Evolutionary Significant Unit
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An Evolutionary Significant Unit (ESU) (often lowercased: evolutionary significant unit) is a population of organisms that is considered distinct for purposes of conservation. Delineating ESUs is important when considering conservation action.
This term can apply to any species, subspecies, geographic race, or population. Often the term "species" is used rather than ESU, even when an ESU is better considered a subspecies or variety, rather than a biological species. In marine animals the term stock is often used.
Definitions of an ESU generally include at least one of the following criteria:[1]
- Current geographic separation
- Genetic differentiation at neutral markers among ESUs caused by past restriction of gene flow
- Locally adapted phenotypic traits caused by differences in selection.
Criteria 2 considers the gene flow between populations, measured by FST. A high degree of differentiation at neutral markers (differences in allele frequencies) implies a lack of gene flow, showing that random drift has occurred in isolation from other populations. Very few migrants per generation are needed to prevent strong differentiation of neutral markers. Even a single migrant per generation may be enough for neutral markers to show gene flow between populations, making it difficult to differentiate the populations through neutral markers.
Criteria 3 does not consider neutral genetic markers, instead looking at locally adapted traits of the population. Local adaptations may be present even with some gene flow from other populations, and even when there is little differentiation at neutral markers among ESUs. Reciprocal transplantation experiments are necessary to test for genetic differentiation for phenotypic traits, and differences in selection gradients across habitats. Such experiments are generally more difficult than the fixation index tests of criteria 2, and may be impossible for very rare or endangered species.
For example, Cryan's buckmoth feeds only on the herb Menyanthes trifoliata, 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. In this case gene flow was sufficient to reduce differentiation at neutral markers, but did not prevent local host adaptation.
Both criteria 2 and 3 have the problem that there is no clear dichotomy between ESU and not ESU, as genetic differentiation between populations forms a continuum.[2]
[edit] United States Endangered Species Act
For the purposes of the Endangered Species Act a "species" is defined to include
- any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature.
However the act does not define what constitutes a "distinct population segment", but it is generally considered to be synonymous with an evolutionary significant unit, so that is it must:
- be substantially reproductively isolated from other conspecific populations, and
- represent an important component in the evolutionary legacy of the biological species[3]
[edit] Equivalent terms
The equivalent term used by COSEWIC is Wildlife Species, or for brevity just species, which is used to refer to biological species, subspecies, varieties, or geographically or genetically distinct populations of organisms. [1]
[edit] References
- ^ Jeffrey Conner, Daniel Hartl. A Primer of Ecological Genetics. 2004.
- ^ Crandall et al. 2000. Considering evolutionary processes in conservation biology. Trends in Ecology and Evolution. 15:290-295.
- ^ Waples, R. S. 1991. Pacific salmon, Oncorhynchus spp., and the definition of "species" under the Endangered Species Act. Mar. Fish. Rev. 53(3):11-22.
