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Re: All in the family...
"Going back one step up the tree, (Kingdom, Division, Class, Order, Family,
Genus, Species), all three genera are considered to be members of the Order
Alismatales (and this is the same whether Najas is considered to lie within
the Hydrocharitaceae or is placed within Najadaceae). So there is SOME
relationship between all three genera (i.e. they ALL share a common ancestor
at some time in the distant past)."
I should have qualified that...... in SOME classification schemes (there are
several) all three genera are placed within the Order Alismatales. In other
schemes they can end up in totally different Orders.
Some, not all. Over the past 20 years, there have been several comprehensive
classification schemes proposed for the flowering plants. They differ in the
total number of families recognized (lumpers vs splitters) and how these
families are grouped into higher level arrangments, depending upon the point
of view of the botanist responsible for the scheme. Both the names used for
and the "rank" certain groups of plants get recognized at can differ from
scheme to scheme but neither the name nor the rank is really important. Rank
is relative and only has meaning within a particular scheme - a ranking in
one scheme doesn't necessarily mean the same thing in another scheme and the
names used for the various groupings can vary all over the place. For the
casual reader, the only important thing to ask is WHICH scheme is being used
by the book or reference you are using. Once you know that, you at least
have a frame of reference from which to work.
For example, I have a copy of Christopher D.K. Cook's "Aquatic Plant Book,
2nd edition", published in 1996. It purports to list and describe all the
aquatic plants of the world, as recognized as "aquatic" by Cook. He follows,
in general, the classification scheme proposed during the 1980's for the
flowering plants by Rolf Dahlgren, a botanist from Copenhagen. This scheme
is different from the one proposed by Arthur Cronquist, an American, who's
scheme became much more "popular" in North America than did Dahlgren's, for
various reasons, not necessarily because it was "better".
Many of the families recognized by Dahlgren differed slightly from those
listed in Cronquist's scheme (splitter vs. lumper). In Cook's book, he notes
which genera are included within most of the families where such differences
of opinion exist as well as which families are specifically excluded. This
provides a good frame of reference and allows the reader to better
understand what is meant by the various names used. Both Dahlgren and
Cronquist recognized Hydrocharitaceae and Najadaceae as separate families.
Both agreed that they were monocots, which Dahlgren called "Liliidae" and
Cronquist called "Liliopsida". Cronquist isolated Hydrocharitaceae in its
own Order, the Hydrocharitales. He placed Najadaceae in a different (but
related) Order, the Najadales, together with Aponogetonaceae,
Scheuchzeriaceae, Juncaginaceae, Potamogetonaceae, Ruppiaceae,
Zannichelliaceae, Posidoniaceae, Cymodoceaceae and Zosteraceae. Dahlgren
also placed Najadaceae in a different order from Hydrocharitaceae but the
membership of the two Orders (Hydrocharitales and Najadales) differs in the
two author's work, with many of the families Cronquist lists in the
Najadales showing up in Dahlgren's Hydrocharitales. Pure difference of
The most recent attempt at arranging the family tree of the flowering plants
is currently being undertaken by an international group of researchers (it
isn't the work of any one person) and is using information gathered by DNA
analysis, together with the more traditionally used morphological data and
fossil evidence, to try and determine how the flowering plants evolved the
diversity that exists today.
In the APG classification, the Family Najadaceae is included within the
Family Hydrocharitaceae (i.e. the genus Najas is a member of the family
Hydrocharitaceae), and this family in turn belongs to the Order Alismatales,
which are monocots.
As it currently exists, the family tree that is being mapped out by this
group (Angiosperm Phylogeny Group) uses a combination of "official" names
and "informal" names for different groups of related plants. Within this
classification scheme, only groups which are monophyletic, including all and
only the descendants of a common ancestor, ought to receive "official"
names. They also emphasize the difference between "grouping" and "ranking",
holding that "ranking" is relative.
There are several websites which provide details on the current status of
the APG classification but you have to be aware that it truly is a work in
progress and subject to change as new DNA studies get done. Most of the web
based sites refer to:
APG [Angiosperm Phylogeny Group]. 1998. An ordinal classification for the
families of flowering plants. Ann. Missouri Bot. Gard. 85: 531-553.
A new paper, based upon this earlier study and newer ones, will be published
--. 2002. An update of the Angiosperm Phylogeny Group classification for the
orders and families of flowering plants: APG II. Submitted.
O.K. enough already - this is reading like a lecture, and we all know how
some people don't like lectures........