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.
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.
This type of speciation occurs when similar species develop in the same habitat, and there are no physical barriers present (Nat Geo Education).
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