Final answer:
Natural selection is the key mechanism of mammalian evolution, acting on phenotypic variations and driving the diversity of species through survival and reproduction advantages.
Step-by-step explanation:
The evolution of mammals is best explained by natural selection, one of the major mechanisms of evolution. Natural selection operates on the phenotypic variations within populations, favoring traits that enhance survival and reproduction. Over time, this process can lead to significant evolutionary change, resulting in the diverse array of mammalian species we see today. Evidence of this evolution includes a rich fossil record, anatomical structures, and embryological development that suggests common ancestry but shows modification over time.