species/subspecies query

Don McAllister mcall at SUPERAJE.COM
Wed Nov 15 09:17:27 CST 2000


Thomas Lammers wrote:

> Isn't this a classical "ring species"?  How are these typically handled?
>

I will leave that with those who have dealt with them, but two extremes occur -
a continuous cline with no obvious steps in which case a single monotypic
species would be indicated, and one with clear step-wise variation which could
be broken down into two or more subspecies.

> Coming from a botanical background, I tend to read the BSC in an
> intransitive fashion.  If two morphologically distinguishable individuals
> can form viable progeny, they *may* belong to the same species or may
> not.   If they cannot form viable progeny, they probably don't belong to
> the same species (I have to leave room for the various incompatibility
> systems some plants have to ensure xenogamy).

The crux is formation of viable progeny. Where the forms are sypatric, there is
less problem in deciding whether they will form viable progeny, or whether
genetic, behavioral, ecological, morphological, etc. barriers will function to
prevent crossing beyond simple hybridization (hybrids can be sterile and so no
genes intogress into the parental populations.  The allopatric case is much more
of a challenge. Either you make a judgement about whether or not two populations
have comparable differences to valid sympatric pairs, or you try and perform an
experiment.

In many cases the experiment would be difficult to carry out - think of bringing
together two trees, deepwater fishes, canopy courting birds, etc.  Even if one
can bring two allopatric forms together one has to be careful.  Equal numbers of
mates of both forms have to be brought together otherwise sexual drive may
overcome barriers. Are the conditions in the experiment suitably close to those
in nature?  If not, perhaps crossing would be facilitated that would not occur
in a more natural situation.

So the theory is simpler than the practice. And that is why the indirect
biological test is the one usually applied.  Are the populations sufficiently
morphologically, genetically, ecologically, behavirorally, etc. different enough
so that one could logically deduce that the forms are sufficiently different in
heritable characters that they comprise what would be valid species when
comparing them with known sympatric pairs.

don
Don McAllister

>
>
> Thomas G. Lammers, Ph.D.
>
> Assistant Professor and Curator of the Herbarium (OSH)
> Department of Biology and Microbiology
> University of Wisconsin Oshkosh
> Oshkosh, Wisconsin 54901-8640 USA
>
> e-mail:       lammers at uwosh.edu
> phone:      920-424-7085
> fax:           920-424-1101
>
> Plant systematics; classification, nomenclature, evolution, and
> biogeography of the Campanulaceae s. lat.
> -----------------------------------------------------------
> "Today's mighty oak is yesterday's nut that stood his ground."
>                                                  -- Anonymous




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