[Taxacom] barcode of life

Steve Manning sdmanning at asub.edu
Mon Jul 19 11:10:46 CDT 2010


Hi Rich,

Don't worry about the lengths of your posts - we can read as much or 
as little as we want to or have time to.

I wonder if an elephant in the room here is that taxonomists tend to 
anticipate the flak they are likely to take if they synonymize two 
morphological species that are well-known, at least to other 
taxonomists.  Just a matter of normal human psychology.  Might this 
explain why we don't see more synonymizing when two allopatric or 
parapatric species are shown to hybridize when given the chance?  For 
example, I have not heard of any attempts to synonymize dogs and 
coyotes (forgive me if I am wrong) and I know this was one factor (I 
like to think not the deciding factor) when I briefly considered 
synonymizing the Chinese and American Catalpa species when evidence 
was published that they were spontaneoulsly and unintentionally 
producing hybrids in a nursery where both were kept for sale as 
ornamentals/shade trees, even though in nature they are clearly 
morphologically distinguishable as well as geographically isolated 
from each other.

Best,
Steve

At 05:18 PM 7/4/2010, Richard Pyle wrote:
>Hi Stephen,
>
> > It is an in principle
> > refutable hypothesis that they are distinct species.
>
>In some cases, I would agree.  Those are cases where the two forms are
>sympatric, but show no evidence of hybridization; or cases where
>occasionally strays from one population find their way into the other
>population, and no evidence of hybridization occurs.
>
>But because the BSC pivots on the ability to produce fertile hybrids *in
>nature*, in the vast majority of cases (i.e., allopatric sister-species or
>sister-subspecies or whatever you want to call them), it's not really a
>refutable hypothesis until a natural event allows us to refute it.  We can
>certainly test (in many cases) whether viable hybrids can be produced when
>individuals from the two populations are artificially given the opportunity
>to do so, but that's not really how the BSC is defined.
>
>Also, even when they do have the opportunity to reproduce in nature, there
>may be a spectrum of circumstances ranging from totally uninhibited
>reproduction, to highly assortative/disproportionate mating, whereby viable
>offspring from the two different forms/populations result from only a very
>small percentage of the opportunities to do so.  And in the case of rare
>vagrants from one population to another, how rare does it need to be to
>cross the threshold from "gene flow between two populations of the same
>species" to "rare hybridization event"?
>
>Following the lead of Curtis, I'll illustrate with a couple of examples from
>the genus I worked on for my PhD.
>
>Centropyge ferrugata Randall & Burgess in Burgess & Axelrod 1972 is
>restricted to a small region in the western Pacific, from southern Japan to
>the northern Philippines, and a population in the Ogasawara Islands. It
>looks like this:
>http://nsdb.bishopmuseum.org/popup.asp?id=-1075464259&pg=i
>
>Centropyge shepardi Randall & Yasuda 1979 is restricted in distribution to
>the Mariana and Ogasawara Islands. It looks like this:
>http://nsdb.bishopmuseum.org/popup.asp?id=-906582009&pg=i
>
>These two species have only been known for a few decades, yet their status
>as distinct species has already been questioned. Several people have
>informally suggested that they should be regarded as the same species, based
>on color/morphology alone.  A genetic investigation currently in progress
>found identical haplotypes between the two forms.  It would be extremely
>tempting to treat them as different forms of the same species.  Except for
>one thing:  they both occur in the Ogasawara Islands, and despite two field
>trips there, and many dives searching specifically for hybrids, none were
>found.  You could often find harems of both species within a meter of each
>other, so *clearly* they have opportunity to hybridize. Yet, without
>exception, the harems are composed of one form or the other, without any
>apparent inter-mixing or hybridization.
>
>Now, another example in the same genus:
>
>Centropyge flavissima (Cuvier in Cuvier & Valenciennes 1831) is widespread
>throughout the tropical central Pacific, and looks like this:
>http://nsdb.bishopmuseum.org/popup.asp?id=1306009233&pg=i
>
>Centropyge vroliki (Bleeker 1853) is widespread throughout the tropical
>western Pacific, and looks like this:
>http://nsdb.bishopmuseum.org/popup.asp?id=388967542&pg=i
