How can small variations produce large differences in organisms?

In the case of genes, one change in one sequence in a nucleotide can lead to major changes in the entire being of a living being. For example, a difference in two nucleotides could cause a a genetic disease like cystic fibrosis. The number of ways nucleotides can be arranged and the number of nucleotides differ from one organize to the next. This alone could explain how large differences between organisms are produced.