Final answer:
Natural selection acts on the population's heritable traits to select for beneficial alleles and adapt to the environment. This process is known as adaptive evolution.
Step-by-step explanation:
Natural selection acts on the population's heritable traits: selecting for beneficial alleles that allow for environmental adaptation, and thus increasing their frequency in the population, while selecting against deleterious alleles and thereby decreasing their frequency.
Scientists call this process adaptive evolution. Natural selection acts on entire organisms, not on an individual allele within the organism.
An individual may carry a very beneficial genotype with a resulting phenotype that, for example, increases the ability to reproduce (fecundity), but if that same individual also carries an allele that results in a fatal childhood disease, that fecundity phenotype will not pass to the next generation because the individual will not live to reach reproductive age.