Humans love to categorize things. It makes thinking easier if one can create convenient huristics instead of dealing with everything one at a time. I'm gonna have to whisper this but sometimes stereotypes aren't bad.
The definition of species is like that. To draw an analogy that will be missed by the people who need it most, if you wake up one morning and it's 55 degrees outside, and by mid-afternoon your thermometer reads 95 degrees, at what temperature would you say the day went from "cool" to "hot"?