Languages fall into a rough nested hierarchy. Obviously, there is cross pollination between languages, especially in the modern era, but languages are a decent metaphor for biological evolution.
As you probably know, changes in spelling, pronunciation, and jargon enter into a language each year. These are equivalent to mutations. As long as a population all speaks the same language, they are like a species. Any changes spread through the population so that everyone understands each other even if they couldn't understand someone from many generations ago (e.g. try reading middle english). In this way, you have mutation and selection.
Speciation is something different, but works with the other two mechanisms. Speciation is two populations that no longer speak to each other, for whatever reason. When you have two populations that don't speak to each other you get DIFFERENT changes occuring in each population. When enough differences accumulate, all of the sudden you have two populations that can't understand each other. That is speciation, and that is exactly what happened to French, Italian, and Spanish, three of the languages that are descendents of Latin. And wouldn't you know it, languages form a branching hierarchy just like evolution: