[Taxacom] Grades. clades, and predictivity

[Curtis Clark]

Expanding on my previous post, any classification containing both grades 
and clades, with no way of distinguishing them, cannot be predictive in 
the sense we've been discussing. I don't think this has ever been 
successfully refuted.

Although I am a proponent of classification based on clades, I think it 
is possible to have a "pure" grade classification, which would be 
predictive. My allusion to taxa based on plesiomorphies is one way of 
looking at it, but perhaps not the most satisfactory. It might be better 
to characterize grade boundaries with contrasting plesiomorphy/apomorphy 
pairs, so that, for instance, vascular plants would be distinguished 
from bryophytes (s.l.) by presence vs. absence of lignified secondary 
walls of water-conducting cells, and bryophytes would be distinguished 
from green algae by presence vs. absence of multicellular embryonic 
(dependent) sporophytes.

One of the things that made grades so appealing in the past was 
ignorance: the difference between birds and reptiles was seemingly so 
great that many plesiomorphy/apomorphy pairs marked the boundary. As we 
learn more, these polythetic boundaries are less and less tenable. It 
seems to me that the only useful grade boundary is a key innovation, one 
that was necessary, if not sufficient, for the derived grade to 
diversify. Lignified secondary cell walls are thus a good choice for 
vascular plants, and perhaps mammae for mammals, but we've learned that 
feathers don't really do it for birds.

So there's a conceptual framework lurking in there for anyone who would 
like to be the "Hennig of grades".

-- 
Curtis Clark                  http://www.csupomona.edu/~jcclark/
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