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Re: Sterna, Whales, and Evolution



In a message dated 96-02-15 14:07:03 EST, ornstn@inforamp.net (Ronald
Orenstein) writes:

>Which presents me with a problem I have never quite understood.  Let us say
>that you have, say, a genus of ten closely-related and very similar species.
>One of those species - and only one - becomes the ancestor of a new and
>highly-differentiated radiation.  The other ten [you mean nine] go extinct.
>
>Now - is the base species of the new radiation excluded from the genus with
>the other ten species?  Must the whole of the new radiation, no matter how
>differentiated and diverse it becomes, be included in the stem genus of
>which it forms a nested subset?  If not, why not?

Cladistic taxonomies do not handle this kind of situation very well at all,
which is why I don't use them; in MM #2 I call this the Ancestral-Species
Problem of cladistic taxonomy. Typically, a cladistic taxonomy would relegate
the nine extinct lineages to stem-group status, probably giving some or all
of them their own genera (solely to avoid paraphyletic supraspecific taxa),
and give the tenth species and its slew of descendants their own named
higher-level clade.

Reducing your problem to a situation between just two species, A and B, there
is only one cladogram that relates those two species, namely the trivial
cladogram with two branches A and B. But there are _six_ different ways A and
B can be related to each other: (1) they can both arise from a different
common ancestral species; (2) B can branch off from A; (3) A can branch off
from B; (4) A can become B; (5) B can become A; or (6) A and B are not
closely related at all. The cladogram clearly fails to distinguish among
these cases, and indeed only cases (1) and (6) resemble the cladogram itself
to any extent.

Six relationships, but only one possible cladogram. I trust you see why
cladistic taxonomy is ridiculously inadequate to deal with situations such as
the one you described.