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The mission of the International Association of Bryologists (IAB), as a society, is to strengthen bryology by encouraging interactions among all persons interested in byophytes.

Friday, October 9, 2009

BRYONET: IAB blog : Many species just are paraphyletic

BRYONET


Thanks, Efraín, for the nice response. I did not vote, since neither
choice was acceptable. Paraphyly "as I understand it"? Apparently only a
phylogeneticist understands this word, just as I was once assured that
only true Christians can read confusing and contradictory passages in
the Bible with perfect understanding. But I certainly see the effects of
rejecting paraphyly on classification.

You say that "species as lineages are monophyletic." Following Brent's
careful distinction between clades and lineages in his in-proof paper
vouchsafed to us through Bryonet, this is an ancestor-descendant inference.

(1) Cladistics assigns no information to inferred shared ancestors, just
that they are shared by sister groups. One can map morphological traits
to shared ancestors but never name the ancestors. Thus, when speaking of
lineages, what taxa or populations or organisms are you talking about? A
nested set of terminal taxa (((A, B) C) D) ... is a complete substitute
for a cladistic tree, and there are NO shared ancestors named in a Venn
diagram. The only way phylogenetic lineages can be given substance is
recognition of paraphyletic groups as ancestors. There is no
ancestor-descendant information in phylogenetic trees except some
implication that a shared ancestor is lurking.

(2) Do you mean if you lump two species together because one apparently
gave rise to another, and therefore comprised an ancestor-descendant
relationship (or lineage), that every species that gives rise to another
species is necessarily to be lumped together with the daughter species?
This would be true only if no ancestors survived and all products after
a split necessarily gradually diverged. Yet there is much evidence that
species remain in evolutionary stasis (except for nonsense mutations in
DNA sequence) when isolated over long geological periods of time.

Richard

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Missouri Botanical Garden
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