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Guidelines for Recognizing Designatable Units

Approved by COSEWIC in November 2015

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 to assess a DU of a biological species considered potentially at risk.

COSEWIC may assess DUs when a single status designation is thought not to reflect the extent of evolutionary significant diversity within a 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. Disjunctions that are a product of human disturbance (as opposed to natural factors) do not qualify as discrete.

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 (examples shown in Figures 1, 2, and 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 loss of the discrete population or group of populations would result in an extensive disjunction in the range of the species in Canada. Here, a disjunction is defined as two or more regions of species’ occurrence that are widely separated geographically by regions of non-occupancy such that natural dispersal of the species among such regions is unlikely to occur.

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.

Some Practical Considerations

  • DUs can be designated following a weight of evidence approach where different lines of evidence (genetic, behavioural, morphological, geographic distribution, etc.) are evaluated for discreteness and then significance. Rather than consider lines of evidence or criteria one by one, evaluate the full body of evidence together for discreteness and significance
  • .

  • Some lines of evidence may be relevant for both discreteness and significance. For example, eco-geographic boundaries can generate isolation (discrete), but also may set the stage for significance, because meaningful (e.g., phylogeographic or adaptive) genetic differences will be promoted if there is isolation. The key for DU designations is that the zones each generate distinct environments that could plausibly be argued to represent different selective regimes that could also drive "significance". Reports should rationalize the relevance of the zones if they are to be used as a basis for designating DUs. Some brief explanation is required in terms of the relevance of the zones to generate isolation and distinct selective pressures. Use the most appropriate map for the wildlife species under consideration (examples shown in Figures 1, 2, and 3).
  • Genetic distinctiveness by itself is not sufficient for DU designation; nor is it necessary for DUs to demonstrate genetic differences. Genetic evidence must be carefully examined for the geographic coverage of sampling and genetic information should be consistent across DUs (e.g., in terms of the character of loci being used for DU recognition).
  • DUs should not be distinguished/defined on the basis of threats, or relative conservation status. Similarly, DUs are not necessarily equivalent to management units (of which there may be more than one within or across DUs).

Fig.1. COSEWIC National Ecological Areas

Fig.2. COSEWIC National Freshwater Biogeographic Zones

Fig.3. COSEWIC Terrestrial Amphibians and Reptiles Faunal Provinces