Evolutionary Significant Unit
An Evolutionarily Significant Unit (ESU) (often lowercased where used without abbreviation, as "evolutionarily significant unit") is a population of organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...

s that is considered distinct for purposes of conservation
Conservation ethic
Conservation is an ethic of resource use, allocation, and protection. Its primary focus is upon maintaining the health of the natural world: its, fisheries, habitats, and biological diversity. Secondary focus is on materials conservation and energy conservation, which are seen as important to...

. Delineating ESUs is important when considering conservation action.

This term can apply to any species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...

, geographic race
Race (biology)
In biology, races are distinct genetically divergent populations within the same species with relatively small morphological and genetic differences. The populations can be described as ecological races if they arise from adaptation to different local habitats or geographic races when they are...

, or population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

. Often the term "species" is used rather than ESU, even when an ESU is more technically considered a subspecies or variety rather than a biological species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 proper. In marine animals
Marine biology
Marine biology is the scientific study of organisms in the ocean or other marine or brackish bodies of water. Given that in biology many phyla, families and genera have some species that live in the sea and others that live on land, marine biology classifies species based on the environment rather...

 the term "stock" is often used as well.

Definitions of an ESU generally include at least one of the following criteria:
  1. Current geographic separation,
  2. Genetic differentiation at neutral markers (see below) among related ESUs caused by past restriction of gene flow
    Population bottleneck
    A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....

    , or
  3. Locally adapted phenotypic traits
    A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

     caused by differences in selection
    Natural selection
    Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....


Criterion 2 considers the gene flow between populations, measured by FST. A high degree of differentiation at neutral markers (differences in allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

 frequencies) implies a lack of gene flow, showing that random drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...

 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.

Criterion 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 criterion 2, and may be impossible for very rare or endangered species
Endangered species
An endangered species is a population of organisms which is at risk of becoming extinct because it is either few in numbers, or threatened by changing environmental or predation parameters...


For example, Cryan's buckmoth (Hemileuca maia subsp.) feeds only on the herb Menyanthes trifoliata
Menyanthes is a monotypic genus of flowering plant in the family Menyanthaceae containing the single species 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. 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, prompting a contention for consideration of both genetic and ecological processes in identifying ESUs . Because the different approaches to designating ESUs each have their benefits, and the need and form of management prescriptions may vary across contexts, some support an "adaptive" approach to identification of ESUs, for instance suggesting consideration of facets from numerous designation methods.

United States Endangered Species Act

For the purposes of the Endangered Species Act
Endangered Species Act
The Endangered Species Act of 1973 is one of the dozens of United States environmental laws passed in the 1970s. Signed into law by President Richard Nixon on December 28, 1973, it was designed to protect critically imperiled species from extinction as a "consequence of economic growth and...

 a "species" is defined to include "any distinct population segment
Distinct population segment
A distinct population segment is the smallest division of a taxonomic species permitted to be protected under the U.S. Endangered Species Act. Species, as defined in the Act for listing purposes, is a taxonomic species or subspecies of plant or animal, or in the case of vertebrate species, a...

 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 this is generally considered to be synonymous with an evolutionarily significant unit, so that it must:
  1. be substantially reproductively isolated from other conspecific populations, and
  2. represent an important component in the evolutionary legacy of the biological species

Other equivalent terms

The equivalent term used by COSEWIC
Committee on the Status of Endangered Wildlife in Canada
The Committee on the Status of Endangered Wildlife in Canada ; French: Le Comité sur la situation des espèces en péril au Canada, is an independent committee of wildlife experts and scientists whose "raison d’être is to identify species at risk" in Canada...

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. http://www.cosepac.gc.ca/eng/sct0/assessment_process_e.cfm
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