My guess would be that it happened gradually. Someone living in a place where the minimum temperature is 15 might be willing to move north to a place where the minimum temperature is 13 as long as other factors are better for him. Couple of generations later you have someone who’s living in a place where the minimum temperature is 13 willing to go north to a place where the minimum temperature is 10, so on and so forth.
My guess would be that it happened gradually. Someone living in a place where the minimum temperature is 15 might be willing to move north to a place where the minimum temperature is 13 as long as other factors are better for him. Couple of generations later you have someone who’s living in a place where the minimum temperature is 13 willing to go north to a place where the minimum temperature is 10, so on and so forth.