[Taxacom] The 'reality' of species boundaries -- Once Again (UGHHH!)
s.thorpe at auckland.ac.nz
Fri Sep 11 22:37:45 CDT 2009
>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"! My second answer is the same answer, just not given in the form of an analogy! 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? The fact that there isn't a perfectly precise objective boundary to Australia doesn't make it as artificial as U.S.A. See the analogy?
>Ummm...no. That is not the context
Ummm...well, yes, that IS the context in which I'm having this discussion! But I will play along with your preferred context below:
>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! 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. Different issue, though ...
>But the outcome depends also on abiotic factors. Does the "realness" of existing species boundaries at this point in time depend on *future* abiotic factors such as climate change?
No, not at all. Two populations are distinct species provided that they would maintain sufficiently high levels of reproductive integrity in the future UNDER PRESENT CONDITIONS! If present conditions change, then we can't simply look to the future to see if they are distinct species, but we don't do that anyway - instead we look at things like, for example, genitalic morphology, and take it as EVIDENCE for reproductive isolation or not. Something of an analogy: The "realness" of the ice caps in the present can't be negated by global warming resulting in them melting in future! If conditions change, then species EVOLVE, one can become two, and maybe two can become one, or else two can become extinct leaving a new one of hybrid origin...
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. This seems to be based on your own first hand explorations in the field. Am I right? If so, I can do little more than say that my "experience in the field" screams just the opposite! 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! I disagree! 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. That is why we see lions and tigers, and why ligers and tigons are rare curiosities. As a matter of fact, things that look distinct typically just don't interbreed with each other to a significant extent (unless they are males and females of dimorphic species - but that is why it is dimorphism and not two species!). It could have been different. Yes, there are a few problem cases, which you seem to see as negating the general rule, 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. 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!
From: Richard Pyle [deepreef at bishopmuseum.org]
Sent: Saturday, 12 September 2009 2:30 p.m.
To: Stephen Thorpe
Subject: RE: [Taxacom] The 'reality' of species boundaries -- Once Again (UGHHH!)
> The answer to your question
> is that there isn't a (completely precise) answer to your
> question, but there doesn't have to be one!
If that's your answer to the question, then why did you give me the bogus
answer on statistics the first time around?
> demarcated areas of land like Australia don't have perfectly
> precise boundaries either, but that doesn't stop them from
> being perfectly meaningful and useful features of objective
I'm not asking for perfectly precise. I just want a rough ballpark. If we
had perfect knowledge, how would we decide if we were looking at one
species, or two?
> If I have a motto, it would be "context is
> everything" (not to be taken out of context! :), and the
> overall context here, as I see it, is about bioinformatics
> involving computers (databases) requiring more precision than
> actually exists in biological taxonomy.
Ummm...no. That is not the context. Let's pretend that computers don't
exist. What is the property of organisms (or populations of organisms) that
make them inherently the same species, or inherently different species?
> >No, not the same form as my Centropyge example
> Did I not just say that it WASN'T intended to be of the same
Sorry -- I mis-read your original post.
> If we knew all the relevant biological facts, we could model
> the situation and see in advance what the outcome would be
> given enough time.
But the outcome depends also on abiotic factors. Does the "realness" of
existing species boundaries at this point in time depend on *future* abiotic
factors such as climate change?
> It certainly depends on a complex of
> factors, so there is no simple answer.
I'm assuming that none of these complex of factors are arbitrary, or at the
mercy of human subjectivity -- is that fair to say? I'm not looking for
precision -- fuzziness is fine. I just want to know if this complex set of
factors are all "natural", and independent of human interpretation.
> But either there will
> be something keeping the species apart (except for rare
> hybrids) or not. It might have something to do with inherited
> habitat preferences, and/or genitalic differences leading to
> a low probability of fertilization, and/or ...
But if we have a complete spectrum of examples, ranging from discrete
populations that have near-zero hybridization between them, to overlapping
populations that have near-complete gene-flow between them, then how do we
decide where along that continuum a species boundary exists? I don't mean
"how do we decide based on our limited knowledge". I mean "how do we decide
based on perfect knowledge"?
> >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?
> If that were to happen, the result would be unchanged! If
> they were distinct species before the allopatric populations
> disappeared (=they would never have lost that broad integrity
> had they not died out), then they are still distinct species,
> albeit extinct ones, after they disappear, because had they
> not gone extinct they would have maintained their integrity
> (apart from the few hybrids) ...
But...what if the hybrid populations continue to persist? Are they a third
species? Or no species at all?
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