Hybrid Speciation

Daniel L. Geiger dgeiger at SCF-FS.USC.EDU
Fri Apr 17 08:57:11 CDT 1998

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

Good point to start with.

>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.

Here I disagree. I work in the marine snail family Haliotidae (= abalone,
ormers). On the West American coast pretty much all 7 "species" hybridize,
at least most of them have been documented in laboratory cross
fertilization studies and a good number of them have been observed in the
field, though in low frequencies. So then if all species hybridize and the
fact that they form a monophyletic group, that then means that at least
some are sister species; hence we have one west coast species according to
the above definition. This is clearly nonsense in the abalone, where
postive Darwinian selection has been demonstrated for some sperm acrosomal
protein (Vacquier and co-workers at SCIRPPS, UCSD). The animals as well as
the shells are distinct with rather little variation.

So I think the non-sister species argument is certainly powerfull, but it
shall not exclude hybrids of "good" sister species. A frequency argument
may be helpful to understand a particular case. Where we have some
estimates on the frequency in abalone it is on the order of 0.5%, i.e.,
rather low.

I second the point that cladistic or phenetic methodologies cannot identify
reticulate speciation, so some other approach needs to be taken.

Daniel Geiger

Daniel L. Geiger
Department of Biological Sciences
Allan Hancock Foundation Building 233
University of Southern California
Los Angeles, CA 90089-0371

phone  (213) 740 5783
fax    (213) 740 8123
e-mail dgeiger at usc.edu
www    http://nhm.org/~dgeiger

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