ESU or evolutionary significant unit is a term used in a situation when there are few subspecies of animals (or plants) exist, but it is impossible to maintain living populations of all of them. Thus sometimes we must prioritize some species for conservation and maintain only one or two populations. There are three main frameworks for identifying conservation units:
1) reproductive isolation and adaptation;
2) reciprocal monophyly;
3) “exchangeability” of populations.
Term reproductive isolation means that ESU species should be a unique product formed evolutionary through hundreds of generations and will never re-evolve.The other characteristic is adaptation which implies that the species should possess unique genetic and phenotypic variation which can be useful for future evolutionary processes.
“Reciprocally monophytic” means that DNA lineages of ESUs must have more recent common ancestor with each other than with sequences from other ESUs. Such DNA sequences are usually mtDNA (mitochondrial DNA) for animals and cytoplasmic markers for plants, because they both lack recombination.
The idea of ecological and genetic exchangeability means that individuals are able to freely move between populations and can take the same ecological niche playing the same ecological role as resident individuals without any fitness reduction.