There are several different types of speciation (how a new plant or animal species comes to be). Two of these types include allopatry and sympatry.
Allopatry[]
Allopatric speciation happens when a species is divided into 2 separate groups which do not come into contact with one another (they become isolated groups) (Nat Geo Education).
A physical barrier (like a valley or a river) could isolate the two groups, which would cause each group to evolve separately, unable to breed with the individuals of the other group.
Sympatry[]
This type of speciation occurs when similar species develop in the same habitat, and there are no physical barriers present (Nat Geo Education).
References[]
Basic concepts: Allopatry and sympatry – Evolving Thoughts. (n.d.). Retrieved November 21, 2013, from http://scienceblogs.com/evolvingthoughts/2007/03/11/basic-concepts-allopatry-and-s/
speciation - National Geographic Education. (n.d.). Retrieved November 21, 2013, from http://education.nationalgeographic.com/education/encyclopedia/speciation/?ar_a=1