Hybrid Speciation

Fri Apr 17 11:53:02 CDT 1998

Dear Abdul Ghafoor and others,

I have done some thinking about this problem with regard to the genus
Rhododendron where hybridisation is rife.

I believe one first has to ask the question 'What is a hybrid?'

The mundane definition of this is that it is a cross between two
species but as species boundaries are often a matter of opinion and
may certainly be debated this is not much use. I therefore define a
hybrid in a cladistic sense - as a cross between two non-sister taxa
(i.e. taxa that would form a paraphyletic group).

If a cross occures between sister taxa then it suggests that these
taxa should be considered as a single entity - how could we show that
these two taxa (OTUs in our analysis) aren't segregates of a larger
taxon? 'Hybrids' of this type should not be formally recognised.

If the patents of a hybrid are non-sisters (i.e. can be demonstrated
to have different evolutionary histories) then the hybrid can truely
be said to be a hybrid.

All this becomes clear if you draw out the trees. Something I won't
attempt to do on e-mail.

The problem with all this theory is that it relies on an analysis
having been carried out on a data set that excludes all the hybrids.
This is tricky!! The only suggestion I have here is to use intuition
and leave out obviously phenetically intermediate taxa. This is not
a solution to the problem just a means of getting on with something.

Whether a species is of hybrid origin can not be detected
retrospectively using phenetic or cladistic analyses alone, really
one needs to employ the techniques of population/molecular biology
and cytology to answer this kind of question.

At the alpha-taxonomic level one could say that it doesn't really
matter. Describe every distinct entity as a taxon and, unless there
is very good circumstantial evidence (e.g. sympatric distributions
and deformed pollen) keep speculation on the origins of the group in
a comments or notes section of the publication. Some one can always
put an X infront of the name at a later date.

Hope this helps out. I am sure that there are taxacomers who will
strongly disagree with part or all of what I have said and voice
different opinions.


Dr Roger Hyam
Royal Botanic Garden,
Inverleith Row,
Edinburgh, EH3 5LR
Scotland, U.K.
Tel. 0131 248 2893 (direct)
Tel. 0131 552 7171 (switchboard)
Fax. 0131 248 2901
Please Note New Phone & Fax Numbers !

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