[Taxacom] Diagnosing species

[B.J.Tindall]

Bob Mesibov wrote:
"Microbiologists are very comfortable with this approach to taxonomy, even if
many botanists are still not."

Don't agree here. Microbiologsits are aware of 
the problems of gene transfer etc. but my 
impresssion is that one is still trying to force 
the taxa onto a dichotomous "phylogenetic" tree, 
and when it doesn't fit one removes the data that 
is causing the problem. Judging from the 
publications I see many microbiologists are very 
uncomfortable with the situation.

In the case of genera, yes they, and other taxa 
need to be defined, but this is either poorly 
done in microbiology or one relies heavily on a 
single gene. However, as they say it will all come out in the wash.

Brian

At 06:06 24.06.07, Bob Mesibov wrote:
>Richard Zander's recent TAXACOM posts have pointed to regarding species as
>diagnosable entities for taxonomic purposes, rather than as historical
>entities. By "historical" I mean "phylogenetic" as understood by most
>TAXACOM readers: a species as a single branch on a dichotomous tree of life.
>Zander's diagnosis would include a range of characters not commonly thought
>of as species-defining.
>
>At first glance this suggestion seems to be an abandonment of the Darwinian
>goal of a taxonomic system which accurately reflects the genealogical
>history of life.
>
>At second glance, it does no such thing. Botanists like Zander are aware
>that a very large proportion of plants have hybrid origins, and they have no
>reason to think that hybridisation hasn't been going on for a very long
>time. Even if hybridisation was less frequent among the ancestors of today's
>plants than it was in recent times, the multitude of hybrid ancestors that
>must have existed should encourage botanists to throw strictly dichotomous
>trees on the No Longer Useful heap. The genealogical history of plants is a
>network, and "reconciling" incongruent gene trees into a dichotomous
>structure is not unlike a Ptolemian trying to "save the circles" to keep the
>Earth at the centre of the Universe. It perpetuates a falsehood.
>
>Defining genera as historical entities makes just as little sense. The
>reality is that depending on which gene tree you look at, a plant species
>could logically belong to several historical genera at one and the same
>time. Genera also need diagnosing.
>
>Microbiologists are very comfortable with this approach to taxonomy, even if
>many botanists are still not. Most zoologists don't want to think this way
>at all, even though they would admit they come from a hybrid ancestor, i.e.
>the first eukaryotic cell. They prefer to think that animal evolution has
>been largely dichotomous since the earliest metazoans appeared, and if gene
>exchange happens at all, it occurs mainly during slow sympatric or
>parapatric speciation, and less frequently afterwards, as reproductive
>isolation gradually becomes a no-exception policy. (I'm thinking here of
>speciation not driven by chromosomal rearrangements. These isolate animal
>lineages immediately.)
>
>However, I wonder how many zoologists actually test their molecular
>phylogenies for evidence of hybridisation or introgression, as botanists do
>more and more? If you don't look for signs of hybridisation you won't find
>it, and a successful hybrid species looks just like a (excuse me!) real one.
>
>Getting back to seeing species as diagnosable entities, that seems an
>eminently sensible approach to dealing with a Network of Life. Arguments
>about adhering to strict monophyly vs. allowing some paraphyly could be
>restricted to those bits of the Network where they have some logical
>relevance. (I'm thinking here of higher taxonomic divisions, although I read
>that monophyly has run into serious trouble at the Domain level in recent
>years.)
>
>As for the argument that phylogenetic software doesn't (yet) deal adequately
>with reticulate evolution, so until it does we should continue to believe
>that animal evolution is strictly dichotomous... Hmmmm. Hey, Procrustes,
>maybe you should learn more about your guests before settling them down in
>that bed of yours?
>---
>Dr Robert Mesibov
>Honorary Research Associate, Queen Victoria Museum and Art Gallery
>and School of Zoology, University of Tasmania
>Home contact: PO Box 101, Penguin, Tasmania, Australia 7316
>(03) 64371195; 61 3 64371195
>
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>http://www.utas.edu.au/spatial/locations/index.html
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>http://biodiversitysalvage.blogspot.com/
>---
>
>
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