Paraphyly and names
skala at INCOMA.CZ
Fri Jan 18 12:57:55 CST 2002
OK, we seem to agree that it is not useful to have as many taxa as
clades. Then, we can proceed:
>The 'truncated clade' Amphibia does not have reality, in the sense that
we cannot make statements about it
>except about what it does NOT have, like the amniote egg - and what it
SHARES with others, like the tetrapod
>condition. This is why Ken is mistaken ...
It is a usual, but rather unfounded idea that apomorphies mean "having
something" and plesiomorphies "not having something", so the latter are
less informative and/or less real. There are two aspects of this, one
intuitive and one logical:
(1) objection based on the intuitive meaning of "having something", that
plesiomorphy is simply missing of some feature.
Answer: an apomorphy can well be a loss of previously acquired feature.
For example, some angiosperm plant genera have secondary lost
chloroplasts; this is their synapomorphy but in this case plesiomorphy
mean "to have something" instead, i.e. chloroplasts.
(2) objection based on the logical meaning: apomorphies are by
definition derived character states, so their respective plesiomorphic
character states are "not to have something", i.e. "not to change".
Answer: while there is a straightforward logic why synapomorphies are
crucial for constructing cladogram, there exists no logical reason why
they should be *exclusively* used for splitting the cladogram into taxa.
If, e.g., we have a pectinate cladogram of species A,B,C,D,E and clades
A+B and A+B+C+D are supported by many synapomorphies, we will gain the
most informative taxa by grouping ((A+B)+(C+D))+E. This is a measurable
fact unlike the rather vague notion that plesiomorphy is "nothing".
Once again: *constructing* cladogram and *splitting* it into taxa are
basically two different operations which can - and even must - differ in
method. Those who disagree should explain clearly why they think so.
skala at incoma.cz
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