Darwin's postulate are that 1) there is variation among organisms in a population and that 2) some of this is heritable. 3) This leads to significant and 4) non-random differences in survival and reproductive success among individuals. To answer the question, think of how these mice would fit these postulates in a bright, hot, sandy, sunny habitat likes dunes in Florida.
Work Step by Step
1) The mice in the sand dunes will vary in how they are suited to live in a bright, hot, dry, salty habitat. 2) Some of this will be heritable, such as mice with a coat color that matches the color of the sand and camouflages them from the sight of predators. Better adapted mice will produced significantly more offspring (3,) and this will be because the offspring will also be naturally selected, not due to random chance (4). These more reproductively successful, better adapted mice would include such examples as lighter colored individuals.