paraphyly and polyphyly

Derek Sikes dsikes at UCALGARY.CA
Wed Nov 10 12:17:21 CST 2004


Dear Taxacomers,

I'm preparing to teach an introductory course in systematics and would
like to hear back on the following issue:

I've had trouble teaching students the difference between paraphyly and
polyphyly before and have had debates with systematist colleagues about
these terms. I have also noticed what I think are occasional miss-uses
of these terms in the literature. At the last evolution meetings,
although  many people presented results of "non-monophyly" most called
it simply that:  "non-monophyly" and avoided calling it either
polyphyly or paraphyly - perhaps because they are uncertain themselves
of the distinction?

I also think that most texts confuse the matter because they always
show trees with certain tips circled to demonstrate the different terms
(e.g. Gullan & Cranston 2005 fig. 7.1).

According to my understanding any such tree that is illustrative of
paraphyly can also be illustrative of polyphyly - if there is no
mention of characters and/or the common ancestor  indicated on the
tree.

As I see it, the distinction between these 2 terms must be made based
on whether the common ancestor of the members in the group in question
is included (paraphyly) or not (polyphyly).  [ Wiley 1981 has a nice
set of definitions, I know, but I'm interested in your opinions, not
definitions from texts]

And how is the determination regarding the ancestor made? By looking at
not just the tree & the group but also looking at the CHARACTERS that
define the group! (this is what is often absent from text book
explanations of these terms) If the group's apomorphies are homologous
among its members then the common ancestor had those apomorphies and
the group is paraphyletic (if it's missing some descendants of that
ancestor) whereas if the apomorphies of the group are a result of
homoplasy then the ancestor did not have those apomorphies and wouldn't
be in the group and thus it is polyphyletic.

A good example to discuss this confusion is Nikaido et al 2001
"Retroposon analysis of major cetacean lineages: The monophyly of
toothed whales and the paraphyly of river dolphins"

http://www.pnas.org/cgi/content/abstract/98/13/7384

Their tree shows river dolphins being non-monophyletic. But are they
paraphyletic (as the title suggests) or are they polyphyletic?

Polyphyly would imply that the characters that define the family
Platanistidae (river dolphins) are the result of convergence, ie there
was no common river-dolphin ancestor of all of today's river dolphins.

Paraphyly would imply that all river dolphins evolved from a common
river-dolphin ancestor but that various lineages of other non-river
dolphin cetaceans evolved (reversal) from an ancestral life of being a
river dolphin to a life of being a large marine predator.

So, to properly distinguish among these terms requires some estimation
of ancestral states - (is the ancestor in or out of the group?) and we
all know that a single tree can result in various, different mappings
of ancestral states, thus making this a far more challenging task than
simply looking a topology and "eyeballing" the answer as we often ask
students to do on exams.

My take is that the river dolphins are polyphyletic. I think this is a
good example of confusion with this term. I'm interested to hear from
any of you who disagree or have comments on this issue.

Thanks,
Derek Sikes
++++++++++++++++++++++++++++++++++++++++++++++
Derek S. Sikes, Assistant Professor
Division of Zoology
Department of Biological Sciences
University of Calgary
2500 University Drive NW
Calgary, Alberta, Canada, T2N 1N4

dsikes at ucalgary.ca
http://homepages.ucalgary.ca/~dsikes/sikes_lab.htm

phone: 403-210-9819
FAX:  403-289-9311

"Remember that Truth alone is the matter you are in Search after; and
if you have been mistaken, let no Vanity reduce you to persist in your
mistake." Henry Baker, London, 1785

Entomological Society of Alberta:
http://www.biology.ualberta.ca/courses.hp/esa/esa.htm
++++++++++++++++++++++++++++++++++++++++++++++




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