Two Mice of the Same Species Have Different Ear Shapes

Short Answer

Two mice of the same species have different ear shapes. You find that one mouse, having ears of normal shape, was caught in a field in Kenya. The other mouse, with curled ears, was caught in the frozen tundra of Greenland. You have determined that both mice have identical genotypes at the gene loci controlling ear shape. How would you explain the differences in ear shape?