When applied to a particular allele, this theory posits that mutation carriers have a selective advantage, perhaps in a particular environment.

Selection occurs when the fitnesses of particular alleles are inequal, hence selection always exerts a load.

For example, leuA is one of the genes of the leucine biosynthetic pathway, and leuA273 is a particular allele of this gene.

It is calculated as the frequencies with which particular alleles at particular genes occur across all of the members of a population that interbreed with each other.

The second line sought to explain the origin and maintenance of sequence variation within populations, and the fixation of particular alleles between species.

The frequency of one particular allele will become more or less prevalent relative to other forms of that gene.

Some Scottish Collies (and other collie breeds) have a particular allele of the multi-drug resistance gene, MDR1.

Formal definition: Allele or gene frequency is the percentage of all alleles at a given locus in a population gene pool represented by a particular allele.

However, if this particular allele becomes common, selection pressure on parasites to avoid recognition by this common allele increases.

In these terms evolutionary adaptation may occur most obviously through the natural selection of particular alleles.