[Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)

[Stephen Thorpe]

Hi Richard,
>Given that the resolution of strong disagreements is extraordinarily valuable in science
The "pot of gold at the end of the rainbow", perhaps?!
>Using reproductive compatibility as a criterion for species delimitation is okay only when it is practical, such as with birds and other big blundering beasts
Well, perhaps also with small blundering bugs! Explains the entomological fixation on genitalia. I can't really speak for plants, but I suspect that a proper understanding of BSC would in fact hold up.
>As far as I'm concerned, big changes in the genome that do not affect expressed traits are not important in classification, even if they interfer with reproductive compatibility
Then, what sense do you make of cryptic species? Particularly sympatric cryptic species, if you consider such a thing to be even a possibility?
Also, depends what you mean by classification. I don't count species-level taxonomy as classification per se. Classification is what we do to the species once we have them.
I would agree that big genomic changes that do not affect expressed traits are not important in classification or species circumscription, PROVIDED that they do NOT interfere (significantly) with reproductive compatibility
Cheers,
Stephen

________________________________
From: Richard Zander [Richard.Zander at mobot.org]
Sent: Wednesday, 16 September 2009 5:53 a.m.
To: Stephen Thorpe; Edwards, James; taxacom at mailman.nhm.ku.edu
Subject: RE: [Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)

Given that the resolution of strong disagreements is extraordinarily valuable in science, why don't I contribute my guerrilla bit?

Stephen Thorpe says "Note that high levels of morphological dissimilarity, particularly of genitalia, are EVIDENCE for, but not DEFINITIVE of species boundaries according to BSC. Similarly, high levels of GENETIC dissimilarity are also evidence of species boundaries according to BSC. Only high levels of reproductive incompatibility are DEFINITIVE of species boundaries according to BSC."

I think "genetic" is too vague. The genome can change vastly but if stabilizing selection clamps down on those genes that affect evolution as expressed traits, such as when a species is in expressed-trait-stasis for hundreds of thousands of years, then the only value of what genetic change is tolerated is in tracking lineage continuity, which we do in molecular cladograms. I think what we (each of us) mean by evolution is critical. As far as I'm concerned, big changes in the genome that do not affect expressed traits are not important in classification, even if they interfer with reproductive compatibility. Using reproductive compatibility as a criterion for species delimitation is okay only when it is practical, such as with birds and other big blundering beasts.

Remember the fuss when Bradshaw and others demonstrated large populations of plants that had no barriers to gene exchange but exhibited rather stable morphological differences based on genetic differences among parts of the population separated merely distance?

R.
_______________________
Richard H. Zander
Missouri Botanical Garden
PO Box 299
St. Louis, MO 63166 U.S.A.
richard.zander at mobot.org<mailto:richard.zander at mobot.org>


________________________________
From: taxacom-bounces at mailman.nhm.ku.edu on behalf of Stephen Thorpe
Sent: Sun 9/13/2009 6:26 PM
To: Edwards, James; taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)


Jim,

At the risk of being accused of further "ad hominem" attacks against you, I simply don't think that you have a proper understanding of the Biological Species Concept! This is somewhat ironic for the executive director of EoL, don't you think?! :)

The Biological Species Concept (BSC) says that, for sexually reproducing organisms, species boundaries (between populations) are defined by high levels of reproductive incompatibility.

By contrast, a morphological species concept (MSC) would say that species boundaries are defined by high levels of morphological dissimilarity.

Note that high levels of morphological dissimilarity, particularly of genitalia, are EVIDENCE for, but not DEFINITIVE of species boundaries according to BSC. Similarly, high levels of GENETIC dissimilarity are also evidence of species boundaries according to BSC. Only high levels of reproductive incompatibility are DEFINITIVE of species boundaries according to BSC.

