How does natural selection occur?

A population of organisms exists with a set range of alleles. In an organism, a random mutation can result in the formation of a new allele. Some mutated alleles can be beneficial to an organism, and increase their ability to survive. As these organisms are more likely to survive, they are more likely to reproduce successfully and pass on their advantageous alleles to their offspring. This is called differential reproductive success in a population. Therefore, the number of organisms within the population possessing the advantageous allele increases, thus the proportion of organisms surviving increases. This leads to evolution, as the proportion of organisms with the beneficial allele continues increasing and becomes more common.