[Taxacom] Phylogenetic Classification?

Alexander.Schmidt-Lebuhn at biologie.uni-goettingen.de Alexander.Schmidt-Lebuhn at biologie.uni-goettingen.de
Sun Jul 26 07:27:12 CDT 2009

>>>Consider the fact that before spermatophytes evolved, pteridophytes were
>>> the only tracheophytes in existence.  In other words, pteridophytes were
>>> a clade before one of them happened to give rise to spermatophytes.<<
> This oft-ignored 500-lb Gorilla in the Room is one of the strongest bits
> of evidence that (pardon the mixed metaphor) the Emperor Has No Clothes.
> This "sliding scale", the extreme "relativity" of relationships is
> troubling.  It bothers me that a group of organisms, *through no "fault"
> of its own* can lose its status as a clade and become anathema simply
> because of what its descendents have done.  In my opinion, *meaningful*
> taxa are defined on the basis of aspects and features integral to them,
> not by accidents of history, not by who their relatives are.

Er, what? The pteridophyte/tracheophyte clade of olden times is still a
clade and will always be - it is the clade including *all* offspring of
the pteridophytes of old, today's "pteridophytes" and their seed plant
sisters. That is the beauty of cladist thinking, that you *never* chop off
a huge chunk of organisms from the taxon that their ancestor belonged to.
This approach may have one or two practical problems of its own, which
should be weighed against the myriad problems you get if you accept
paraphyletic groups, but what both of you have described here is precisely
the travesty of paraphyly: that a clade, through no fault of its own, can
lose its status as an acceptable taxon over the course of evolution just
because some of its members have evolved an additional apomorphy.

Alexander SL

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