Now, you claim that the only significance of morphological dissimilarity of genitalia, is that it may provide a "good character" on which to base species distinctions, but that any morphological dissimilarity in any other body part might be equally "good"

In cases where there is a high level of morphological dissimilarity in genitalia, the BSC and the MSC both give the same answer (distinct species), all other things being equal*. Not so when there is near identity of genitalia, but high levels of morphological dissimilarity in other body parts. The MSC has no choice under such circumstances but to give these other dissimilarities equal weight to genitalic ones, and thereby deem the species to be distinct on grounds of high levels of morphological dissimilarity! The BSC can go either way, depending on additional evidence for the level of reproductive incompatibility. Perhaps, there is no reproductive incompatibility between two given allopatric populations with high levels of (non-genitalic) morphological dissimilarity! If so, the BSC says they are the same species, but the MSC says that they are distinct species! My understanding of species in biology allows for the possibility of two allopatric populations of the SAME SPECIE
 S to diverge morphologically to a high degree in genitalic and/or non-genitalic characters. The MSC does not. This is, in fact, the very reason why, as YOU SAY, differences in genitalia don't prove reproductive isolation. This isn't a weakness of BSC, but rather its strength! Using BSC, high levels of morphological dissimilarity (in genitalia or other) can be overthrown by facts about reproductive compatibility, but not so for MSC, according to which the morphological facts are DEFINITIVE of species boundaries!
*Again, BSC is sensitive to other (non-morphological) evidence for the species not being distinct, after all. MSC is not, because the morphological evidence is DEFINITIVE! Again, my understanding of species in biology allows for the possibility of two allopatric populations of the SAME SPECIES to diverge morphologically to a high degree in genitalic (and other) characters, if there is other (non-morphological) evidence for reproductive compatibility.

4 quick examples:

(1) Michael Heads' duck: there is a highish level of non-genitalic morphological dissimilarity between mallards and grey ducks - sufficiently high that we are pretty confident that they are distinct species, and high enough for the MSC to deem them to be distinct. Does the BSC force us to say that they are the same species, just because the grey duck is being hybridised out of existence by the mallard? Answer: no! It looks like it does, but only if you make the mistake of taking the fact that the grey duck is being hybridised out of existence by the mallard to indicate very low levels (zero, in fact) of reproductive incompatibility. In fact, we haven't a clue from the example what the levels of reproductive incompatibility are, except that they are not 100% What matters is something like the proportion of mating events that lead to fertile offspring, and this could be very low (=very high levels of reproductive incompatibility), for all we know!

(2) Richard Pyle's Centropyge: some details are unclear, but the jist seems to be that there are high enough levels of (non-genitalic) morphological dissimilarity between largely allopatric populations to make most taxonomists consider them to be distinct species, but a significant amount of hybridisation in a small overlap zone - enough hybridisation to seemingly force the BSC to declare same species. In this case, it appears, the BSC fails precisely because of its sensitivity to non-morphological data! So, I need to show that either BSC can allow distinct species here, or else that it is just plain wrong to think that they are distinct. I think the first option is best. One possibility is that we really don't know what the levels of reproductive incompatibility are, as with the duck. The proportion of hybrids is just as much a function of time and other factors as it is a function of reproductive incompatibility.

(3) Beverley Holloway's (2007) description of Geodorcus sororum as a new species distinct from G. capito: The Geodorcus on the tiny Sisters Islands in the Chathams are the most distinctive looking in the whole genus, and very different looking to G. capito in the rest of the Chathams. There is huge non-genitalic morphological dissimilarity, but no genitalic morphological dissimilarity, and apparently even no significant genetic dissimilarity! She justifies her erection of the new species on BSC grounds. Clearly, on the face of it, it seems to contradict BSC and require MSC. Her logic is that a non-genitalic morphological dissimilarity of such immense magnitude is in itself strong prima facie evidence for there being high levels of reproductive incompatibility, even though the "usual" evidence (genitalia, genes) is lacking. She considers that the onus of proof lies with anybody who disagrees with her to come up with convincing positive evidence of reproductive compatibility.

