Paleoendemism along with neoendemism is a possible subcategory of endemism. Paleoendemism refers to species that were formerly widespread but are now restricted to a smaller area. Neoendemism refers to species that have recently arisen, such as through divergence and reproductive isolation or through hybridization and polyploidy in plants.
The first part of the word, paleo, comes from the Greek word palaiós, meaning "ancient". The second part of the word, endemism is from New Latin endēmicus, from Greek ενδήμος, endēmos, "native". Endēmos is formed of en meaning "in", and dēmos meaning "the people".
Changes in climate are thought to be the driving force in creating paleoendemic species, generally due to habitat loss. Regions where the climate has remained relatively stable form refugia which are more likely to be endemic hotspots today. This applies to both neoendemism and paleoendemism. However, paleoendemism differs as it does not require additional factors such as barriers and ecological opportunities as it does not rely on adaptive radiation like neoendemism does. It instead relies on the instability of other regions' climate, which may limit the range of a species to a more stable region, thus turning that species paleoendemic. Limited ability for dispersal is also important in the creation of endemic species. The two terms can essentially be defined as "cradles" of new species (neoendemism), or "museums" of old species (paleoendemism).
It is not always clear whether a particular species is paleoendemic or neoendemic.
Paleoendemism on islands
Islands as harbors for endemic species are explained by the theory of island biogeography. However, in order to be considered a paleoendemic on an island, the species must have had a widespread distribution previously, thus eliminating newly formed islands as potential refuges of paleo-endemics.
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