Ref for biological species and genus

[Mike Taylor <mike@miketaylor.org.uk>]

Sorry for the ignorant question.  I am looking for the definitive
reference for the the biological species concept (a population capable
of interbreeding to produce fertile offspring).  I understand that
this notion has been extended to define a "biological genus" as a
population capable of interbreeding to produce live offspring that may
not be capable of reproducing further -- is there a reference for
this, too?  Many thanks.

 _/|_    ___________________________________________________________________
/o ) \/  Mike Taylor  <mike@miketaylor.org.uk>  http://www.miketaylor.org.uk
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