An evolutionarily significant unit (ESU) is a population of
organism
In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...
s that is considered distinct for purposes of
conservation
Conservation is the preservation or efficient use of resources, or the conservation of various quantities under physical laws.
Conservation may also refer to:
Environment and natural resources
* Nature conservation, the protection and manageme ...
. Delineating ESUs is important when considering conservation action.
This term can apply to any
species
In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...
,
subspecies
In biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics ( morphology), but that can successfully interbreed. Not all specie ...
,
geographic race, or
population
Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction usi ...
. 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 the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
proper. In
marine animals the term "stock" is often used as well.
Definition
Definitions of an ESU generally include at least one of the following criteria:
[Jeffrey Conner, Daniel Hartl. ''A Primer of Ecological Genetics''. 2004, ]
# Current geographic separation,
# Genetic differentiation at
neutral markers among related ESUs caused by
past restriction of gene flow, or
# Locally adapted
phenotypic traits
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
caused by differences in
selection.
Criterion 2 considers the gene flow between populations, measured by
FST. A high degree of differentiation between two populations among genes that provide no adaptive advantage to either population (known as ''neutral markers'') 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.
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
An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching and inv ...
.
For example,
Cryan's buckmoth (''Hemileuca maia'') feeds only on the herb ''
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
The Endangered Species Act of 1973 (ESA or "The Act"; 16 U.S.C. ยง 1531 et seq.) is the primary law in the United States for protecting imperiled species. Designed to protect critically imperiled species from extinction as a "consequence of ec ...
a "species" is defined to include "any
distinct population segment {{no footnotes, date=February 2018
A distinct population segment (DPS) 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 ...
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
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.
See also
FSTAT (software)
References
{{Reflist
Conservation biology
Environmental law in the United States