>
>The two species are parapatric, and within the vast majority of their
>respective ranges, their coloration is extremely consistent.
>
>However, in a few localities, along the border between their respective
>ranges, the freely form hybrids:
>http://nsdb.bishopmuseum.org/popup.asp?id=1922645980&pg=i
>http://nsdb.bishopmuseum.org/popup.asp?id=1690520359&pg=i
>
>In fact, in these specific areas, they form hybrid swarms -- where there is
>a complete spectrum of forms from one pure parent to the other. Indeed,
>"pure" parents are very hard to find.
>
>So, by anyone's metric, there are effectively no biological reproductive
>barriers between these two species.  When given the opportunity to
>cross-breed in nature, they do so seemingly without restraint.  But for the
>vast, vast, vast majority of the individuals of each species, there *is* a
>*geographic* barrier to reproduction.  That is, looking at both species in
>full, only a tiny, tiny percentage of individuals of either species
>participate in hybridization episodes.  Moreover, both species have been
>known since the early/mid 19th century, yet nobody, ever, in that 150-year+
>history of taxonomists, has ever suggested that the two species should be
>synonymies.
>
>Would it best serve the communicative needs of biologists to regard them as
>the same species?
>
>So here's my point:  When we have direct information that, when given the
>opportunity to interbreed in nature, two closely-related forms fail to do
>so; then we have good reason to regard them as distinct species, no matter
>how similar they are morphologically or genetically.  This is the strength,
>in my opinion, of the BSC as a guide to defining species boundaries.
>
>However, the converse is not necessarily true (and, I suspect, this is where
>we may disagree).  That is, there are some cases (e.g., C. flavissima and C.
>vroliki) where, despite a demonstrated ability to produce viable offspring
>in nature, there is still good reason to regard them as distinct species.
>You might want to maintain that these *are* the same species, according to
>the BSC.  And I would have a hard time refuting that.  But I can also tell
>you that if I published a paper declaring C. flavissima and C. vroliki to be
>the same species, *NOBODY* in the ichthyological community would follow my
>lead. Why? Because the communicative needs of biologists are best served by
>regarding the two forms as distinct species.
>
>Now, if it were the case that the two forms (C. flavissima and C. vroliki)
>were broadly overlapping, with hybrid swarms representing more than a tiny
>fraction of the total set of populations of both species; or if there was a
>continuum of variation from one end of the spectrum to the other, then it
>would make much more sense that the communicative needs of biologists would
>be better met by regarding them as the same species, and perhaps as
>different subspecies.
>
>One final thing:  The point has been raised several times in the past (in
>print, at least -- if not on Taxacom), that the question of whether or not
>two populations represent different species is separate from the question of
>whether or not those populations should be labeled with different species
>epithets.  I *think* I understand where this sentiment is coming from, but I
>don't think I buy it.  I'm not aware of any case where biologists have said
>"these are clearly the same species, but we'll continue to call them by
>separate species names"; or "these are clearly distinct species, but we
>think that people should continue calling them by the same name".  Much more
>often we see the softer: "our evidence clearly suggests that these should be
>treated as distinct species, but we'll leave it to someone else to change
>the name".  In other words, while one may be able to make a philosophical
>argument that whether they *are* distinct species is separate from whether
>they should be *named* as different species; in practical terms, I think
>there is no meaningful distinction.
>
>But in this context, perhaps one's perspective on how lines should be drawn
>between species depends on whether one is looking at the issue through the
>eyes of a nomenclaturist, or through the eyes of an evolutionary biologist.
>
>Aloha,
>Rich
>
>P.S. Apologies for the length of this post, which will (no-doubt) reinforce
>my reputation as a voluminous poster to this -- despite my best efforts in
>recent years to establish a track record to the contrary.
>
>
>
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