paraphyly

Bengt Oxelman bengt.oxelman at SYSTBOT.GU.SE
Fri Jun 13 01:04:54 CDT 1997


>>>Exactly!  "Budding off" of peripheral isolates is extremely common mode o=
f
>>>allopatric speciation.  When it happens, a formerly monophyletic species
>>>becomes paraphyletic.
>
>
>>This is nonsense and the result of a confusion of species as taxa (Linnean=
=2E..
>>
>>... For example evolution in the genus Mus would make Eucaryota paraphylet=
ic.
>
>
>Bengt's response is incorrect.  Evolution within Mus would not affect
>Eukaryota.  There is still a chain of common origins from genus on down to
>whatever level "eukaryota" occupies.  Budding off does not affect higher
>(vertical) taxonomic designations, only horizontal ones.  If Mus newus evol=
ved
>from Mus oldus, but populations of M. oldus still exist, then M. oldus beco=
mes
>a paraphyletic species, but genus Mus is not affected.
>

OK, If I go out in the field and discover two Mus species, which one should
be called M. oldus and which is M. newus? In other words, sister taxa have
equal age! My point is that any attempt to place temporal aspects on taxa
(including species) will lead to unsolvable paradoxes. The monophyletic
group of Mus oldus and M.newus is equal to their common ancestor! The
Linnean categorical rank species may be used for the least inclusive group
or potential ability to interbreed may be used as ranking criterion, but
that is arbitrary if we insist in a taxonomic system that is consistent.
Monophyly is a criterion that is consistent with our perception of nature
and it can be applied consistently (at least in theory) if we learn to not
confuse process with pattern. Especially, we must get rid of the idea the
species rank is special. If we actually saw a taxon split into two, then
what we had seen would not be a taxon 'budding off' a new taxon of the same
rank, but the birth of a new hierarchical rank. Note that this would not
necessarily lead to the recognition of a new Linnean taxon, which are
constrained by convenience (which is fortunate!). It would not be necessary
to name a new genus or section or any other supraspecific taxon.

--------------------------------------------------------------------------
Bengt Oxelman
Department of Botany
Stockholm University
Lilla Frescativ=E4gen 5
S-106 91
Sweden

Phone: +46 8 161215
=46ax: +46 8 165525
Internet e-mail: bengt.oxelman at systbot.gu.se




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