Dissimilation is the process whereby one of two identical or very similar sounds is changed so as to make the two less similar. In a few English words there are changes in both the pronunciation and the spelling. For instance, Latin peregrinus became in Italian pelegrino, which became English pilgrim, and earlier English berfrey became our belfry, in both words the first [r] sound and <r> letter changing to [l] and <l>. On the other hand, Latin melimelum became our marmalade, with the first [l] and <l> changing to [r] and <r>.
Sometimes, though the pronunciation changes, the spelling does not. Thus,
colonel is prounced as if it were kernel, with [l] becoming [r], though the spelling keeps the two <l>’s. And diphthong, with [fth] in the middle, has an accepted pronunciation as if it were spelled dipthong – that is, with the [f] in the string [fth] dissimilated to [p]. February also has an accepted variant pronunciation as if it were spelled Febyuary.
You may have noticed that most of the cases of dissimilation involve the sound [r] and the letter <r>. In a word with two [r] sounds sometimes one of the two is simply deleted in both pronunciation and spelling, as in English prow, which comes ultimately from Latin prora. Sometimes the deletion occurs with no change in spelling – for instance, caterpillar has an dissimilated and accepted pronunciation as it were spelled catepillar.
But usually the dissimilated pronunciation, though quite common in everyday speech, is not an accepted variant. The following are some examples: reservoir is often pronounced as if it were spelled resevoir; southerner is often pronounced as if it were spelled southener; berserk as if it were spelled beserk; surprise as if spelled suprise; particular as if spelled paticular; governor as if spelled govenor; and library as if spelled libary. These simplified pronunciations can be seen as examples of dissimilation.