What change would air mass modification cause in a continental polar air mass that moves south over the central united states

 Changes hot and dry as it moves, but for an air mass to take uniform properties it is necessary to park, more or less, for a certain number of days over a large region where the underlying surface have uniform characteristics. This region is called the air mass generating region.

It is evolution, because we all have some of the same traits like eye color or shape or ears.