[Taxacom] Paraphyletic species

[Stephen Thorpe]

Dear Ken 'n' all:

Yes, the Rieseberg and Brouillet paper is a good one (thanks to Bob  
for the link). It seems to confirm some of my earlier suggestions. To  
quote the last sentence of the abstract:

a species classification based on the criterion of monophyly is  
unlikely to be an effective tool for describing and ordering  
biological diversity

Note the words 'a species classification'. It does NOT say:  
classification based on the criterion of monophyly is unlikely to be  
an effective tool for describing and ordering biological diversity

In other words, it is inappropriate to apply the concepts on  
monophyly/paraphyly to species. These concepts apply only to groups of  
species, where the species themselves are "taken as given". Who knows  
how speciation happens? Quite likely it involves hybridisation in some  
cases - and that would really confuse the cladists!

However, I would take Rieseberg and Brouillet's conclusions a step  
further and say that maybe whole monophyletic groups are normally  
formed by paraphyletic processes, i.e., from a single isolated  
population of a parent species which remains unchanged. And the parent  
monophyletic group minus the newly formed monophyletic subgroup might  
have no synapomorphies. In other words, a monophyletic subgroup can  
perhaps evolve from a parent monophyletic group, leaving a  
paraphyletic residue. Example: birds evolved from a subgroup of  
reptiles. Birds have obvious apomorphies, but do the remaining  
reptiles? Maybe we can split the remining reptiles into several  
monophyletic groups, but what if there are a bunch of taxa left over  
which are so plesiomorphic that they share no synapomorphies, and yet  
it would be silly to elevate each species to the level of first  
division reptile because they have only trivial species-level  
characters to separate them!

Cladistics would be easier if rates of divergence were equal for  
parent and daughter taxa, but this might not be so! Imagine a daughter  
taxon evolving great novelty in a geological instant and diversifying  
rapidly into a big group while its highly plesiomorphic parent species  
remains static and has only trivial species-level differences with  
other species in its group! In other words, maybe one nondescript  
average species in the right place at the right time can give rise to  
a whole megadiverse monophyletic group of quite different looking  
things! Example: perhaps all birds are derived from one rather  
ordinary species of lizard?

Cheers,

Stephen

Quoting Kenneth Kinman <kennethkinman at webtv.net>:

> Dear All,
>        Following up on my response on Friday (to John Boggan's
> question), I would suggest reading the paper cited below.  It would also
> be relevant to Stephen's concern whether the term paraphyletic should be
> applied to species (rather than just groups of species).
>       The conclusion of the authors was that large numbers of plant
> species are indeed paraphyletic.  I obviously agree with them.  Here is
> the citation:
>      Rieseberg and Brouillet, 1994.  Are many plant species
> paraphyletic?  Taxon, 43:21-32.
>
>           ---------Ken Kinman
>
>
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