How does natural selection change thephenotypes within a population over time?

Phenotype is the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. Essentially its physical characteristics. According to natural selection, some characteristics are more likely to survive and thus pass on to children of a generation. Therefore, over time said characteristics such as the ability for an organism to blend in with its environment get passed along and become more prominent within a population ultimately making the appearance (or phenotype) of the population more synonymous with the characteristics "chosen" by natural selection.    

Explanation:

Individuals with “better” more adapted phenotypes(characteristics) are naturally selected by the environment and they’re genetic information, thus phenotype, gets passed on more than others because individuals with those certain traits will reproduce more and survive longer.

From DNA to RNA to proteins. Many genes encode protein products, meaning that they specify the sequence of amino acids used to build a particular protein.