Genetic variation is one of the principles for natural selection. This is the individuals in a population or group differ in some trait of interest. With these two mice they both have a different color in their fur
Overproduction and struggle for survival are for some individuals with a particular trait more likely to survive and or to have more offspring compared to the individuals that do not have that trait. The dark fur makes the mouse stand out ,but it is good if they live under dark rock and is bad living out in sandy desert.
Different survival and reproduction is like the trait that helps individuals survive or leave more offspring will increase in the population. The different survival fur that helps with camouflaging and not to stand out for prey to see it.
Change in the gene pool are beneficial traits that are passed on to future generations over time, allow for the gene pool (all the genes in a population) to change. The mouse with the black fur will stand out and eaten. But the Tan mice will blend in with the sand.