Genetic distance

Genetic distance is a measure of the genetic divergence between species or between populations within a species. Populations with many similar alleles have small genetic distances. This indicates that they are closely related and have a recent common ancestor.

Genetic distance is useful for reconstructing the history of populations. For example, evidence from genetic distance suggests that African and Eurasian people diverged about 100,000 years ago. Genetic distance is also used for understanding the origin of biodiversity. For example, the genetic distances between different breeds of domesticated animals are often investigated in order to determine which breeds should be protected to maintain genetic diversity.

In the genome of an organism, each gene is located at a specific place called the locus for that gene. Allelic variations at these loci cause phenotypic variation within species (e.g. hair colour, eye colour). However, most alleles do not have an observable impact on the phenotype. Within a population new alleles generated by mutation either die out or spread throughout the population. When a population is split into different isolated populations (by either geographical or ecological factors), mutations that occur after the split will be present only in the isolated population. Random fluctuation of allele frequencies also produces genetic differentiation between populations. This process is known as genetic drift. By examining the differences between allele frequencies between the populations and computing genetic distance, we can estimate how long ago the two populations were separated.

Although it is simple to define genetic distance as a measure of genetic divergence, there are several different statistical measures that have been proposed. This has happened because different authors considered different evolutionary models. The most commonly used are Nei's genetic distance, Cavalli-Sforza and Edwards measure, and Reynolds, Weir and Cockerham's genetic distance, listed below.

In all the formulae in this section, ${\displaystyle X}$ and ${\displaystyle Y}$ represent two different populations for which ${\displaystyle L}$ loci have been studied. Let ${\displaystyle X_{u}}$ represent the ${\displaystyle u}$^th allele at the ${\displaystyle l}$^th locus.

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