[Taxacom] double-peaked mountains (was: Wilkins on species, again

[Richard Pyle]

Lord knows I shouldn't jump into this fray, 12 hours before heading to the
airport for several weeks in the field.  But...

>         I have to disagree with Kirk here when he says "that the more
relevant
> question is, what are taxa?" I say this is confusing apples and oranges.
Higher
> taxa (genera and above) are human constructs, if they contain more than
> one species.  However, species taxa are special.  They are real, but they
are
> fuzzy, and thus the endless debate about them.

Hmmm.... So the gaps between "species" are somehow more objectively defined
(on average) in nature (i.e., less in the realm of human constructs) than
gaps at higher taxon ranks.  Is that your assertion?  What about the gaps
between "population", "geographic variety", "subspecies", "species group"
and the like -- are they less fuzzy, or more fuzzy, than those between
subgenera, genera, tribes, and the like?  For example, does the fuzziness
continue to diminish directionally as one moves from, say, subgenera towards
population?  Or does the objectiveness of the gap somehow peak in the area
of what we call "species", and then the fuzziness increases in both
directions (towards more inclusive groups in the higher taxon direction, and
more exclusive groups in the population direction)?

> In many cases, it clearly is, because the gap between the species
"American
> robin" and any species closely related to it is great enough to make the
> answer very clear.  It is a REAL, clear cut species.  Many species are
even
> more distinct and less likely to ever hybridize with any related species.

But are there any gaps within the scope of organisms you would call
"American robin"?  Have people done studies on populations structure across
the entire range of the species?  If there is some sort of structure, might
some taxonomists lean in the direction of recognizing more than one
"species"? And, more fundamentally, are examples like the "America robin"
the exception, or the rule?

>        The trouble comes in the minority of species in which the species
is in the
> evolutionary process of splitting.  

What is the basis of your assertion that this represents the "minority"
state among the trajectory of speciation?  You seem to imply that speciation
happens in a punctuated fashion, with relatively rapid and brief speciation
"events", separated by relatively long periods of relative stasis.  If this
were the norm in speciation processes, then I would agree with you that the
"fuzziness" should apply to a minority of species.  But I'm not so sure
that's true.  Indeed, it appears to me that the farther (evolutionarily) one
gets form ourselves (moving from mammals, birds, terrestrial vertebrates,
fishes, and so on down to the realm where the vast, vast, vast majority of
"species" have been asserted), the less punctuated the pattern appears.
Even among our fellow vertebrates the fishes (at least the ones I'm most
familiar with -- which live on coral reefs), the more I travel to different
locations, the less punctuated the pattern appears to be -- even among the
"good" species (analogues to your American robin).  In fact, the "norm" as I
am beginning to see it is that speciation process seems to be one in which
"fuzzy" is the norm.  I see cases where there are "nested" clusters -- such
that a particular "species" might have close relatives that the most ardent
lumpers would collapse, while at the same time there is morphological and
genetic variation across geographic scales within the "species" that the
most ardent splitters would assert as distinct species.  In other words,
before a definitive gap within a pair of species has been finalized, newer,
finer gaps are already starting to appear within the clusters. In other
words, the next rounds of fuzziness seem to already be starting before the
previous rounds have finished.

I wonder how well we really understand such patterns among insects,
nematodes, and the like.

> It is sort of like a double-peaked mountain
> with summits very close together.  Whether it is a single mountain with
two
> peaks (two subspecies if you will) or should be classified as two separate
> mountains, depends on both the history of how they formed (which can be
> investigated) and their future (which can only be the subject of educated
> guesses).  

That may be the pattern you see, but the pattern I see is much more fractal.
Mountains have ridges and peaks, and those ridges an peaks have hills and
valleys, and those hills and valleys have mounds and gullies.... and so on.
>From a certain perspective, it seems easy to say where the lines should be
drawn.  But look a bit closer, and what seemed like sharp lines tend to blur
-- so much so that I think blur is the norm, and sharp is the exception.

>        You can argue about it until the cows come home, but it often
largely
> depends on future events.  If one summit eventually collapses into a
> shoulder of the other, then those arguing that it was just one mountain
claim
> victory, but if the summits continue to separate intact, the two mountain
> adherents can claim victory.  We won't know for sure until such future
events
> occur.

I agree that in many cases, the "correct" answer will only become evident in
the future (for species anyway -- I'm not enough of a geologist to know on
the mountain thing). Maybe that's what people mean by "species are
hypotheses" -- the hypothesis being that the amount of gene flow (and shared
genetic and morphological characters) between the collective set of
descendants of one group and the collective set of descendants of another
group will attenuate over time into the future; with the alternate
prediction being that the genetic pool will ultimately converge over time.
Unfortunately, these are hypotheses we won't be able to test for many
thousands of years (in most cases).

>       HOWEVER, my main point is that we either have one REAL species or
two
> REAL species.  To argue that they are NOT REAL just because they are fuzzy
> (are there two species or one) does not make sense to me.  

I'm fine with this statement (I've given up on the whole "real vs. non-real"
thing).  Where I'm not sure I agree with you is that this rank we refer to
as "species" is an apple to the relative orange of higher (and lower?)
clusters of organisms with shared ancestry and characteristics. I tend to
think of it more as comparing cultivars of apples
(http://en.wikipedia.org/wiki/List_of_apple_cultivars).

Aloha,
Rich