(4) cryptic species: these have identical morphology, but high levels of genetic dissimilarity, interpreted as strong evidence for high levels of reproductive incompatibility. Hence, cryptic species are consistent with BSC, but not with MSC.

Cheers,

Stephen

________________________________________
From: Edwards, James [EDWARDSJL at si.edu]
Sent: Monday, 14 September 2009 5:52 a.m.
To: Stephen Thorpe; taxacom at mailman.nhm.ku.edu
Subject: RE: [Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)

Stephen -

Ad hominem attacks are not the best way to get your points across. In my "not perfectly logical (=rational)" way, I'll respond to a couple of your contentions.

>The biological species concept is a great theoretical construct, but it has very little relevance, I believe, to what taxonomists do in their day-to-day work
(You replied) I disagree! There are countless examples like the one I previously gave from Holloway (2007), suggesting that the true species boundaries are determined by facts of reproductive integrity, and justifying postulated boundaries in terms of the BSC. I suggest that if it is not made as explicit as in this example, it is implicit in what taxonomists do in their day-to-day work, and if they are not doing that, then I'm not sure what they are doing!

Edwards' comment:  The taxonomists I know are focused on identifying good characters for distinguishing species. If they invoke the Biological Species Concept, it is after the fact.

>we taxonomists are agonizing over where we think species-level differences fall within the diversity we see
(You replied) this statement is devoid of any meaning!

Edwards' comment:  This statement was in response to your contention that taxonomists "agonise over evidence for or against reproductive isolation". As I have said before, in my opinion taxonomists agonize over where species boundaries lie. We use characters that "work", and do not necessarily infer a direct connection between them and reproductive isolation. Sure, insect genitalia appear to be good characters that *may (or may not) be* good species-level characters. That's why taxonomists, even those who do not invoke the BSC, use them. So what? We also use wing venation and a host of other characters, too, without making any explicit reference to the BSC. The BSC is invoked after the fact, in an attempt to explain the character distribution we have already inferred from the specimens examined, i.e. after we have already agonized over where the species boundaries lie.

            --  Jim

Dr. James L. Edwards
Executive Director
Encyclopedia of Life
National Museum of Natural History
Smithsonian Institution
P.O. Box 37012, MRC 106
Washington, DC 20013-7012

Phone:  1 202 633 8730
Mobile:  1 571 230 4098
Fax:      1 202 633 8742
Email:   edwardsjl at si.edu

-----Original Message-----
From: Stephen Thorpe [mailto:s.thorpe at auckland.ac.nz]
Sent: Saturday, September 12, 2009 9:24 PM
To: Edwards, James; taxacom at mailman.nhm.ku.edu
Subject: RE: [Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)

Hi Jim,

I fully agree with your premises, but fully disgree with the conclusions that you draw from them! Dare I suggest that you are not a perfectly logical (=rational) being? Well, nobody is, of course, but the "real" measure of a "man's" (=human's) rationality is how willingly they engage in rational debate, so you can still "redeem yourself" by reading, thinking about, and then replying to what I am about to say! Sorry, I'm in a slightly facetious mood! :)

> differing genitalia per se are not proof of reproductive isolation. You are *inferring* such isolation
I couldn't have said it better myself! Yes! Absolutely! You have stated MY position with the utmost (almost) clarity! Differing genitalia per se are EVIDENCE for reproductive isolation. Reproductive isolation is a real thing in the world (given a chosen threshold - see the analogy with statistics?) that we pay attention to when we do SPECIES LEVEL taxonomy. Not so for genera, etc.

>But until you have done the experiments to show that hybridization does not occur between two groups of organisms with different genitalia, you have not discovered any boundaries due to reproductive isolation
This statement is all over the place! My view is that we don't know WITH CERTAINTY where the species boundaries are, but we accumulate EVIDENCE, such as very different genital morphology!

