I love this stuff: research about how and why certain words change while others do not. 

Partha Niyogi, author of the book The Computational Nature of Language Learning and Evolution and a professor of computer science and statistics at the University of Chicago, says these empirical findings are consistent with theoretical models on the lexical evolution. “Languages are constantly changing,” he notes. “In biological evolution that fact has been given a lot of attention, but the fact is that in languages this is happening all the time, [as well]. Darwin in [The Descent of Man] commented that languages were evolving over time, and it was just like speciation.”