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FW: Re: FW: Re: FW: Re: FW: Help with Cladistics
>In a message dated 95-08-28 17:00:51 EDT, chris brochu writes:
>> A taxon cannot leave a
>>group because it posesses some new character state.
In strict cladistic methodology, only monophyletic groups (an ancestor and
all of its descendents) are recognized. As such, once a member of a group,
always a member.
Cladists reject the notion that paraphyletic groups - groups arbitrarily
excluding one or some descendent lineages - on two bases:
1. Such groups are inherently arbitrary and subjective. Birds share a
common ancestor with dinosaurs. A natural group defined by their common
ancestor thus exists. The name we choose to fix to this group is
arbitrary, but the group itself exists regardless of our ability to recover
it. The practice of separating groups on the basis of some "key character"
is very subjective - who, after all, is to decide which characters are
"key?" Why are feathers so much more important than a wishbone or a hollow
skeleton - characters that are arguably requisite for avian flight, but
which are distributed more generally among theropod dinosaurs.
2. We will always be finding fossils that smear the transition between
such groups. When the hands and feet of Ichthyostega were found back in
the '80's, they were seen to have 7 or 8 digits and to resemble fins more
than legs. Was it still so obviously a non-fish? As our knowledge of the
fossil record improves, our ability to resolve paraphyletic assemblages
Christopher A. Brochu
Department of Geological Sciences
University of Texas at Austin
Austin, TX 78712
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