In biology, ataxon with adisjunct 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.[1]
Also called range fragmentation, disjunct distributions may be caused by changes in the environment, such asmountain building andcontinental drift orrising sea levels. It may also be due to an organism expanding its range into new areas, by such means asrafting, or other animals transporting apropagule to a new location; seeds consumed by birds and animals can be moved to new locations during migrations, and those seeds can be deposited in fecal matter. Other conditions that can produce disjunct distributions include flooding, changes in wind, stream, and current flows, and other anthropogenic introduction ofintroduced species either accidentally or deliberately through agriculture and horticulture.
Disjunct distributions can occur when suitablehabitat isfragmented, 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;[2] 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.[3]
There are many patterns of disjunct distributions at many scales: Europe - East Asia, Europe-South Africa (e.g. genusErica),Mediterranean-Hoggart disjunction (genusOlea),amphi-Pacific distribution (Australia - South America),Asa Gray disjunction (eastern North America and East Asia), etc.
This kind of disjunct distribution of a species, such that it occurs inIberia and in Ireland, without any intermediate localities, is usually called "Lusitanian" (named after the Roman ProvinceLusitania, corresponding roughly to modern-day Portugal).
Examples of animal species with a Lusitanian distribution are: the Kerry slugGeomalacus maculosus and thePyrenean glass snailSemilimax pyrenaicus. Plant species with this kind of distribution include severalheather species (Calluna spp.) and thestrawberry 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 arefugium off of the south-west of Ireland during theQuaternary (last)glaciation. In this refugium, relic fauna and flora from a previous ice-free period survived until the present warmerinterstadial 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 ofIreland'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)[4] examined the genotypes ofEurasian pygmy shrew, a small mammal, across its range in Europe. The Irish population showed close genetic affinity to a population from Andorra but not to that ofBritain 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 singlefounder event. The authors concluded that it had been introduced in the early (Paleolithic) or middle (Mesolithic)Stone Age, by boat, probably from south-west Europe. This coincides with work on human populations, which found[5][6] a stronggenetic 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 or earlyMesolithic.
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.
The Asa Gray disjunction is a frequently-seen disjunct distribution where plant species in Eastern North America have their closest relatives in East Asia, but are otherwise absent from Europe or western North America. This distribution is best understood as arelict of theArcto-Tertiary Geoflora, a plant assemblage that covered much of the Northern Hemisphere until the lateNeogene, when climactic changes fragmented it. This is supported by fossil specimens of such plants being found in western Northern America, where they are no longer found in the modern day.[7][8]