Disjunct distribution
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In biology, a
taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...
with a disjunct distribution is one that has two or more groups that are related but considerably separated from each other geographically. The causes are varied and might demonstrate either the expansion or contraction of a species' range.


Range fragmentation

Also called range fragmentation, disjunct distributions may be caused by changes in the environment, such as
mountain building Mountain formation refers to the geological processes that underlie the formation of mountains. These processes are associated with large-scale movements of the Earth's crust (tectonic plates). Folding, faulting, volcanic activity, igneous int ...
and
continental drift Continental drift is the hypothesis that the Earth's continents have moved over geologic time relative to each other, thus appearing to have "drifted" across the ocean bed. The idea of continental drift has been subsumed into the science of pla ...
or rising
sea level Mean sea level (MSL, often shortened to sea level) is an average surface level of one or more among Earth's coastal bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical datuma standardised g ...
s; it may also be due to an
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 ...
expanding its range into new areas, by such means as
rafting Rafting and whitewater rafting are recreational outdoor activities which use an inflatable raft to navigate a river or other body of water. This is often done on whitewater or different degrees of rough water. Dealing with risk is often a ...
, or other animals transporting an organism to a new location (plant seeds consumed by birds and animals can be moved to new locations during bird or animal migrations, and those seeds can be deposited in new locations in fecal matter). Other conditions that can produce disjunct distributions include: flooding, or changes in wind, stream, and current flows, plus others such as anthropogenic introduction of alien
introduced species An introduced species, alien species, exotic species, adventive species, immigrant species, foreign species, non-indigenous species, or non-native species is a species living outside its native distributional range, but which has arrived there ...
either accidentally or deliberately (agriculture and horticulture).


Habitat fragmentation

Disjunct distributions can occur when suitable
habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
is fragmented, which produces fragmented populations, and when that fragmentation becomes so divergent that species movement between one suitable habitat to the next is disrupted, isolated population can be produced. Extinctions can cause disjunct distribution, especially in areas where only scattered areas are habitable by a species; for instance, island chains or specific elevations along a mountain range or areas along a coast or between bodies of water like streams, lakes and ponds.


Examples

There are many patterns of disjunct distributions at many scales: Irano-Turanian disjunction, Europe - East Asia, Europe-South Africa (e.g. genus ''
Erica Erica or ERICA may refer to: * Erica (given name) * ''Erica'' (plant), a flowering plant genus * Erica (chatbot), a service of Bank of America * ''Erica'' (video game), a 2019 FMV video game * ''Erica'' (spider), a jumping spider genus * E ...
''), Mediterranean-Hoggart disjunction (genus ''
Olea ''Olea'' ( ) is a genus of about 40 species in the family Oleaceae, native to warm temperate and tropical regions of the Middle East, southern Europe, Africa, southern Asia, and Australasia. They are evergreen trees and shrubs, with small ...
''), etc.


