[Taxacom] The Reality (or not) of Species (again!)`

Neil Bell neil.bell at helsinki.fi
Mon Sep 15 05:50:37 CDT 2008


Richard Pyle wrote:

> I stand by my assertion that the image of the billowing
> smoke is a MUCH closer representation of the "reality" of the 
> contunuum of
> life through the ages than the stick-figure cladograms we see so often.
> Sure, in the broadest/crudest sense, the stick-figures can be 
> characterized
> as "representations of reality". But to those of us coming from the other
> perspective, such a notion is patently, laughably, absurd. Don't get me
> wrong -- cladograms are a GREAT tool for communication, in the same 
> way that
> taxonomic nomenclature is a great tool for communication.  And they're a
> very convenient and intuitive short-hand method for representing 
> hypotheses
> about gross historical patterns of descent.  But the danger, as others 
> have
> already pointed out, is in allowing oneself to believe these stick 
> figures
> represent "reality".
>   

The problem I have with this (I didn't have a problem with most of what 
you said) is that it seems to conflate perspective with a continuum of 
reality/unreality, or of accuracy of representation. It's not that the 
phylogenetic perspective is "broader and cruder" (and so further removed 
from "reality"), but rather that it *really is* the more accurate 
representation of relationships at the level at which it operates. The 
relationships between a group of single exemplars (even if they are 
treated as individuals)  from different families are nearly always 
genuinely hierarchical. It doesn't matter in this  context if some of 
them are members of ambiguously differentiated species complexes; any 
individual you selected from that complex would have the same 
relationship to the exemplars from the other families as any other 
(irrespective of whether you could accurately reconstruct it). To claim 
that this perspective is "cruder" seems to me just a bias of 
perspective. The hierarchical structure is genuine (a product of the 
fact that at *some* level of differentiation there is a permanent 
severing of gene flow between lineages, lateral transfer excepted) and 
you need the tools of phylogenetics to observe it. The fact that the 
relative relationships of these individuals ultimately derive from 
processes that cannot be characterised in the same manner does not make 
them less "real", or justify this statement:


> Yes, but I think that the *point* some of us are making is that the 
> premise
> of the so-called "hypotheses" generated by cladograms may be 
> fundamentally
> flawed.


Of course a phylogenetic study conducted within a single genus in which 
half of the supposed species are hybrids or populations having 
significant gene flow between them will be fundamentally flawed, but the 
error is in the application of the method, not in the method.


> I do have a disagreement with this -- particularly in regards to the 
> use of
> the word "usually".  A lot depends on what you mean by "boundary".  At 
> any
> given snapshot in time, you will certainly find clusters of organisms 
> that
> have much more recent shared ancestry than other clusters; and very often
> (perhaps even "usually"), you will find very few or perhaps no individual
> organisms that fall in the space between the clusters.  So in that sense,
> it's easy to see a boundary.  But in the context of time, if you trace 
> the
> ancestors of those two clusters backward, generation after generation, 
> they
> will eventually meet at the same cluster. And in this sense, I would 
> argue
> that there is NOT usually (I would guess practically never) an 
> unambiguous
> boundary (even if you argue from the perspective of vicariance) between
> species.  

Surely this applies just as clearly to individuals, the only difference 
being the relative lengths of time an entity is distinct or not 
distinct. So the difference is actually just the one that you expressed 
in this way:


> In my world, I see plenty of examples where the fuzziness of one
> speciation event has not yet attenuated while another apparent speciation
> event is already beginning.


I agree that in groups undergoing rapid radiations this will often be 
the case; there are some very clear examples of this in the organisms I 
study. In these cases, perhaps species boundaries must either be 
arbitrary or should be applied at at level far above what most 
morphological taxonomists would be comfortable with. In most other 
groups the rate of speciation must be much less however, otherwise we 
wouldn't observe the discontinuities that we do.


> And I can say that about a group of vertebrates -- which are typically 
> MUCH
> "cleaner" in terms of having what we might perceive as "real" species 
> than
> many/most other groups (e.g., plants).


Interestingly and surprisingly, not according to this study:

Loren H. Rieseberg, Troy E. Wood and Eric J. Baack (2006). The nature of 
plant species. Nature 440: 524-527.


Neil Bell.



Richard Pyle wrote:

> I've broken this out into another thread, so as not to distract from the
> other (very interesting) discussion started by Bob Mesibov.
>
> Although we've had this discussion many, many times on this list 
> before, I
> have some new ideas about how to articulate my own perspective, so I will
> try again.
>
> As with so many things we argue about, a lot of the aspects of this 
> argument
> boil down to perspectives and semantics. 
> I think most of us will agree that, as far as we know, most/all life on
> Earth has a common origin.  Working on that premise, then all living 
> things
> today are connected to each other by a *continuous* and *unbroken* 
> chain of
> reproductive events.  Unless we regard all of Life on Earth as a single
> species (not very practical for purposes of communicating about
> biodiversity), then we must draw boundaries between species at points 
> along
> this *unbroken* continuum of reproductive events.
>
> >From the perspective of someone who is primarily interested elucidating
> phylogenies (which, I presume are intended to be congruous with some 
> sort of
> evolutionary history -- though after reading Bob's earlier post 
> re-launching
> the tracking characters thread, I'm not so sure anymore) -- species are
> "real" entities because surrounding us in nature there are seemingly
> unambiguous "sets" of organisms that cluster very tightly together in 
> their
> characteristics, and we do *NOT* see continuous spectra between these 
> sets.
> For example, we cannot find in nature today a continuous spectrum of
> organisms representing intermediates between humans, chimpanzees, and
> orangutans (Please, John -- I imply no phylogenetic affinities here).  
> The
> "fuzziness" between species, from this perspective, can be thought of
> primarily as a short-live phenomenon that happens only during periods of
> speciation (which are assumed to be very short, relative to periods 
> between
> speciation events). In the example given, this "fuzzy" period lasted only
> during the time that the shared ancestor between humans and chimps (or
> between humans and orangs, or between orangs and chimps) was diverging 
> into
> distinct species, and thereafter the three lineages can be thought of as
> "real" units of nature. From this perspective, it's quite appropriate to
> think of species as "real", which can be authentically represented by 
> nodes
> at the tips of cladograms.
>
> The perspective of someone who operates in the world of alpha taxonomy is
> quite different.  From this perspective, the "non-reality" of species 
> is not
> about comparing humans to chimps (or to orangs, or whales, or palm 
> trees),
> it's about making decisions that seem utterly arbitrary.  People in this
> world are constantly operating in the realm of the "fuzzy" -- dealing 
> with
> boundary effects between "putative" species.  They don't spend their time
> thinking about whether we should treat humans as a different species from
> chimps (...or orangs, or whales, or palm trees....).  They worry about
> things like "how many species of orangutans are there?"  I'm not a 
> primate
> taxonomist, but as I understand it, until recently there was only a 
> single
> species, with two recognized subspecies (one on Borneo, the other on
> Sumatra).  Now (I guess) those are recognized as full species, but some
> maintain that the Bornean species can be further divided into three
> subspecies.  Are there two species of orang? Four? One?  The notion that
> nature has the "correct" answer to this question, and that it's our 
> job as
> taxonomists to continue studying the situation until we "discover" the
> "correct" answer, seems utterly naïve from this perspective.  Rather, 
> from
> this perspective the "correct" answer is the one that best facilitates
> communication among humans.  That is, species boundaries are *defined* 
> by us
> (not "discovered" by us), and as such, are not "real" entities in nature.
> Species are "real" only in the sense that at any given moment of time, 
> the
> derivitives of evolution (i.e., organisms) form a clumpy pattern, rather
> than smooth continuum.  But when you look at it across time, the 
> pattern is
> extremely smooth -- down to the resolution of a single generation 
> against a
> background of billions of years.
>
> So which perspective is correct?  The obvious answer is "both" (or
> "neither", depending on whether you're the glass-half-full type, or the
> glass-half-empty type).  I know what perspective I have, and it 
> reflects the
> nature of the world I live in, and the questions that interest me.
>
> As I tried to express in my previous post, it boils down to the 
> question of
> what proportion of all evolutionary history is characterized by the 
> "fuzzy"
> regions of speciation.  I already said that the "species are real"
> perspective generally assumes that the "fuzzy" zones are proportionally
> small on the grand scale of things.  [The only way you can eliminate the
> "fuzziness" altogether is to maintain that every single speciation event
> begins with an individual offspring that is not conspecific with its
> parent(s).]  But when you talk to people who operate from the other
> perspective, you find that many of them see the "fuzziness" as the 
> dominant
> mode.  In my world, I see plenty of examples where the fuzziness of one
> speciation event has not yet attenuated while another apparent speciation
> event is already beginning.  In such cases, the entire pattern is fuzzy.
> And I can say that about a group of vertebrates -- which are typically 
> MUCH
> "cleaner" in terms of having what we might perceive as "real" species 
> than
> many/most other groups (e.g., plants).
>
> Sorry, Curtis -- I stand by my assertion that the image of the billowing
> smoke is a MUCH closer representation of the "reality" of the 
> contunuum of
> life through the ages than the stick-figure cladograms we see so often.
> Sure, in the broadest/crudest sense, the stick-figures can be 
> characterized
> as "representations of reality". But to those of us coming from the other
> perspective, such a notion is patently, laughably, absurd. Don't get me
> wrong -- cladograms are a GREAT tool for communication, in the same 
> way that
> taxonomic nomenclature is a great tool for communication.  And they're a
> very convenient and intuitive short-hand method for representing 
> hypotheses
> about gross historical patterns of descent.  But the danger, as others 
> have
> already pointed out, is in allowing oneself to believe these stick 
> figures
> represent "reality".
>
> Aloha,
> Rich
>
>
>
> _______________________________________________
> Taxacom mailing list
> Taxacom at mailman.nhm.ku.edu
> http://mailman.nhm.ku.edu/mailman/listinfo/taxacom
>
>
>   


-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Neil E. Bell
Postdoctoral Researcher
(Bryophyte Systematics)

Kasvimuseo
PO Box 7
00014 University of Helsinki
FINLAND

+358 9 191 24463
neil.bell at helsinki.fi
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Neil E. Bell
Postdoctoral Researcher
(Bryophyte Systematics)
 
Kasvimuseo
PO Box 7
00014 University of Helsinki
FINLAND
 
+358 9 191 24463
neil.bell at helsinki.fi
~~~~~~~~~~~~~~~~~~~~~~~~~~~~





More information about the Taxacom mailing list