The importance of hybrids depends on ones species concept. Botanist
routinely encounter fertile hybrids between species and even between genera.
Some botanical species are thought to be of hybrid origin.
 
In thinking about hybrids, remember that lineages that have diverged have
not necessarily evolved isolating mechanisms that would prevent them from
having fertile offspring.  Suppose two species (or lineages) that have been
geographically separated come back into contact and they are able to mate
with each other and have fertile offspring.  If the offspring are less fit
than the either parent species, then there will be selective pressure for
the evolution of isolating mechanisms that will prevent them from mating
with each other. For example, there could be a shift in mating season,
because individuals that breed during the overlap season are at a
disadvantage.
 
Species A  xxxxxxxxxxxxxxxxxxxx
Species B                 xxxxxxxxxxxxxxxxxxxx
 
What is important is not whether the offspring are fertile, but whether such
fertility leads the species to merge with each other (which would make some
argue that they weren't separate species to begin with) or not.
 
Gary
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Gary Rosenberg, Ph.D.                     [log in to unmask]
Malacology & Invertebrate Paleontology    gopher://erato.acnatsci.org
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