>, you have used the morphological species concept to say that you think that these morphological differences are so great that you believe that the species could not/do not interbreed
No, if you were to use a MSC (as opposed to BSC!), then morphological differences in genitalia would be no more or less significant than differences in colour or any other morphological character. So, ceteris paribus,  two individuals differing hugely in colour but with identical genitalia would be no less distinct species than two of identical colour but with hugely different genitalia!

>The biological species concept is a great theoretical construct, but it has very little relevance, I believe, to what taxonomists do in their day-to-day work
I disagree! There are countless examples like the one I previously gave from Holloway (2007), suggesting that the true species boundaries are determined by facts of reproductive integrity, and justifying postulated boundaries in terms of the BSC. I suggest that if it is not made as explicit as in this example, it is implicit in what taxonomists do in their day-to-day work, and if they are not doing that, then I'm not sure what they are doing!

>we taxonomists are agonizing over where we think species-level differences fall within the diversity we see
this statement is devoid of any meaning!

>However, until and unless we have done hybridization experiments, we cannot truly assert whether or how reproductive isolation might be involved in these differences
it is unclear what you mean here: note that a MSC taxonomist has no reason to do hybridization experiments - it would be irrelevant! If you are just saying that hybridization experiments are stronger evidence for reproductive integrity than genitalic differences, then I can agree with that, no problem!

Cheers,

Stephen

________________________________________
From: Edwards, James [EDWARDSJL at si.edu]
Sent: Sunday, 13 September 2009 4:53 a.m.
To: Stephen Thorpe; taxacom at mailman.nhm.ku.edu
Subject: RE: [Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)

But differing genitalia per se are not proof of reproductive isolation. You are *inferring* such isolation. Because genitalia are so closely involved with reproduction, you and most other insect taxonomists believe that they are good indicators of species boundaries. But until you have done the experiments to show that hybridization does not occur between two groups of organisms with different genitalia, you have not discovered any boundaries due to reproductive isolation. You have inferred that these boundaries exist, but you have not really shown that reproductive isolation is the cause. Instead, you have used the morphological species concept to say that you think that these morphological differences are so great that you believe that the species could not/do not interbreed. I do not find that any more "real" or non-subjective than Rich Pyle apparently does.

The biological species concept is a great theoretical construct, but it has very little relevance, I believe, to what taxonomists do in their day-to-day work. I know of very few taxonomists who "agonise over evidence for or against reproductive isolation". Rather, we taxonomists are agonizing over where we think species-level differences fall within the diversity we see. Some of us then use the BSC to justify those decisions that we have made on morphological or genetic or other grounds. However, until and unless we have done hybridization experiments, we cannot truly assert whether or how reproductive isolation might be involved in these differences.

            --  Jim

Dr. James L. Edwards
Executive Director
Encyclopedia of Life
National Museum of Natural History
Smithsonian Institution
P.O. Box 37012, MRC 106
Washington, DC 20013-7012

Phone:  1 202 633 8730
Mobile:  1 571 230 4098
Fax:      1 202 633 8742
Email:   edwardsjl at si.edu
-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Stephen Thorpe
Sent: Saturday, September 12, 2009 3:27 AM
To: taxacom at mailman.nhm.ku.edu
Subject: [Taxacom] FW: The 'reality' of species boundaries -- Once Again (UGHHH!)

FYI: outcome of debate - stalemate! (see below)
I do think though that Richard's view fails to make sense of what a lot of taxonomists are doing when they agonise over evidence for or against reproductive isolation. Why bother dissecting genitalia?
________________________________________
From: Richard Pyle [deepreef at bishopmuseum.org]
Sent: Saturday, 12 September 2009 7:07 p.m.
To: Stephen Thorpe
Subject: RE: [Taxacom] The 'reality' of species boundaries -- Once Again (UGHHH!)