Lusitanian distribution

This kind of disjunct distribution of a species, such that it occurs in
Iberia The Iberian Peninsula (), ** * Aragonese and Occitan: ''Peninsula Iberica'' ** ** * french: Péninsule Ibérique * mwl, Península Eibérica * eu, Iberiar penintsula also known as Iberia, is a peninsula in southwestern Europe, defi ...
and in
Ireland Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
, without any intermediate localities, is usually called "Lusitanian" (named after the Roman Province
Lusitania Lusitania (; ) was an ancient Iberian Roman province located where modern Portugal (south of the Douro river) and a portion of western Spain (the present Extremadura and the province of Salamanca) lie. It was named after the Lusitani or Lusita ...
, corresponding roughly to modern-day
Portugal Portugal, officially the Portuguese Republic ( pt, República Portuguesa, links=yes ), is a country whose mainland is located on the Iberian Peninsula of Southwestern Europe, and whose territory also includes the Atlantic archipelagos of ...
). Examples of animal species with a Lusitanian distribution are: the Kerry slug '' Geomalacus maculosus'' and the Pyrenean glass snail '' Semilimax pyrenaicus''. Plant species with this kind of distribution include several heather species (''Calluna'' spp.) and the strawberry tree (''Arbutus unedo''). The theory behind the name "Lusitanian" is now discredited; it posited that there was an ice-free land mass that served as a refugium off of the south-west of Ireland during the
Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS). It follows the Neogene Period and spans from 2.58 million years ...
(last)
glaciation A glacial period (alternatively glacial or glaciation) is an interval of time (thousands of years) within an ice age that is marked by colder temperatures and glacier advances. Interglacials, on the other hand, are periods of warmer climate betw ...
. In this refugium, relic fauna and flora from a previous ice-free period survived until the present warmer
interstadial Stadials and interstadials are phases dividing the Quaternary period, or the last 2.6 million years. Stadials are periods of colder climate while interstadials are periods of warmer climate. Each Quaternary climate phase is associated with a Ma ...
period. Although the theory is no longer accepted, the term Lusitanian is still used as a descriptive term for faunal elements such as the Kerry slug. Recently a better explanation of the occurrence of the Kerry slug and similar faunal elements in southwestern Ireland has been developed. This new theory is supported by two recent discoveries: the genetic similarity of much of Ireland's fauna to that of northern Spain, and the genetic similarity of much of Ireland's human population to that of northern Spain. Mascheretti et al. (2003) examined the genotypes of Eurasian pygmy shrew, a small mammal, across its range in Europe. The Irish population showed close genetic affinity to a population from
Andorra , image_flag = Flag of Andorra.svg , image_coat = Coat of arms of Andorra.svg , symbol_type = Coat of arms , national_motto = la, Virtus Unita Fortior, label=none (Latin)"United virtue is stro ...
but not to that of
Britain Britain most often refers to: * The United Kingdom, a sovereign state in Europe comprising the island of Great Britain, the north-eastern part of the island of Ireland and many smaller islands * Great Britain, the largest island in the United King ...
or other places in Europe. The genetic structure of the population further showed that the entire Irish population of the Eurasian pygmy shrew had originated from a single
founder event In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, usi ...
. The authors concluded that it had been introduced in the early (
Palaeolithic The Paleolithic or Palaeolithic (), also called the Old Stone Age (from Greek: παλαιός ''palaios'', "old" and λίθος '' lithos'', "stone"), is a period in human prehistory that is distinguished by the original development of stone to ...
) or middle (
Mesolithic The Mesolithic (Greek: μέσος, ''mesos'' 'middle' + λίθος, ''lithos'' 'stone') or Middle Stone Age is the Old World archaeological period between the Upper Paleolithic and the Neolithic. The term Epipaleolithic is often used synonymous ...
)
Stone Age The Stone Age was a broad prehistoric period during which stone was widely used to make tools with an edge, a point, or a percussion surface. The period lasted for roughly 3.4 million years, and ended between 4,000 BC and 2,000 BC, with t ...
, by boat, probably from south-west Europe. This coincides with work on human populations, which foundMcEvoy B., Richards M., Forster P. & Bradley D. G. 2004. ''The longue durée of genetic ancestry: multiple
genetic marker A genetic marker is a gene or DNA sequence with a known location on a chromosome that can be used to identify individuals or species. It can be described as a variation (which may arise due to mutation or alteration in the genomic loci) that can be ...
systems and Celtic origins on the Atlantic facade of Europe''. Am. J. Hum. Genet. 75: 693-702.
a strong genetic similarity in make-up between populations in western Ireland and in northern Spain. This would be explained by a human migration from Spain to Ireland in the late
Paleolithic The Paleolithic or Palaeolithic (), also called the Old Stone Age (from Greek: παλαιός ''palaios'', "old" and λίθος ''lithos'', "stone"), is a period in human prehistory that is distinguished by the original development of stone too ...
or early
Mesolithic The Mesolithic (Greek: μέσος, ''mesos'' 'middle' + λίθος, ''lithos'' 'stone') or Middle Stone Age is the Old World archaeological period between the Upper Paleolithic and the Neolithic. The term Epipaleolithic is often used synonymous ...
. It seems increasingly likely that much of Ireland's Lusitanian fauna is in reality an artefact of this era of human expansion in the early part of the Postglacial era. In other words, it seems likely that these species were introduced accidentally with trade items or goods brought by boat from Iberia.


See also

*
Geographical isolation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
*
Habitat In ecology, the term habitat summarises the array of resources, physical and biotic factors that are present in an area, such as to support the survival and reproduction of a particular species. A species habitat can be seen as the physical ...
* Habitat fragmentation


References

{{Reflist Biogeography Biology terminology Habitat Ecology terminology