As new changes in the DNA sequence are constantly being generated in a population's gene pool (changing the population's allele frequencies), some of these changes will be beneficial and result in traits that allow adaptation and survival. Natural selection causes evolution of a species as these beneficial traits become more common within a population. Evolution can occur within a species without completely resulting in a new species. Therefore, evolution and speciation are not the same.
In population of mice, young mice could be born with one of three colors. They were born with grey, black or white fur. Initially the color was random. However black coloring made the mice easy targets for fox and cats during the day while white coloring made them easy targets for owls at night.
What color mouse would become more common over time?