Codons - each represents an amino acid. The information of the genetic code is contained as a long sequence of nucleotides which code for proteins. E.g. the peppered moth. Evolution is caused by changes in gene frequencies over time. We can measure the proportion of different alleles in a species.
Populations are the sub-units of species.
When populations are separated, they often do not contain the same gene frequencies. Gene flow brings new genetic frequencies to populations and serves to homogenize the distribution of alleles. Gene flow is caused by individuals from one population interbreeding with another population.