In biology, a taxon

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 with a disjunct distribution is one that has two or more groups that are related but widely 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

Orogeny

Orogeny refers to forces and events leading to a severe structural deformation of the Earth's crust due to the engagement of tectonic plates. Response to such engagement results in the formation of long tracts of highly deformed rock called orogens or orogenic belts...

 and continental drift

Continental drift

Continental drift is the movement of the Earth's continents relative to each other. The hypothesis that continents 'drift' was first put forward by Abraham Ortelius in 1596 and was fully developed by Alfred Wegener in 1912...

 or rising sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

s; it may also be due to an organism

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...

 expanding its range into new areas, by such means as rafting

Rafting event

Oceanic dispersal is a type of biological dispersal that occurs when organisms transfer from one land mass to another by way of a sea crossing on large clumps of floating vegetation. Such matted clumps of vegetation are often seen floating down major rivers in the tropics and washing out to sea,...

, 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 animals 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

Introduced species

An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...

 either accidentally or deliberately (agriculture and horticulture).

Habitat fragmentation

Disjunct distributions can occur when suitable habitat

Habitat

* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

 is fragmented

Habitat fragmentation

Habitat fragmentation as the name implies, describes the emergence of discontinuities in an organism's preferred environment , causing population fragmentation...

, 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

Erica ,the heaths or heathers, is a genus of approximately 860 species of flowering plants in the family Ericaceae. The English common names "heath" and "heather" are shared by some closely related genera of similar appearance....

), Mediterranean-Hoggart disjunction (genus Olea

Olea

Olea is a genus of about 40 species in the family Oleaceae, native to warm temperate and tropical regions of southern Europe, Africa, southern Asia and Australasia. They are evergreen trees and shrubs, with small, opposite, entire leaves...

), etc.

Lusitanian distribution

This kind of disjunct distribution of a species, such that it occurs in Iberia

Iberia

The name Iberia refers to three historical regions of the old world:* Iberian Peninsula, in Southwest Europe, location of modern-day Portugal and Spain** Prehistoric Iberia...

 and in Ireland

Ireland

Ireland is an island to the northwest of continental Europe. It is the third-largest island in Europe and the twentieth-largest island on Earth...

, without any intermediate localities, is usually called "Lusitanian

Lusitanian

Lusitanian may refer to:*Lusitanians, an ancient people of western Iberian Peninsula.**Lusitanian language, the language of the ancient Lusitanians.**Lusitanian mythology, the mythology of the ancient Lusitanians....

" (named after the Roman Province Lusitania

Lusitania

Lusitania or Hispania Lusitania was an ancient Roman province including approximately all of modern Portugal south of the Douro river and part of modern Spain . It was named after the Lusitani or Lusitanian people...

, corresponding to modern day Portugal

Portugal

Portugal , officially the Portuguese Republic is a country situated in southwestern Europe on the Iberian Peninsula. Portugal is the westernmost country of Europe, and is bordered by the Atlantic Ocean to the West and South and by Spain to the North and East. The Atlantic archipelagos of the...

).

Examples of animal species with a Lusitanian distribution are: the Kerry slug Geomalacus maculosus and the Pyrenean

Pyrenees

The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...

 glass snail Semilimax pyrenaicus

Semilimax pyrenaicus

Semilimax pyrenaicus is a species of air-breathing land snail, a terrestrial pulmonate gastropod mollusk in the family Vitrinidae.-Distribution:This species is known to occur in:* Ireland* France-References:* AnimalBase info at:...

. Plant species with this kind of distribution include several heather

Calluna

Calluna vulgaris is the sole species in the genus Calluna in the family Ericaceae. It is a low-growing perennial shrub growing to tall, or rarely to and taller, and is found widely in Europe and Asia Minor on acidic soils in open sunny situations and in moderate shade...

 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

Quaternary

The Quaternary Period is the most recent of the three periods of the Cenozoic Era in the geologic time scale of the ICS. It follows the Neogene Period, spanning 2.588 ± 0.005 million years ago to the present...

 (last) glaciation. In this refugium, relic fauna and flora from a previous ice-free period survived until the present warmer interstadial 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

Fauna of Ireland

The fauna of Ireland comprises all the animal species inhabiting the island and surrounding waters.-Mammals:Only 26 land mammal species are native to Ireland, because it was isolated from the European mainland by rising sea levels after the Midlandian Ice Age...

 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

Eurasian pygmy shrew

The Eurasian Pygmy Shrew , often known simply as the Pygmy Shrew, is a widespread shrew of northern Eurasia. It is the only shrew native to Ireland....

, a small mammal, across its range in Europe. The Irish population showed close genetic affinity to a population from Andorra

Andorra

Andorra , officially the Principality of Andorra , also called the Principality of the Valleys of Andorra, , is a small landlocked country in southwestern Europe, located in the eastern Pyrenees mountains and bordered by Spain and France. It is the sixth smallest nation in Europe having an area of...

 but not to that of Britain

Great Britain

Great Britain or Britain is an island situated to the northwest of Continental Europe. It is the ninth largest island in the world, and the largest European island, as well as the largest of the British Isles...

 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. The authors concluded that it had been introduced in the early (Palaeolithic) or middle (Mesolithic

Mesolithic

The Mesolithic is an archaeological concept used to refer to certain groups of archaeological cultures defined as falling between the Paleolithic and the Neolithic....

) Stone Age

Stone Age

The Stone Age is a broad prehistoric period, lasting about 2.5 million years , during which humans and their predecessor species in the genus Homo, as well as the earlier partly contemporary genera Australopithecus and Paranthropus, widely used exclusively stone as their hard material in the...

, by boat, probably from south-west Europe. This coincides with work on human populations, which found a strong genetic

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 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

Paleolithic

The Paleolithic Age, Era or Period, is a prehistoric period of human history distinguished by the development of the most primitive stone tools discovered , and covers roughly 99% of human technological prehistory...

 or early Mesolithic

Mesolithic

The Mesolithic is an archaeological concept used to refer to certain groups of archaeological cultures defined as falling between the Paleolithic and the Neolithic....

.

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.