[Taxacom] Diagnosing species

Sun Jun 24 03:38:53 CDT 2007

Thank you, Bob Mesibov, for capturing so effectively in one email the core
essence of a fundamental point I have tried again and again to make on this
forum in recent years.  And, for doing so much more effectively than I ever
have.

First, a short soapbox commentary, followed by directly addressing one of
the points/questions in Bob's post.

<Rant>
This history of life on earth seems, for the most part, to represent a
single unbroken continuum spanning some 4 billion years (give or take).  The
resolution of that continuum is individual reproduction events, giving a
granularity of "smoothness" to of something on the order of 10^-7 to 10^-9.
If the timeline were printed at a resoultion of 4000dpi (a high-resolution
scanned image, ten times higher resolution than a typical image in a typical
publication), it would be something like sixteen miles long (give or take a
couple of orders of magnitude).  Everything we now consider to be a
"species" parted company through historical time from its nearest sibling
"species" somewhere along that very high-resolution gradation.  It is that
smoothness of historical transition -- that high resolution -- that utterly
defies attempts to objectivly identify where a "species" begins.

The smoothness of the continuum further increases, and the objective
demarcation of "species" from one another is rendered ever more futile, in
consideration of the patterns of assortative
mating/introgression/hybridization/reticulate evolution that are already
very evident in organisms like plants and corals; and perhaps increasingly
so elsewhere in the Eukaryotic realm.
</Rant>

Which brings me to my direct response to Bob's question regarding these
sorts of patterns in the animal world.

For the past 15 years or so, I have gradually begun to see more and more
anecdotal/cirumstantial evidence within my animal group (coral-reef fishes)
for a prevelance of reticulate patterns of evolution at the "species" level
(i.e., at the level at which modern ichthyologists typically tend to draw
species lines).  These patterns were further illustrated to me during a
month-long cruise across part of the western Pacific, where we saw clear
examples of species "A" in the central Pacific being traditionally
unambiguously different from species "B" in the western Pacific, in fact
being represented by a continuum of forms along intervening areas.  I'm
gradually collecting examples of this sort of thing, and hope to start
looking at it through molecular techiques in time.  I may be totally wrong
about this, but if I'm not wrong, then at least within the realm of
coral-reef fishes, the historical pattern may in many cases look less like
the branches of a maple tree, than the "trunk" of a Banyon tree.

Aloha,
Rich

Richard L. Pyle, PhD
Database Coordinator for Natural Sciences
  and Associate Zoologist in Ichthyology
Department of Natural Sciences, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
Ph: (808)848-4115, Fax: (808)847-8252
email: deepreef at bishopmuseum.org
http://hbs.bishopmuseum.org/staff/pylerichard.html

> -----Original Message-----
> From: taxacom-bounces at mailman.nhm.ku.edu 
> [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Bob Mesibov
> Sent: Saturday, June 23, 2007 6:07 PM
> To: TAXACOM
> Subject: [Taxacom] Diagnosing species
> 
> 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
> 
> Australian Millipedes Checklist
> http://www.qvmag.tas.gov.au/zoology/millipedes/index.html
> Tasmanian Multipedes
> http://www.qvmag.tas.gov.au/zoology/multipedes/mulintro.html
> Spatial data basics for Tasmania
> http://www.utas.edu.au/spatial/locations/index.html
> Biodiversity salvage blog
> http://biodiversitysalvage.blogspot.com/
> ---
> 
> 
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