BSC

[Curtis Clark]

At 10:13 AM 11/12/99 -0400, Bill Shear wrote:
>Do all botanists misunderstand the BSC to this extent?

and in a separate message:
>Curtis, can you elaborate?  I have to confess that I do not understand
your
>point.

I guess I did dash it off in a hurry. Reproductive isolation is indeed an
attribute of most if not all sexual species. I would argue that it is still
not the best criterion for recognizing species, but that is a separate
issue, and I agree that it can be a useful criterion.

However, in my experience, most biologists (not just botanists :-) use the
ability to form fertile hybrids as a quick-and-dirty measure of
reproductive isolation. This simply does not work. In some groups of
plants, such as Quercus and the asteraceous shrubs that I work on, it fails
to work in a spectacular manner. But even among vertebrates, the ability to
interbreed is often used as an excuse to lump species.

All of this results in part from BSC advocates, even the best of them,
refusing to accept ecological isolation mechanisms. In a genus I study,
Encelia of the Asteraceae, F1 hybrids are common in the wild, F2 and
backcross are only in areas of human or severe natural disturbance, and
there is no evidence of introgression, even (so far) at the molecular
level. Progeny tests in one case show that the F2 and backcross
recombinants are formed and exist as seeds, but some unknown ecological
factor prevents their showing up as mature plants.

Ledyard Stebbins once said of this situation: "What you have is a
comparium." He meant syngameon, a group of interfertile "semispecies" (see
the archives recently for some posts on alternate meanings of this term).
He went on to say that the species could "breed themselves out of
existence" at any time.

There is a case in California where two oaks hybridize commonly in one part
of the state (I don't remember the species offhand), and I seem to remember
Stebbins giving that as an example. I did some research and found that one
of the species has been in the fossil record since the Pliocene, the other
might go back to the Oligocene, and hybrids are found in the fossil record
as far back as the Miocene. So for 15 million years these species have
hybridized and still not done themselves in as separate entities.

I gave a paper at an AIBS meeting, Clark, Curtis. 1984. Selection against
hybrid recombinants as an isolating mechanism: the syngameon reassessed.
Amer. J. Bot. 71(5), Part 2, p. 161. (I don't have the text of the abstract
on disk, but there is a .pdf at
http://www.csupomona.edu/~jcclark/pdf/a1984aspt1.pdf) where I explored this
issue in greater detail. My conclusion is that morphology does mean
something, and that the mere presence of hybridization, no matter how
extensive, is not a surrogate for either gene flow or reproductive
isolation.

Maybe we botanists just don't get it. But it's not necessarily from lack of
trying.


----------------------------------------------------------------
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu