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i) Disjoint Distributions
Continuous distributions of living organisms are anomalous in nature because even the environmental variables that determine them are highly variable. What is discussed as once continuous distribution of certain, organisms that was split by continental drifts and plate tectonics, is therefore questionable. Fossil records on the flora of Africa suggest that neighbouring populations converge during favourable periods but separated at wide intervals when conditions were harsh. The degree of convergence is still questionable. For instance, it may occur in the form of a small strip. However on the local scale, one
can talk about the distribution of a species population being continuous. Naturally, distributions are disjoint. Thus, a species may occur in areas, which are geographically far apart. Unravelling the causes of disjoint distribution is one of the biogeographical problems. Evolutionary disjoints are said to be species which were once widely distributed but were split or made to contract into isolated patches because of climatic changes. The survivors of these species today occur in limited numbers in different continents or on one continent. A good example is the distribution of the African Gorilla.
ii) Endemic Distributions
These are distributions, which are confined to areas of origin or to refugia. Tectonic processes and past climatic changes have been predicated as being responsible for wiping out world vegetation and fauna population. If changes are gradual, some organisms will avoid death and take refuge in suitable habitats. If they are erratic, the organisms will become extinct. The Pleistocene ice age has similar effects in USA and Europe. They wiped out populations of vulnerable species while in southern warmer
areas and on mountains on which ice formation was limited some organisms migrated behind retracted ice; others remained confined to refugia probably due to their inability to overcome barriers. Most flora on sea islands or mountain tops have high levels of endemism. On the East African mountains, for instance, the lobelia species on Mt Kenya is endemic. Alternatively, endemism is associated with organisms that have recently evolved and have therefore not had enough time to spread.
Lastly, some endemic species could be on their way to extinction as changes in their habitats have reached such an intolerable state that they cannot compete successfully for soil and space.
francis1897 answered the question on October 5, 2022 at 13:53
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