Population Differentiation Index
The Population Differentiation Index (PDI) is an estimate of the percentage of genetic drift variance retained. In other words, it is the extent to which random drift in each population is independent and is therefore an objective criterion for the relatedness of populations.
About genetic relatedness
Populations of the same species in animal farming are always genetically related, at least to some extent. A common origin, exchange of breeding stock between population, a similar breeding goal and a similar selection environment create genetic similarity between populations. For endangered animal breeds, it is desirable to have objective criteria to establish the genetic relatedness with similar populations in other countries and other breeds in the same country. Several measurements of genetic relatedness of populations exist, but making recommendations is cumbersome, as these measurements are highly dependent on the specific case.
Random drift is defined as change in allele frequencies by chance. These changes are more pronounced in smaller populations. In related populations, random drift tends to work in the same direction, for example because of a common genetic origin or regular exchange of breeding individuals. The Population Differentiation Index (PDI) is an estimate of the percentage of genetic drift variance retained. In other words, it is the extent to which random drift in each population is independent and is therefore an objective criterion for the relatedness of populations. PDI ranges from 0% for completely related populations to 100% for completely unrelated populations.
Factsheet
Population Differentiation Index
Applications
Two breeds in neighbouring countries have a similar appearance, breeding goal and living conditions. Should they be conserved as two separate populations or as a single one?
Two populations have a low level of exchange of breeding males. Are they two breeds or one breed?
An endangered breed occasionally uses specifically selected breeding males of a large breed. When is it an opportunity and when does it change into a threat?
A subpopulation of an existing breed was used to form a new breed or create a new synthetic breed. After how many generations can the new breed be considered as sufficiently unrelated to the breed of origin?
Recommended use
PDI is intended as a piece of objective information to aid decision making on genetic conservation, not as the sole criterion. The tool requires that the two populations have pedigrees with multiple generations. This may be a set of real pedigrees or a set of pedigrees that is simulated given the actual breeding structure. If there is only a limited number of generations of pedigree available, it is advised to do both. A simulation tool is also available with the PDI tool.
In order to use this method consistently across species, we recommend to use a fixed time horizon, for example 40 years. Divide this time horizon by the generation interval of the breed or species to obtain the target number of generations to consider in the simulation.