Thank you for that.  I now feel I understand where you are coming from on
this (much better than before), and that we are at the point where we simply
must agree to disagree.  In your world view, based on your experience with
nature, you have confidence that there are natural barriers between species
groups.  My world view, based on my experience with nature, leads me to a
different conclusion.  Neither of us can be "proven" right or wrong -- we
just have different perceptions of the natural world.  I doubt that anything
else I say would change your mind, and I doubt that anything else you say
would change my mind. Not because we are both stubborn, but because we've
thoroughly explained our respective positions to each other, and we seem to
both understand where each other is coming from, but we've both failed to
provide sufficient evidence to compel the other to change their world-view
on this issue.

I can think of worse outcomes than that!

Thanks for the exchange.

Aloha,
Rich

> -----Original Message-----
> From: Stephen Thorpe [mailto:s.thorpe at auckland.ac.nz]
> Sent: Friday, September 11, 2009 7:19 PM
> To: Richard Pyle
> Cc: taxacom at mailman.nhm.ku.edu; Gurcharan Singh
> Subject: RE: [Taxacom] The 'reality' of species boundaries --
> Once Again (UGHHH!)
>
> Richard,
> >Where is the analogy in that?
> I may have misinterpreted you, but you seemed to be
> suggesting that because we need to choose a threshold of
> reproductive isolation (90%, 95%, etc.) from a continuum of
> possibilities, in order to define species boundaries
> according to the BSC,  the species boundaries themselves were
> therefore purely subjective. The analogy was that you do in
> fact have to choose a significance level for a statistical
> test from a continuum of possibilities, but the test itself
> is not thereby rendered purely subjective.
>
> >just explain to me how we determine if there is sufficient
> >hybridization (with as much implied fuzziness and imprecision as you
> >feel necessary) to decide whether we are seeing two separate species
> >with rare hybridization, or one single species with
> imperfect gene flow
> OK, I will: here we go: choose whatever threshold you like
> (!), but better make it consistent with uncontroversial
> cases. Having chosen it, the species boundaries are now
> determined by the world, not by us! We must now look to the
> world to see what they are. We look for evidence of
> mechanisms that would result in X % reproductive isolation.
> If we had chosen differently, then the species boundaries
> would be different in some cases, but probably very few, as
> most cases probably have virtually 100% reproductive
> isolation, which is why chimp and human aren't the same
> species by any "definition". So there is some leeway, but not
> much. This is how it is done IN THEORY. IN PRACTICE, we don't
> decide on a precise threshold and take it from there.
> Instead, we look for evidence of "very high" levels of
> reproductive isolation, such as very different genitalic
> morphology, etc. What is "very high"? That is fuzzy.
> Nevertheless, we look to the objective world to discover
> species boundaries in a manner that is totally lacking for
> genera or other levels. Species are more like Australia, ...
> Genera are just "convenient monophyletic groups", unless you
> believe in some sort of well-defined objective notion of
> "overall similarity", in which case a particular threshold
> could be chosen and then the genera would also be determined
> by the world. I don't ...
>
> >> but the very practice of biological taxonomy for sexually
> reproducing organisms involves trying to discover the natural
> boundaries of reproductive integrity and calling them species.
> >Yes, I know this is how you see it.  But I, and many other
> taxonomists,
> >see it a different way
> I believe that I have just explained in what sense 'the very
> practice of biological taxonomy for sexually reproducing
> organisms involves trying to discover the natural boundaries
> of reproductive integrity and calling them species'  is
> consistent with your question 'just explain to me how we
> determine if there is sufficient hybridization (with as much
> implied fuzziness and imprecision as you feel necessary) to
> decide whether we are seeing two separate species with rare
> hybridization, or one single species with imperfect gene flow?'
>
> Regarding hybridisation, and with particular reference to the
> most recent post by G. Singh, we must be careful to
