Most of the time in most groups of organisms, species are formed when one splits into two, or rarely more than two. Sometimes they form by hybridizing, or crossbreeding, to form a new species more or less in one go. Speciation is also referred to as macroevolution, an increase in biodiversity, or as taxonomic multiplication.

Two organisms, or two populations, or two varieties, species or taxa, can live in relation to each other in two ways, geographically speaking. They can live in the same region (sym-meaning “together”, plus patria, gives sympatry), or they can live in different regions (allo-meaning “other”, plus patria gives allopatry).

Allopatric speciation means that speciation occured in different regions.The key with allopatric speciation is geographical separation. For example, say you have a squirrel population in a mountain, and over years a canyon forms a physical barrier between the two sides so that what was once 1 species of squirrel is split up into two different populations on different sides Over time, each population will adapt to its region and eventually speciate. This actually took place in I beleive the grand canyon between Abert and Kaibab squirrels. You can look that up if you would like.

Sympatric speciation means that one population of one species became two species while in the same geographic region with no physical separation. This usually occurs because different inviduals of the same species begin to occupy a distinct niche and eventually become specifically adapted for that role. Look up African tilapia which is a good example if you want to know more.

Scoures: http://scienceblogs.com/evolvingthoughts/2007/03/basic_concepts_allopatry_and_s.php

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