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

[Stephen Thorpe]

Hi Richard,

I will try not to be offended that you consider it more important to be clear to the list, than you do to me - the very person you were writing the message to! :)

>Obviously (I hope it's obvious), my world view very-much recognizes the reality of reproductive barriers between organisms
Yes, it is obvious to me, anyway!

>My contention to Stephen is that the average "fuzziness" zone (which he acknowledges exists) surrounding many species is broader than he makes it out to be (we do occassionaly see hybrids between wolves and coyotes), to the point where a non-trivial fraction of species boundaries are potentially (and legitimately) contentious, and are at the mercy of human subjectivity (even with perfect knowledge of the biology).
Yes, this is my understanding also of the point of contention. But "fleshing it out" in detail is tricky! Let me try:

Consider two allopatric populations, cut off from each other by a river, and unable to swim or fly. There will be some level of reproductive incompatibility between them, maybe 0%, maybe 100%, or maybe somewhere in between. Whether or not they are different species, on my (BSC) view, depends on which threshold level you choose, and what the actual level is (i.e., whether it is above or below the chosen threshold). So, if the actual level is 100%, then they are different species regardless of which threshold level you choose. This is the "nice neat case", along with actual level=0% (=same species regardless of chosen threshold). In these two cases, the species boundary is not "subjective" in your sense of the word. In the other cases, it is. The reason for this "subjectivity" is that even if we knew exactly what the actual level of reproductive incompatibility was (e.g. 63%), the species boundary would still depend on the "subjective" choice of threshold value. You, Richard, seem now to be agreeing with my view, as just outlined, but claiming a diagreement concerning the proportion of cases which are "in between" the 0 and 100% range - I say few, you say many. It therefore now appears that you have no objection to a BSC view of species boundaries, even though it is in stark contrast to what goes on at generic and other levels! The terms of the debate seem to have shifted somewhat, don't you think???

I think I can agree with you that if you are looking at allopatric populations of very similar morphotypes, the actual level of reproductive incompatibility could be anywhere, and typically may not be near 0 or 100%. So, the species boundary may be very sensitive to the chosen threshold level, which is why there is so much "disgreement" between taxonomists regarding the specific distinction of very similar allopatric populations! So, I'm not at all sure that we disagree about that, after all!!!

What I thought you were saying, and what I do disagree with, is that there is no difference between species and genera (or any other level), i.e., they are all just equally "subjective" human constructs, and the BSC doesn't apply! But, on my view (above), the BSC does apply (to species), genera (and other levels) are very different, and yet I still get the same "subjectivity" as you concerning species boundaries, even though reproductive isolation is on my view the objective criterion for drawing species boundaries, unlike genera, etc!!! So, where are we???

At any rate, there are still problems! Michael Head's duck example still poses a threat to my view. In this case, there could well be 0% reproductive incompatibility between the two populations of Anas, even though most taxonomists would want to continue to call them distinct species! This is a threat to the BSC! I'm not sure I believe my own attempts to "save both species"!

Cheers,

Stephen

________________________________________
From: taxacom-bounces at mailman.nhm.ku.edu [taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of Richard Pyle [deepreef at bishopmuseum.org]
Sent: Sunday, 13 September 2009 7:29 a.m.
To: taxacom at mailman.nhm.ku.edu
Subject: Re: [Taxacom] FW: The 'reality' of species boundaries -- Once Again    (UGHHH!)

Now that Stephen has decided to forward my off-list message to this list, I
feel I should clarify one of my comments that could be misunderstood out of
context.

I said:

> 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.

Obviously (I hope it's obvious), my world view very-much recognizes the
reality of reproductive barriers between organisms.  We don't have many
hybrids between wolves and palm trees; or even between wolves and mountain
lions. The reason we don't see such hybrids is, of course, the existence of
one or more barriers (behavioural, mechanical, biochemical, genetic, etc.)
to gene exchange.  My contention to Stephen is that the average "fuzziness"
zone (which he acknowledges exists) surrounding many species is broader than
he makes it out to be (we do occassionaly see hybrids between wolves and
coyotes), to the point where a non-trivial fraction of species boundaries
are potentially (and legitimately) contentious, and are at the mercy of
human subjectivity (even with perfect knowledge of the biology).

My one wish is that this simple point of clarification does not launch yet
another wave of debate. If it does, it certainly won't be perpetuated by me.

Aloha,
Rich



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