How I remember being told over and over how to spell tongue! I didn't understand it then; I don't understand it now. What evolution might put a silent "ue" at the end of a word?
The spelling comes from the French, and indicates that the g was once pronounced -- in other words, it wasn't always just an engma (ŋ). However, the word tongue doesn't come to us from French; the Old English and some Middle English used tunge. It appears that the French style of spelling won out somewhere in the 14th century, but since that time the pronunciation has changed. For that matter, the sorts of French words, like langue, that influenced the English spelling have also (mostly) lost the g sound over time.
A case of pronouncing the word the way it originally was, even when the spelling changed.
Interesting to note that this word was originally spelt "tounge":
The "e" was always silent, and I suppose they just moved the "u" to the back when they standardized spelling. This doesn't mean that all "-ue" endings are silent. It was spelt "tongue", but it is pronounced "tunge", because it came from "tunge":
This doesn't mean that all "-ue" endings are silent, it's just that in this special case, it is pronounced the way it was originally spelt, even when the spelling changed.
One case of "-ue" being pronounced is "Montague", pronounced: