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Guidelines for Recognizing Designatable Units

Approved by COSEWIC in November 2011

It is widely recognised that status assessments and the conservation of biological diversity require that units below the species level (using “species” in the accepted sense of the taxonomic hierarchy) be considered when appropriate. The Species at Risk Act includes “subspecies, varieties or geographically or genetically distinct population” in its definition of wildlife species. This recognizes that conservation of biological diversity requires protection for taxonomic entities below the species level (i.e. Designatable Units or DUs), and gives COSEWIC a mandate to assess those entities when warranted.

Approach to the status assessment of DUs:

COSEWIC’s usual approach to assigning status is, first, to examine a species as a whole. In these cases, the species itself is one DU.

COSEWIC may assess DUs when a single status designation is thought not to reflect the probability of extinction of the species.

Designatable Units should be discrete and evolutionarily significant units of the taxonomic species, where “significant” means that the unit is important to the evolutionary legacy of the species as a whole and if lost would likely not be replaced through natural dispersion.

Following is a set of guidelines to assist in the identification of Designatable Units for the purpose of status assessment by COSEWIC. The guidelines should be seen as aids for identifying DUs and not as rigid criteria.

Guidelines for the identification of DUs:

1)  Subspecies or varieties:

A unit may be recognized as a DU if it represents a named subspecies or variety identified in accordance with COSEWIC’s guidelines for naming subspecies and varieties. COSEWIC may choose not to recognize a named subspecies or variety as a DU if current scientific data do not support its validity.

2)  Discrete and evolutionarily significant populations:

A population or group of populations may be recognized as a DU if it has attributes that make it “discrete” and evolutionarily “significant” relative to other populations.

The first step in identifying DUs is to ask whether the population or group of populations is discrete from other populations.


A population or group of populations may be considered discrete based on one or more of the following criteria:

1.  Evidence of genetic distinctiveness including, but not limited to, inherited traits (e.g. morphology, life history, behaviour) and/or neutral genetic markers (e.g. allozymes, DNA microsatellites, DNA restriction fragment length polymorphisms (RFLPs), DNA sequences).

2.  Natural disjunction between substantial portions of the species’ geographic range, such that movement of individuals between separated regions has been severely limited for an extended period of time and is not likely in the foreseeable future and where the disjunction is likely to favour the evolution of local adaptations.

3.  Occupation of differing eco-geographic regions that are relevant to the species and reflect historical or genetic distinction, as may be depicted on an appropriate ecozone or biogeographic zone map (Figs. 1 - 3). Some dispersal may occur between regions, but it is insufficient to prevent local adaptation.


If a population or group of populations is considered discrete, based on one or more of the above criteria, then its significance may next be considered. A population may be considered significant based on, but not limited to, one or more of the following criteria, each of which can be considered a measure of evolutionary significance:

1.  Evidence that the discrete population or group of populations differs markedly from others in genetic characteristics thought to reflect relatively deep intraspecific phylogenetic divergence. Such differences would typically be manifested as qualitative genetic differences at relatively slow-evolving markers (e.g. fixed differences in mitochondrial or nuclear DNA sequences or fixed differences in alleles at multiple nuclear loci). Quantitative (frequency) differences of shared alleles, especially for rapidly-evolving markers such as microsatellites, generally would not be sufficient to meet this criterion.

2.  Persistence of the discrete population or group of populations in an ecological setting unusual or unique to the species, such that it is likely or known to have given rise to local adaptations.

3.  Evidence that the discrete population or group of populations represents the only surviving natural occurrence of a species that is more abundant elsewhere as an introduced population outside of its historical range.

4.  Evidence that the loss of the discrete population or group of populations would result in an extensive gap in the range of the species in Canada.

It is important to recognize that some criteria provide more compelling evidence of “discreteness” and “significance” than others; hence, when identifying a DU, it is important to present the best available evidence for all criteria that are met.

Fig.1. COSEWIC National Ecological Areas

Fig.2. COSEWIC National Freshwater Biogeographic Zones

Fig.3. COSEWIC Terrestrial Amphibians and Reptiles Faunal Provinces