Natural selection can serve to stabilize a population if the new traits, called mutations, are eliminated when they appear because they are not as well-adapted to the environment. The opposite effect is obtained when a new trait is introduced which allows individuals to adapt better over time, the species will change as this mutation becomes more widespread in the population. In human beings, increased brain size helped individuals to adapt better, and so brain size increased gradually in the species. Changes in the overall genetic makeup of a population are normal when there are environmental changes, especially sever environmental disruptions. Specialized adaptations to specific environments can lead, over time, to the development of subpopulation of individuals, ones who are better adapted to particular soil conditions food sources and so on. Given enough time, these subpopulations may develop into separate species, such as zebras and horses, living in distinct environments and not interbreeding.
Individuals in a population differ from one another because______.

A. they have different success generation by generation
B. they are ill-adapted and eventually die out
C. they have different genetic inheritances
D. they want to survive and reproduce successfully