Most species cannot crossbreed because the genetic, behavioral and ecological barriers are too great to overcome.
2.
Local populations have diverged through vicariance, because climate change after the last ice age generated ecological barriers between Mafra and Gr�ndola.
3.
These two populations are separated by the Sierra de Bahoruco ( Massif de la Selle in Haiti ), with three peaks exceeding that form an extensive ecological barrier.
4.
Ultimately, however, it turned out that much land west of the Missouri River was not ecologically suited for homesteading because of mountainous terrain, poor soils, lack of available water and other ecological barriers to significant settlement.
5.
The fact that there were few ecological barriers like mountains also made it much easier for livestock and, eventually, writing, the wheel and other inventions to spread than in the Americas or Africa, which both have a north-south axis.
6.
In contrast to'geographic'vicariance ( see above ) where physical or biotic barriers lead to the fragmentation of a population or taxon, ecological vicariance is the process by which an initially continuous and often widespread population ( or taxon ) becomes fragmented through ecological barriers.
7.
According to Shortridge's description, " F . n . nigripes " is smaller and paler than " F . n . thomasi ", but since specimens with characteristics of both assumed subspecies are found close to Kimberley in central South Africa, the existence of subspecies is questioned, as no geographical or ecological barriers to their ranges occur.
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