> distinguish the case where the hybrid population itself
> becomes reproductively isolated from the parents, in a new
> speciation event...
>
> Stephen
>
> ________________________________________
> From: Richard Pyle [deepreef at bishopmuseum.org]
> Sent: Saturday, 12 September 2009 4:32 p.m.
> To: Stephen Thorpe
> Subject: RE: [Taxacom] The 'reality' of species boundaries --
> Once Again (UGHHH!)
>
> > >If that's your answer to the question, then why did you give
> > me the bogus answer on statistics the first time around?
> > Another "politician-like response" from Dr. Pyle! :) My
> first answer
> > was an analogy, and therefore not "bogus"!
>
> No, it was not an analogy.  Here is what you said:
>
> "so, statistics is all just human subjectivity, is it? Funny,
> I thought the whole idea of statistics was to reveal
> objective facts about the world! But wait ... how could I
> have been so stupid? You have to choose a significance level!
> So, statistics is all just human subjectivity after all ..."
>
> Where is the analogy in that?  It's a bogus answer because
> you are misdirecting (dodging) my specific question.  I asked
> you what proportion of hybridization was acceptable in order
> for there to still be a species boundary.  You replied with
> the nonsense quoted above, which I *think* was an attempt to
> (snidely) imply that I was saying statistics were subjective
> (which clearly this is not what I was suggesting).  It seems
> to me that you are the one acting more like a politician here.
>
> > The fact that there isn't a perfectly precise objective answer to
> > "which level of significance should we use?" in statistics, doesn't
> > make statistics any the less objective. See the analogy?
>
> No, I don't see how this is an analogy.  I already addressed
> the precision issue in my later response.  Don't give me a
> precise answer, just explain to me how we determine if there
> is sufficient hybridization (with as much implied fuzziness
> and imprecision as you feel necessary) to decide whether we
> are seeing two separate species with rare hybridization, or
> one single species with imperfect gene flow.
>
> > >But...what if the hybrid populations continue to persist?
> > >Are they a third species? Or no species at all?
> > That wasn't your original question!
>
> Yes, it was!  I asked what would happen if the two allopatric
> populations disappeared, and all that was left was the hybrid
> populations:
>
> "Now, let's say that the broad allopatric populations all die
> out, leaving only the hybrid populations, such that 100% of
> individuals form hybrids.
> Are they now suddenly the same species, simply because the
> allopatric populations disappeared?"
>
> When I asked "Are they now suddenly the same species" -- the
> "they" was referring to the remaining extant individuals in
> the hybrid zone.  I'm sorry if this was not clear.
>
> > I'm not sure that there
> > is an answer to that, other than just "they are hybrids of extinct
> > species".  I would lean to the third species option, since
> some of the
> > species we recognise today could have been formed as hybrids with
> > subsequent extinction of parents.
>
> OK, that's a fair answer.  I don't agree with it, but at
> least it is an answer to my question.
>
> > Anyway, I think I understand you as arguing that there is a
> complete
> > (or nearly complete) spectrum of interbreeding in the world
> today, so
> > no non-arbitrary place to draw species boundaries.
>
> To be precise, I am saying there is a nearly complete
> spectrum of *examples* of different levels of interbreeding
> in the world at all times (not just today).
>
> > This seems to be based on your own first hand explorations in the
> > field. Am I right?
>
> In part -- but just as much (if not more) from my
> conversations with other taxonomists working with different
> groups of organisms, who also have many years of experience
> observing nature.
>
> > If so, I can do little
> > more than say that my "experience in the field" screams just the
> > opposite!
>
> Fair enough.  Perhaps it is simply a function of our
> different experience.
>
> > I suspect that you have seen a few cases where things are less than
> > clear, and this leads you to make the assertion that there is a
> > complete continuum!
>
> No, that's not my assertion.  My assertion is that if you
> look across *all* groups of sexually-reproducing organisms,
> you will find *many* examples that confound a simply BSC view
