Real species and ideology

[Richard Pyle]

> They differ in the process that gives rise to them. In the first case,
> there was never a full cessation of gene flow. In the second, it stopped,
> and then started.

How would you define "stopped" in this case?  How many generations?  There
have probably been more than a few generations since genes flowed between my
ancestors and your ancestors.  Would that be enough of a gap to constitute
"full cessation of gene flow" between the lineage composed of your
ancestors, and the lineage composed of mine?  How many more generations
would need to pass among the future descendants of your lineage, and the
future descendants of my lineage, before we can officially declare a full
cessation?

Actually, humans are a bad example to use (for several reasons).  But I hope
you get the gist of my point.  When you imagine an evolutionary lineage
"split" in the unambiguous sense of a fork in a river, it's easy for me to
understand the basis of your point (in this case, was there an island in the
middle of the river, or was it just a shallow shoal?)  But I think the river
analogy is not a fair representation.  I think a better one would be to
imagine a long and stretched out delta, with a complex network of
intertwining flows.  In some places, the patches of land among certain
sections of the delta get gradually larger and larger, and the tendrils
joining two sections of the delta get fewer and thinner.  In some cases, the
tendrils eventually disappear completely, and we have two seemingly
disconnected delta networks.  At what point did the original delta become
two?  For what distance must the two deltas remain unconnected before a
stream connecting them is seen as an anomalous bridge, rather than an
indication that the two deltas are still the same, except with an unusually
large dry island within it?

> Of course one could imagine a continuum between the end
> cases, but the end cases surely exist. And just because we don't currently
> have the methodology to measure or characterize something, that doesn't
> mean that it isn't improtant.

I agree -- as with species, I see no problem in artificially/arbitrarily
defining some metric (proportional gene flow?) that stands as a *defined*
division between what we should call a cline within one evolutionary unit
(imperfect gene flow), vs. an anomalous exchange of information between two
separate evolutionary units (hybridization).  Indeed, whatever this
arbitrary point is, it should also serve as the definition of "species".
And then we can go out and gather data on populations and patterns of gene
flow, and confidently assign each case to one side or the other of the
arbitrary demarcation point.  But this is entirely different from suggesting
that there is some intrinsic property of the evolutionary process that
defines this demarcation point in a "natural" (beyond the scope of human
definition) way.

> That's a road best not taken. Something is "real" if scientists can study
> it from different angles and get consistent results. The philosophical
> issues are interesting in their own right (my degree does say *Ph*D, after
> all), but I don't see that they directly affect what we do as scientists.

I think we've all been too distracted on the semantics of the word "real"
(myself included).  What I'm saying is that we (scientists/taxonomists)
don't seem to be any closer to discovering some intrinsic aspect of patterns
of gene flow that identify a self-evident distinction between "mini-clades"
(e.g., my family lineage vs. my wife's lineage) that we would think of as
intra-specific variation; and "least inclusive monophyletic units" (or
whatever you want to call them) that we would think of as distinct species.

In retrospect, I think this thread would have been more focused (at least in
terms of what I had hoped to get out of it), if I had used the words
"natural" vs. "artificial"; rather than "real" vs. "artificial".  A
convenient definition of "natural" in this case would be "exists
independently of the existence of humans", or something like that.  So...do
boundaries between species exist intrinsically in nature, independent of the
existence of humans?  Or do we all agree that, ultimately, "a species is
what a taxonomist or community of taxonomists says it is"?

After writing the above, I scanned down the rest of your messages, and
found:

> That's the sense in which I use the word "natural" (since "real" has so
> much baggage :-).

I guess great minds do think alike!!!

So, using the word "natural" in place of "real", do you see any difference
between your perspective, and the one that I have attempted to describe?

> Clades are "real". Subspecies can't be clades; if they were, they'd be
species.

Ughhh!!!!  O.K., how, then, do you define the word "clade"???  Is there
something in the definition that involves total and permanent cessation of
gene flow between ALL future descendants of two different groups of
organisms?  If so, can a single fertile hybrid individual -- who manages to
reproduce with individuals from one or both of the 'parent' lineages,
thereby providing a single instance of gene flow between the two lineages --
"break" the clades, and force us to consider them as a single clade?  If so,
then....yikes!  If not, then what about two hybrids?  A hundred? What about
1% of the combined populations of both parent lineages?  Or maybe 1% per
generation?  Where is the "natural" demarcation that allows us to determine
when one clade becomes two?

So....back to the question:  When do we stop calling it "gene flow", and
start calling it "hybridization"?

Also, what is a "subspecies" by your definition?  Do (should) they exist at
all? Is there any communicative value to be had by assigning the rank of
"subspecies"?

I have suggested that "subspecies" should be used in cases where one clade
is in the process of dividing into two, and we don't have enough confidence
that two clades will, indeed, emerge many generations hence (a.k.a., "a
waffle in ambiguous situations.").  Does that seem reasonable from your "if
they were clades, they'd be species" perspective?

Aloha,
Rich

=======================================================
Richard L. Pyle, PhD
Ichthyology, Bishop Museum
1525 Bernice St., Honolulu, HI 96817
Ph: (808)848-4115, Fax: (808)847-8252
email: deepreef at bishopmuseum.org
http://www.bishopmuseum.org/bishop/HBS/pylerichard.html