According to a New Scientist article I came across the other day, frequently-used words tend to be more resistant to change then words that are used less often.
A team at the University of Reading lead by Mark Pagel, an Evolutionary Biologist, compared the words used to express 200 different meanings in 87 different Indo-European languages. They found that the more frequently a word is used in speech, the less likely it is to change over time. They also found the conjunctions and prepositions tend to change more readily than numbers, pronouns and question words like who, what, where, etc. The team calculated a ‘mutation rate’ for each of the words the studied and predicted that frequently-used words are likely to resist change for over 10,000 years.
Another study at Harvard University demonstrated that the most frequently-used English irregular verbs have tended to remain stable over time, while most of the least frequently-used ones have become regular.