Real species and ideology

Richard Pyle deepreef at BISHOPMUSEUM.ORG
Tue Apr 20 20:46:10 CDT 2004

Thanks, Michael.

If I understand your point correctly, you're saying that the difference
between a morphocline and introgressive hybridization is that for it to be
thought of as a morphocline, the two differing populations must be
monophyletic with respect to all known extant populations; whereas if the
two populations are not monophyletic (or in some cases paraphyletic), then
it would be considered a case of introgressive hybridization, and not a
morphocline. Do I interpret you correctly?

But this actually touches on another conceptual problem with the reliance on
monophyly as a distinguishing factor for species boundaries.

Suppose we have a case of ((AB)(CD)).  If I follow your line of reasoning,
then a pattern of restricted gene flow observed between A & B, or between C
& D, might be thought of as a morphocline; but if we saw the pattern between
A & D (without involving B or C), then we'd feel confident in treating it as
introgressive hybridization.  (Please correct me if I misunderstood your

I certainly understand the logic in that, but I want to come back to two
critical words I used in my first sentence above: "known extant".  Suppose
that B & C both had restricted population sizes, and both went extinct due
to human-induced habitat destruction.  If that happened, then A & D would
become "sister species" within the context of "known extant" populations.
Would the extinction events of B & C open up the possibility that gene flow
between A & D could be thought of as a morphocline, instead of introgressive
hybridization?  I can't see why it would, because the extermination of all
individuals of B & C has no effect on the evolutionary relationship between
A & D.

Now suppose that B & C went extinct before humans began cataloging the
natural world around us.  We would never have known they existed at all.  As
far as we could tell, A & D would be unambiguous sister species, and we'd be
left with the messy conundrum of deciding whether we were looking at a case
of a morphocline, or of introgressive hybridization.  That B & C ever
existed would have absolutely no impact on our interpretation of the
relationships between A & D.

Now suppose that B & C never actually went extinct, but instead inhabited a
part of the earth that Humans have a very difficult time accessing (a few
prime examples come to mind...)  For 250 years, we never knew the existence
of B & C -- we only knew of A & D, and our genetic data showed them to form
a clear monophyletic grouping with respect to all other known taxa.  The
zone of gene exchange between A & D could have been thought of either as a
morphocline between sister groups, or as introgressive hybridization between
sister groups.  But then some intrepid taxonomist goes out and discovers
populations of B & C, and genetic data show them to have the phylogenetic
relationships as defined above: ((AB)(CD)).  Would we then necessarily
eliminate the morphocline option for A & D altogether? What aspect of our
knowledge of the existence of B & C changed the relationship between A & D?


Richard L. Pyle, PhD
Ichthyology, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
Ph: (808)848-4115, Fax: (808)847-8252
email: deepreef at

> A key part in differentiating the two patterns (morphocline vs.
> introgressive hybridization) can be an understanding of the
> broader relationships of the taxa involved.  You omitted the
> last, perhaps most critical, part of Curtis' statement on the
> mater. As he said:
>  "(It especially helps if the two intergrading species turn out
> not to be sister
> species!)"
> Populations that form part of a differentiating morphocline would
> be monophyletic or possibly paraphylethic - and this could be
> demonstrable with genetic data.
> On the other hand, introgressing taxa (previously distinct
> species that have come together and started to hybridise) need
> not be each others closest relatives - and this could also be
> demonstrable with genetic data (and suitable markers could then
> be used to investigate the extent of introgression).
> If the two "messy" (clinal or introgressing) taxa are most
> closely related (or even paraphyletic with respect to some
> peripheral isolate(s)) the problem is obviously a lot more
> complicated.  Either process could/would produce a similar
> pattern of variation (morphological or genetic), and the
> conundrum you are talking about still raises its head......
> Mike
> ----------------
> Michael Bayly
> Research Scientist in Plant Biosystematics
> Museum of New Zealand Te Papa Tongarewa
> PO Box 467
> Wellington, New Zealand
> Ph: +644 381 7262
> Fax: +644 381 7070
> -----Original Message-----
> From: Taxacom Discussion List [mailto:TAXACOM at LISTSERV.NHM.KU.EDU]On
> Behalf Of Richard Pyle
> Sent: Wednesday, 21 April 2004 6:45 a.m.
> Subject: Re: Real species and ideology
> > It is often a non-trivial task to distinguish between two
> > subspecies separated by a morphocline, on the one hand, and two species
> > separated by an area of introgressive hybridization, on the other, but
> > genetic tools can often be brought to bear to sort out these issues.
> How would you distinguish the two separate patterns (morphocline of
> subspecies vs. introgressive hybridization) -- even with the best
> of genetic
> tools?  Indeed, is there anything fundamentally different between the two
> patterns, other than our artificial imposition of the rank
> "species" in one
> case, and "subspecies" in another?
> This gets right to the heart of my original question to you,
> about defining
> the terms "real" and "artificial" in an evolution/species context, and in
> supporting one perspective over the other.  This thread has taken a
> different direction than what I had hoped it would, but the above
> quote from
> Curtis brings it back to what I'm most interested in understanding.
> Aloha,
> Rich
> =======================================================
> Richard L. Pyle, PhD
> Ichthyology, Bishop Museum
> 1525 Bernice St., Honolulu, HI 96817
> Ph: (808)848-4115, Fax: (808)847-8252
> email: deepreef at
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