An
Evolutionarily Significant Unit (ESU) (often lowercased where used without abbreviation, as "evolutionarily significant unit") is a population of
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s that is considered distinct for purposes of
conservationConservation is an ethic of resource use, allocation, and protection. Its primary focus is upon maintaining the health of the natural world: its forests, fisheries, habitats, and biological diversity. Secondary focus is on materials conservation and energy conservation, which are seen as important...
. Delineating ESUs is important when considering conservation action.
This term can apply to any
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
,
subspeciesSubspecies in biological classification, is 1) a taxonomic rank subordinate to species, or 2) a taxonomic unit, a taxon in that rank...
,
geographic raceIn biology, a race is any inbreeding group, including taxonomic subgroups such as subspecies, taxonomically subordinate to a species and superordinate to a subrace and marked by a pre-determined profile of latent factors of hereditary traits....
, or
populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
. 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 speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
proper.
An
Evolutionarily Significant Unit (ESU) (often lowercased where used without abbreviation, as "evolutionarily significant unit") is a population of
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
s that is considered distinct for purposes of
conservationConservation is an ethic of resource use, allocation, and protection. Its primary focus is upon maintaining the health of the natural world: its forests, fisheries, habitats, and biological diversity. Secondary focus is on materials conservation and energy conservation, which are seen as important...
. Delineating ESUs is important when considering conservation action.
This term can apply to any
speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
,
subspeciesSubspecies in biological classification, is 1) a taxonomic rank subordinate to species, or 2) a taxonomic unit, a taxon in that rank...
,
geographic raceIn biology, a race is any inbreeding group, including taxonomic subgroups such as subspecies, taxonomically subordinate to a species and superordinate to a subrace and marked by a pre-determined profile of latent factors of hereditary traits....
, or
populationIn biology, a population is the collection of inter-breeding organisms of a particular species; in sociology, a collection of human beings. Individuals within a population share a factor may be reduced by statistical means, but such a generalization may be too vague to imply anything...
. 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 speciesIn biology, a species is:* a taxonomic rank or* a unit at that rank ....
proper. In
marine animalsMarine biology is the scientific study of living 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...
the term "stock" is often used as well.
Definitions of an ESU generally include at least one of the following criteria:
- Current geographic separation,
- Genetic differentiation at neutral markers (see below) among related ESUs caused by past restriction of gene flow
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
- Locally adapted phenotypic traits
A phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, or behavior. Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and possible interactions...
caused by differences in selectionNatural selection is the process by which heritable traits that make it more likely for an organism to survive and successfully reproduce become more common in a population over successive generations...
.
Criterion 2 considers the gene flow between populations, measured by F
ST. A high degree of differentiation at neutral markers (differences in
alleleAn allele is one of a series of different forms of a gene. The word is a short form of allelomorph , which was used in the early days of genetics to describe variant forms of a gene detected as different phenotypes...
frequencies) implies a lack of gene flow, showing that
random driftGenetic drift or allelic drift is the change in the relative frequency with which a gene variant occurs in a population due to random sampling and chance: the alleles in offspring are a random sample of those in the parents, and chance has a role in determining whether a given individual survives...
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 speciesAn 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. Also it could mean that due to deforestation there may be a lack of food and/or water...
.
For example, Cryan's buckmoth (
Hemileuca maia subsp.) feeds only on the herb
Menyanthes trifoliataMenyanthes 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 ActThe Endangered Species Act of 1973 is the most wide-ranging of the dozens of United States environmental laws passed in the 1970s...
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 this is generally considered to be synonymous with an
evolutionarily significant unit, so that it must:
- be substantially reproductively isolated from other conspecific populations, and
- represent an important component in the evolutionary legacy of the biological species
Other equivalent terms
The equivalent term used by
COSEWICThe 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