Many plant species have mechanisms to prevent self-fertilization:
http://www.news.cornell.edu/releases...tigma.bpf.html
http://www.news.cornell.edu/releases...stils.bpf.html

Self-fertilization can be a disadvantage because it often causes
inbreeding depression in outcrossing species. However, there can be an
advantage to allow fertilization with other species because crossing
between two species may produce a superior hybrid. Thus, interspecific
hybridization may lead to a new species (Arnold 1997). Levin (2002)
gives a figure that between 6 and 16% of plant genera are capable of
crossing between species but doesn't indicate a source of that
statistic. Many important cultivated plants are interspecific hybrids
including grapefruit (Citrus x paradisi) and strawberry (Fragaria x
ananassa).

A lot of breeding between plant species occurs because people have
removed geographical barriers or use hand pollination to overcome time
of flowering differences or lack of pollinators. Orchids often prevent
breeding with other species because an orchid species has coevolved
with a single insect species that will carry pollen just for that
species. However, there are hundreds of intergeneric orchid hybrids
developed by hybridizers:
http://www.orchids.mu/Glossary/Glossary_A.htm

If breeding between two species occurs at a high frequency, it may
result in extinction of one parent even if the hybrids are infertile
(Levin 2002). This can happen naturally but more frequently occurs
when one species is introduced. For example the English bluebell
(Hyacinthoides non-scriptus) [synonyms: Scilla non-scripta, Endymion
non-scripta] readily crosses with the introduced and larger, more
vigorous Spanish bluebell (Hyacinthoides hispanica) to form hybrids.
Hyacinthoides non-scriptus has become rare in the wild as the hybrids
have crowded it out. Levin (2002) gives other examples of introduced
plants that may eventually cause extinction of native species, such as
cultivated sunflowers (Helianthus annuus) breeding with wild
sunflowers to produce fertile hybrids.

In other cases, cross-breeding between species occurs fairly
frequently but both parents maintain their populations. This is
apparently the case in some of the natural interspecific hybrids in
oaks.

In some oaks and pines, the frequency of crossing between species is
sometimes low so the hybrids are rare. For example, Jeffrey pine can
naturally cross with Coulter pine or Ponderosa pine but hybrids are
rare even under artificial conditions:
http://www.na.fs.fed.us/spfo/pubs/si...s/jeffreyi.htm

References

http://waynesword.palomar.edu/hybrids1.htm

Arnold, M.L. 1997. Natural Hybridization and Evolution. New York:
Oxford University Press.

Levin, D.A. 2002. Hybridization and extinction. American Scientist.
90: 254-261.


"Frank Martin" wrote in message ...
Can someone help me how to show this on a cladic chart, and
perhaps why such breeding does not occur at the molecular
level. Please help, Frank