What is natural selection, and how is it related to changes in allele frequency?

Natural selection describes the selective pressure favouring those species with certain phenotypes (characteristics) which make them better suited to the environment than other species. Those with favourable characteristics are more likely to survive and reproduce, thus passing on the favourable alleles to their offspring. This results in the allele frequency of the favourable allele in the population increasing. The allele frequency of other, less favourable alleles, decreases as those without the favourable allele will find it more difficult to survive and breed, thus they will create fewer offspring.