What causes the significant variation in ear shape, ranging from dropped to erect?

Answer

The dichotomy between the Bichon Frise's dropped ears and the Yorkie's erect ears

The variation in ear shape results from the contrast between the dropped, hanging ears of the Bichon Frise and the small, V-shaped ears that stand erect on the Yorkshire Terrier.

What causes the significant variation in ear shape, ranging from dropped to erect?
appearanceDogbreedYorkieBichon