Answer:
Within a population there is always genetic variation. This will give rise phenotypic variation, which are the observable characteristics of an individual.
The environmental variables will put a certain pressure on an individual. Because all individuals vary to a certain degree, their ability to cope with this pressure and survive will also vary.
Thus, organisms whose traits are better suited to the environment will have a higher chance of survival. These individuals will successfully mate and in this way, pass on their genetic information and advantageous traits.
Because of this selection, in time this trait will become dominant within a population. Given enough time, according to the theory of evolution, under the influence of environmental factors, an organism's body will evolve and become more and more adapted to its environment.