Changes

== Defining species ==

== Defining species ==

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>

There is not a single universally-agreed definition of "species", although all definitions agree that successful interbreeding is a key factor.{{fact}}

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"/>

 

 

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>

 

 

"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"/>

==Ring Species==

==Ring Species==

This is a phenomenon which occurs when a contiguous lineage ancestors co-exist at the same time.  Unlike most species whose direct ancestral species are now extinct, the ancestral species of a ring species still survive.  The existence of ring species make it more difficult to define what a species is.  For example, A can breed with B can breed with C can breed with D... however D cannot breed with A.  The most prominent example of a ring species is the Larus Gull which resides in the north Atlantic.  Neighboring species interbreed counter-clockwise around the arctic, however this ability to interbreed terminates near Britain with the Herring Gull and Lesser Black-Backed Gull.<ref>Harrison, Peter (1988): Seabirds (2nd ed.). Christopher Helm, London ISBN 0-7470-1410-8</ref>

"Ring species" occur when a contiguous lineage ancestors co-exist at the same time.  Unlike most species whose direct ancestral species are now extinct, the ancestral species of a ring species still survive.  The existence of ring species make it more difficult to define what a species is.  For example, A can breed with B can breed with C can breed with D... however D cannot breed with A.  The most prominent example of a ring species is the Larus Gull which resides in the north Atlantic.  Neighboring species interbreed counter-clockwise around the arctic, however this ability to interbreed terminates near Britain with the Herring Gull and Lesser Black-Backed Gull.<ref>Harrison, Peter (1988): Seabirds (2nd ed.). Christopher Helm, London ISBN 0-7470-1410-8</ref>

 

 

==Note==

==Notes==

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