… the assumption that language change happens at a given rate, or that it can be understood regardless of external factors, is a serious mistake that has pervaded historical linguistics for centuries. Evidence for this comes from a variety of sources, e.g. the fact that (relatively) isolated populations, like the ones we find in islands, speak languages with markedly archaic traits if compared to other related languages. This can be seen in Icelandic, for example. There is no internal clock in languages signalling the moment for divergence. In fact, the idea of divergence from the common tree is also very poor when applied to historical linguistics. Languages are not like animal or plant species that can be distributed along a genealogical tree in the form of branches or sub-branches that are the consequence of mutations and adaptation. Languages are something quite different.
Jesús Sanchis: Language Continuity. La velocidad del cambio.