Species
A species is a group of organisms with a shared, closed gene pool. A species is made up of all the organisms that are able to mate with one another and produce fertile offspring. For example- lions can mate with other lions and have fertile offspring and a tiger can mate with another tiger and have fertile offspring. But a lion and a tiger can produce a hybrid called a liger or a tigon, and this hybrid cannot reproduce. This concept of not being able to interbreed is one form of reproductive isolation.
sources:http://www.physicalgeography.net/fundamentals/9d.html
http://eol.org/info/468
sources:http://www.physicalgeography.net/fundamentals/9d.html
http://eol.org/info/468