Hybridization can take many forms.  If two populations freely
interbreed, they seem conspecific.  If there are some limits on
interbreeding, then the question becomes where exactly to draw the
line.  Occasional hybridizing can confuse the DNA differences between
good species.  Some hybrids are infertile and don't have any long-term
significance.  Some hybrids can reproduce themselves but can't breed
with either parent.  This is instant creation of a new species.  One
common form of this is when the hybrid has a different chromosome
number (often multiple sets).  In many animals, including some
mollusks, such hybrids are often able to reproduce
parthenogenetically.  Lasaea, sphaeriids, and thiarids are among the
groups where such is known.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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