I am fascinated by the evolution of language, and how local versions diverge to become dialects like Cornish English and Geordie and then imperceptibly diverge further to become mutually unintelligible but obviously related languages like German and Dutch. The analogy to genetic evolution is close enough to be illuminating and misleading at the same time. When populations diverge to become species, the time of separation is defined as the moment when they can no longer interbreed. I suggest that two dialects should be deemed to reach the status of separate languages when they have diverged to an analogously critical point: the point where, if a native speaker of one attempts to speak the other it is taken as a compliment rather than as an insult.
Richard Dawkins, An Appetite for Wonder: The Making of a Scientist
evolution, language