Natural selection was not a conscious choice. There have been many choices in grammar that have been decided almost arbitrarily by higher authorities. Sure, many of these choices may have been based on their own experiences, but they were not necessarily the best choices. They were decided by a small group of individuals, but again, the choice was not strictly a natural selection of grammar rules. Language more often follows this pattern than grammar because people are more likely to argue over grammar than language itself, excluding etymology, of course.
> There have been many choices in grammar that have been decided almost arbitrarily by higher authorities
Just because somebody makes a decree about grammar doesn't mean it'll be widely accepted, any more than an organism obtaining a genetic mutation means it'll become spread throughout the population in subsequent generations.
My point is just that there wasn't any overarching plan. People design their own use of it, but whether or not that becomes popular enough to be considered "the rule" is analogous to natural selection - messy, convoluted, frequently arbitrary, but good enough.
