Definitions of an ESU generally include at least one of the following criteria:[1] Criterion 2 considers the gene flow between populations, measured by FST.
Reciprocal transplantation experiments are necessary to test for genetic differentiation for phenotypic traits, and differences in selection gradients across habitats.
For example, Cryan's buckmoth (Hemileuca maia) feeds only on the herb Menyanthes trifoliata, commonly known as buckbean, and while indistinguishable morphologically from related buckmoths, and not differentiated at the genetic markers tested, the moth is highly adapted to its host plant, having 100% survivorship on Menyanthes, while close genetic relatives all died when reared on the plant.
[2] In this case gene flow was sufficient to reduce differentiation at neutral markers, but did not prevent local host adaptation.
However, the act does not define what constitutes a "distinct population segment", but this is generally considered to be synonymous with an evolutionarily significant unit, so that it must: The equivalent term used by COSEWIC is "Wildlife Species", which is used to refer to biological species, subspecies, varieties, or geographically or genetically distinct populations of organisms.