Natural selection operates on the whole individual, not on individual alleles. It is the combination of alleles at all gene loci that determines an individualís fitness. In a particular environment the most fit individual in the population may not have the best alleles at all gene loci and the least fit individual may not have the worst alleles at all gene loci. However, the more fit individuals have more good alleles than those with lower fitness. Since the more fit individuals survive better, then, after selection, there will be a higher frequency of good alleles in the population. The offspring of these survivors will also have a higher proportion of good alleles. The natural selection process will increase the proportion of good alleles over generations.