> of the world.  If these examples were confined to just one
> kind of organism (like fishes), or if they were extremely
> rare in nature, I would be more confident that a natural
> barrier exists for species.
> But too many taxonomists in too many different groups have
> conveyed to me too many examples of the sort I have provided
> for you, that it seems to me that the "fuzziness" of your
> "Australia" analogy is, on average, much broader than the
> intertidal zone and a few islands here and there.  In that
> context, your analogy of Australia would be more
> representative of typical species if the intertidal zone were
> hundreds of kilometers wide.
>
> > I
> > claim that in something like 95+% cases of sexually reproducing
> > organisms, the levels of interbreeding are well within the
> bounds of
> > what is allowable according to a proper understanding of the BSC.
>
> I'm assuming that you just made up the "95%" figure -- which
> is fine; let's use it.  That means that 5% of cases rely on
> subjective interpretation by a taxonomist.  I would say that
> 1 in 20 species being ambiguous is a non-trivial proportion
> of biodiversity.  But I think you're a bit over confident in
> your estimation.  From a lumper's view of the world, 95% is
> probably about right.  From a splitter's view of the world, I
> bet it would be more like 60%.
>
> > That is why we see lions and
> > tigers, and why ligers and tigons are rare curiosities.
>
> Yes, but across the spectrum of biodiversity, mammals are
> more the exception than the rule.
>
> > It could
> > have been different. Yes, there are a few problem cases, which you
> > seem to see as negating the general rule,
>
> OK, so at least you concede that you're talking about a
> "general rule" in nature -- and on this point we may actually
> come close to agreement.
>
> > but the
> > very practice of biological taxonomy for sexually reproducing
> > organisms involves trying to discover the natural boundaries of
> > reproductive integrity and calling them species.
>
> Yes, I know this is how you see it.  But I, and many other
> taxonomists, see it a different way.  We see evolution as a
> continuous process, starting some
> 4 billion years ago and continuing through an utterly
> unbroken sequence of reproductive events to the set of extant
> biodiversity we see today.  We recognize that the process of
> "speciation" (i.e., the process by which lineages of
> organisms develop reproductive barriers) is highly imperfect,
> and riddled with exceptions to any "rule" that biologists
> have tried to invent.  We don't see the process of assigning
> names to organisms as the discovery of real entities in
> nature, but rather as a highly effective means of
> communication.  In many cases (at least 60%, and perhaps as
> much as 95%), there is very little dissention among
> taxonomists about how best to label clusters of organisms as
> "species".  In the remaining 5%-40% of cases, there is
> disagreement about how best to label biodiversity.
>
> To be sure, some of this disagreement is due to imperfect
> knowledge.  With perfect knowledge, the disagreements would
> be reduced in number.  But even with perfect knowledge,
> evolution is a continuous process in the sense that every
> organism that has ever lived is connected by an unbroken
> sequence of reproductive events.  If you draw a slice through
> evolutionary history at one particular time (like now), then
> that slice will intersect every single stage of the
> evolutionary process.  Your perception is that the proportion
> of cases in that slice that are ambiguous with respect to the
> BSC is small (5% or less).  My perception -- and the
> perception of others who have spent a long time studying the
> taxonomy of different groups of organisms -- is that the
> proportion is not so small.  This, I suspect, leads to our
> different perceptions of the degree of "realness" of species.
>
> > Not so
> > for genera, etc., which are just "convenient monophyletic groups".
> > Interestingly, I don't see Centropyge as being a problem case: from
> > what you say, the levels of hybridisation are low, with nothing to
> > indicate that the level would increase significantly UNDER PRESENT
> > CONDITIONS. Little wonder then, that "everyone" considers
> them to be
> > distinct!
>
> If I had to guess, I would say that something on the order of
> 10% or so of all (combined) individuals of both species are hybrids.
>
> Aloha,
> Rich=

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