>But aren't most hybrids sterile?

It depends on the amount and type of difference between the parent
species and the abilities of particular individuals to deal with the
differences.

For example, many hybrids are polyploid (more than two sets of
chromosomes).  In other cases, a diploid hybrid comes from parent
species with significantly different chromosomes.  Either way, it's
generally difficult for cells in such situations to sort out a proper
set of chromosomes in order to produce gametes, resulting in sterility.
 However, some have found a way.  Freshwater Corbicula are generally
triploid hybrids with strangely modified sexual reproduction.  Wheat is
a hexaploid hybrid, but has a gene that helps it sort the chromosomes
out and function like a diploid.

In other cases, the hybrids are sterile for sexual reproduction but are
able to reproduce asexually.  Others can reproduce with other hybrids
but not with the parents.  In these cases, the hybrid is functionally
an instant new species.

In other cases, especially in species with some degree of courtship,
hybrids may be reproductively functional but disadvantaged in
attracting mates.  Because they have a mix of genes, their courtship
behavior is typically mixed as well, and sending mixed signals
generally doesn't go over well with a prospective mate.

Similarly, if there are specialized survival traits that differ between
the parent species, the hybrid may be confused between options and thus
be more vulnerable and a worse competitor, even if it theoretically
could reproduce in the unlikely event of its surviving.

----------------------------------------
Dr. David Campbell
425 Scientific Collections
University of Alabama, Box 870345
Tuscaloosa AL 35487
"James gave the huffle of a snail in
danger But no one heard him at all"  A.
A. Milne

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