cladism's greatest weakness

[Ken Kinman]

>   While we are on paraphyly (and just to stir the pot): if
>lineage-splitting commonly proceeds by the branching-off of one population
>(or part of a population) from a multi-population species, are not 50% of
>taxa necessarily paraphyletic from their origin? Any additional appearance
>of monophyly in extant groups must have come about through the extinction
>of all but one portion of the paraphyletic 'rump'. This relationship must
>apply to kingdoms as much as to anything else.
>                John Trueman
John,
     Excellent point.  This speciation process is called "budding" and is
quite common.   It is probably the true Achilles heel of cladism, even
though I doubt most cladists realize this weakness.
     The notion or assumption that an ancestral species becomes extinct at
the moment of cladogenesis (and then becomes two new daughter species) was
proposed by Hennig, but I believe he regarded this as just a methodological
assumption (perhaps to get around the problem of budding).
     But I get the distinct impression from strict cladists today that many
actually believe this is some kind of real evolutionary event.  I guess the
thought that the original species could survive unchanged by the budding off
of a small peripheral population----well, it's just too paraphyletic for
them to contemplate seriously.  By now, I would guess most students are
never presented this dilemma, and Hennig's methodological assumption is
often presented as reality.  They might try to use semantics to get around
this, but as far as I am concerned paraphyly is very real, and strengthens
my belief in the need for a cladisto-eclectic form of classification.
       ---Stirring the pot a bit more, Ken Kinman


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