Nucleotide diversity

Nucleotide diversity is a concept in molecular genetics which is used to measure the degree of polymorphism within a population.

One commonly used measure of nucleotide diversity was first introduced by Nei and Li in 1979. This measure is defined as the average number of nucleotide differences per site between two DNA sequences in all possible pairs in the sample population, and is denoted by ${\displaystyle \pi }$.

It is given by the formula:

where ${\displaystyle x_{i}}$ and ${\displaystyle x_{j}}$ are the respective frequencies of the ${\displaystyle i}$ th and ${\displaystyle j}$ th sequences, ${\displaystyle \pi _{ij}}$ is the number of nucleotide differences per nucleotide site between the ${\displaystyle i}$ th and ${\displaystyle j}$ th sequences, and ${\displaystyle n}$ is the number of sequences in the sample.

...
Wikipedia