Ecotype

Four different ecotypes of Physcomitrella patens, stored at the International Moss Stock Center

In evolutionary ecology, an ecotype,^{[note 1]} sometimes called ecospecies, describes a genetically distinct geographic variety, population or race within a species, which is adapted to specific environmental conditions.

Typically, though ecotypes exhibit phenotypic differences (such as in morphology or physiology) stemming from environmental heterogeneity, they are capable of interbreeding with other geographically adjacent ecotypes without loss of fertility or vigor. ^{[1][2][3][4][5]}

Range and distribution

Experiments indicate that sometimes ecotypes manifest only when separated by great spatial distances (of the order of 1,000 km). This is due to hybridization whereby different but adjacent varieties of the same species (or generally of the same taxonomic rank) interbreed, thus overcoming local selection. However other studies reveal that the opposite may happen, i.e., ecotypes revealing at very small scales (of the order of 10 m), within populations, and despite hybridization.^[1]

In ecotypes, it is common for continuous, gradual geographic variation to impose analogous phenotypic and/or genetic variation.^[1] This situation is called cline. A well-known example of cline is the skin color gradation in indigenous human populations worldwide, which is related to latitude and amounts of sunlight.^[6] But often the distribution of ecotypes is bimodal or multimodal. This means that ecotypes may display two or more distinct and discontinuous phenotypes even within the same population. Such phenomenon may lead to speciation and can occur if conditions in a local environment change dramatically through space or time.^[1]

Examples

Rangifer tarandus caribou, a member of the woodland ecotype.

• Earthworms fall into four different ecotypes. Compost earthworms prefer warm and moist environments with a ready supply of fresh compost material. Epigeic earthworms live on the surface of the soil in leaf litter and tend not to make burrows but live in and feed on the leaf litter. Endogeic earthworms live in and feed on the soil, making horizontal burrows through the soil to move around and to feed and they will reuse these burrows to a certain extent. Anecic earthworms make permanent vertical burrows in soil, feeding on leaves on the soil surface that they drag into their burrows.^[7]
• Tundra reindeer and woodland reindeer are two ecotypes of reindeer. The first migrate (travelling 5,000 km) annually between the two environments in large numbers whereas the other (who are much fewer) remain in the forest for the summer.^[8] In North America, the species Rangifer tarandus (locally known as caribou,^[9][9][10] was subdivided into five subspecies^{[note 2]} by Banfield in 1961.^[11][12] Caribou are classified by ecotype depending on several behavioural factors – predominant habitat use (northern, tundra, mountain, forest, boreal forest, forest-dwelling), spacing (dispersed or aggregated) and migration (sedentary or migratory).^[13][14][15] For example, the subspecies Rangifer tarandus caribou is further distinguished by a number of ecotypes, including boreal woodland caribou, mountain woodland caribou and migratory woodland caribou)—the migratory George River Caribou Herd, for example in the Ungava region of Quebec.
• Arabis fecunda, a herb endemic to some calcareous soils of Montana, United States, can be divided into two ecotypes. The one "low elevation" group lives near the ground in an arid, warm environment and has thus developed a significantly greater tolerance against drought than the "high elevation" group. The two ecotypes are separated by a horizontal distance of about 100 km.^[1]
• It is commonly accepted that the Tucuxi dolphin has two ecotypes – the riverine ecotype found in some South American rivers and the pelagic ecotype found in the South Atlantic Ocean.^[16] Similarly, it is accepted that the common bottlenose dolphin has two ecotypes in the western North Atlantic.^[17]
• The warbler finch and the Cocos Island finch are viewed as separate ecotypes.^[18]
• The Scots pine (Pinus sylvestris) has 20 different ecotypes in an area from Scotland to Siberia, all capable of interbreeding.^[19]
• A very subtle case of ecotype is the following: It has been observed that two populations of the same Helix snail species separated by only a few hundred kilometers prefer not to cross-mate, i.e. they reject one another as mates. This event probably occurs during the process of courtship, which may last for hours.

Terminology

Ecotypes have no main taxonomic rank in modern biological classification. However some scientists consider them "taxonomically equivalent to subspecies". This is true in the sense that ecotypes can be sometimes classified as subspecies and the opposite.

Ecotypes are closely related to morphs. In the context of evolutionary biology, genetic polymorphism is the occurrence in equilibrium of two or more distinctly different phenotypes within a population of a species, in other words, the occurrence of more than one form or morph. The frequency of these discontinuous forms (even that of the rarest) is too high to be explained by mutation. In order to be classified as such, morphs must occupy the same habitat at the same time and belong to a panmictic population (whose all members can potentially interbreed). Polymorphism is actively and steadily maintained in populations of species by natural selection (most famously sexual dimorphism in humans) in contrast to transient polymorphisms where conditions in a habitat change in such a way that a "form" is being replaced completely by another.

In fact, Begon, Townsend and Harper assert that

The notions "form" and "ecotype" may appear to correspond to a static phenomenon, however this is not always the case. Evolution occurs continuously both in time and space, so that two ecotypes or forms may qualify as distinct species in only a few generations. Begon, Townsend and Harper use an illuminating analogy on this:

Thus ecotypes and morphs can be thought of as precursory steps of potential speciation.

See also

• Adaptation
• Biological classification
• Cline (biology)
• Ecotope
• Epigenetics
• Evolution
• Polymorphism (biology)
• Ring species
• Speciation
• Species problem

Notes

1. ↑ Greek: οίκος = home and τύπος = type, coined by Göte Turesson in 1922
2. ↑ Banfield, who worked with both the Canadian Wildlife Service and the National Museum of Canada, in his often-cited 1961 classification, identified five subspecies of Rangifer tarandus: 1) the largely migratory barren-ground caribou subspecies Rangifer tarandus groenlandicus, which are found mainly in the Canadian territories of Nunavut and the Northwest Territories, along with western Greenland; 2) the subspecies Rangifer tarandus caribou which is divided into ecotypes: boreal woodland caribou, (also known as forest-dwelling, woodland caribou (boreal), mountain woodland caribou and migratory woodland caribou) —the migratory George River Caribou Herd, for example in the Ungava region of Quebec; 3) Rangifer tarandus pearyi (Peary caribou), the smallest of the species, known as Tuktu in Inuktitut, found in the northern islands of Nunavut and the Northwest Territories; 4) Rangifer tarandus granti subspecies Grant’s caribou, which are mainly migratory and live in Alaska and the northern Yukon and 5) the R. t. dawsoni subspecies; † Queen Charlotte Islands caribou from the Queen Charlotte Islands (extinct since 1910)

References