Genetic variation in a population allows for phenotypic variation, meaning that different traits exist in the same population. When the environment changes, the members of the population are tested by natural selection with regards to how well adapted their specific traits are to the new environment. If the traits they have do not suit their environment, this results in reduced survival and fitness, leading to less of that trait in the population. However, if the traits they have are adapted to the environment, they become naturally selected for with better survivability and fitness (leading to a higher frequency of that trait in the population). Thus, evolution has occurred.
