What is the measure of the ability to survive and produce more offspring relative to other members of the population?

It is adaption because you need to adapt to certain areas SO your offspring's can survive its the same as in if all of a sudden the world was cove rd in snow and there were 1000,0000 white rabbits and 20000.00000000 black rabbits the black rabbit population would decrees because the rabbits need to adapt to the environment to protect it self from predator's.