some sort of mental adaptation that occurs only in certain cultures?
Are there any languages that have both kinds of L — meaning alveolar or “light” or “clear” on the one hand versus velarized or “dark” on the other hand — as distinct phonemes with minimal pairs rather than as mere allophones?