As Cladistics and "Eclecticism", Aves, paraphyly flow on

[Curtis Clark]

At 05:09 PM 2/4/02, Dave Walter wrote:
> From your dicot example I
>assume that someone has published (I think I remember my wife talking about
>it - she works on orchid pollination as well as mites) a convincing
>analysis showing that monocots were derived from within the dicots,
>rendering the latter paraphyletic?

Some or all of the Magnoliidae comes out as sister to the monocots in
almost every molecular analysis, and that was never surprising from a
morphological standpoint.

>I know that I can refer to
>dicots and monocots and get across the basic dichotomy that I want
>(although it isn't as rich in information as I once assume and is somewhat
>misleading).  I also know that if I immediately familiarize myself with the
>most recent hypothesis and try to adopt any proposed new taxa that a) the
>students will be confused and b) the hypothesis (and proposed taxa) may
>very well change again soon.

Not entirely. There is the clade usually called "eudicots" that are those
dicots with tricolpate or tricolpate-derived pollen, as well as a bunch of
other apomorphies both morphological and molecular. They and the monocots
form two robust, well-marked groups, and in fact most of the features used
by general botany texts to separate monocots and dicots only reliably
separate the monocots from these *eu*dicots. Because the bulk of plants in
temperate regions, and a majority even in the tropics, are either monocots
or eudicots, the standard "distinguishing features" work passably well.

In a sense, the flowering plants that aren't eudicots or monocots are
"ground trash" (figuratively) under the two great branches. This is not an
uncommon feature of well-marked clades. I was talking with an ornithologist
colleague today about the multitude of "basal" orders with only a few
species each, and a few species-rich orders that have most of the
diversity. This is a common theme in evolution: some lineages are wildly
successful and others aren't.

>I still use Reptiles, Dinosaurs (as at least one 'strict cladist' seems to
>do too), and Dicots, but I do try to avoid the latinized versions of group
>names that appear to be paraphyletic

I still talk about bryophytes and gymnosperms, too, because they are
historically recognized groups, although eschewed now even by general
botany textbooks (ironically, some molecular evidence suggests that
gymnosperms might be a clade after all). I even grit my teeth and talk
about Fungi Imperfecti, a polyphyletic group diagnosed by our (previous)
lack of knowledge, because students will encounter the term.

I do explicitly say "non-bird dinosaurs" when I talk about the paraphyletic
group, because it makes such a good example of how our classifications
affect our perceptions. I point out four theories about extinct dinosaurs
that were originally and in some cases still are controversial
(homeothermy, parental care of young, vocalization to defend territories,
and vivipary) and explain that our knowledge of birds and crocodilians
makes the first three unsurprising (though not necessarily true :-) and the
last incredible.

To me, the whole purpose of biological classification is to provide a
framework for understanding life, and I find that lineage groups do that
quite nicely. Paraphyletic (and even polyphyletic) groups are part of our
heritage as biologists, and can't be ignored. But they don't have to be
perpetuated.


--
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu