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Line 1: − + − A tiger and a clam are different species, because they cannot mate. A German Shepard and a Beagle can mate, because they are different breeds within one species.+ + + + + + +
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A '''species''' is a group of living beings or organisms having common ancestry and therefore significant genetic similarities in common. The boundary between one species and another is the ability to mate and produce offspring. <ref>A group of organisms belong to the same biological species if they are capable of interbreeding to produce fertile offspring. [http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-S/species.html Holgrem Lab], Northwestern University, Illinois </ref>
A '''species''' is a group of living beings or organisms having sufficient genetic similarity to be able to interbreed and produce fertile offspring.<ref>A group of organisms belong to the same biological species if they are capable of interbreeding to produce fertile offspring. [http://www.biochem.northwestern.edu/holmgren/Glossary/Definitions/Def-S/species.html Holgrem Lab], Northwestern University, Illinois </ref>
For example, a [[tiger]] and a [[clam]] are different species, because they cannot mate. A [[German Shepherd]] and a [[Beagle]] can mate, because they are different breeds within one species.
However, the definition of "species" has changed over time and is sometimes problematic.
For example, there are cases where two species are not capable of interbreeding, but each is capable of interbreeding with a third species.<ref name="spark">[http://www.sparknotes.com/biology/evolution/speciation/section1.html Speciation] (SparkNotes)</ref>
Additionally, a "species" is sometimes defined in such a way that groups of creatures that do not ''naturally'' interbreed (perhaps because of geological isolation) are classified as separate species despite the fact that they ''can'' interbreed.<ref name="spark"/>
Furthermore, many extinct species known only from fossils have been given species names, despite the impossibility of determining their capability of interbreeding.
==The origin of species==
==The origin of species==