Hybrid Speciation

ROGER HYAM R.Hyam at RBGE.ORG.UK
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.

Roger.



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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
http://www.rbge.org.uk
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