deepreef at BISHOPMUSEUM.ORG
Thu Apr 15 22:17:35 CDT 2004
Thanks for the input, Ken -- but this sort of skirts around the edges of
what I'm driving at. A few scattered comments below...
> I believe that the vast majority of speciations are mother
> species giving rise to a daughter species, but this usually takes
> a very long time, and the incipient daughter "semispecies" can
> be absorbed back into the mother species given the right
How would you distinguish the "Mother" from the "Daughter" in this case?
Population size? Relative morphological divergence from ancestral population
(if so, how quantified)? Relative genetic divergence from ancestral
population (again, how quantified)? Relative genetic/morphologic
> It is thus a judgment call whether full speciation
> has occurred, and even then species can still interbreed under
> unusual circumstances.
At what point is it not a judgment call whether full speciation has
> When the incipient daughter "species" is
> separate from the mother species geographically it can be very
> difficult to tell whether it could potentially be absorbed back
> into the mother species. In the case of a circle of subspecies
> (adjacent subspecies interbreed, but subspecies at the ends of
> the closing circle do not), the mother species is held together
> only by the intervening populations which could eventually be
> extirpated (or not).
Whether or not two populations have achieved the status of distinct
"species" (by most definitions) in many cases depends on future events.
> Therefore, I regard species as real, but often not yet very
> discrete. The boundaries can be fuzzy in both space and time in
> those cases that happen to be caught in the act of crossing from
> semispecies to full species. Speciation is rarely speedy (as can
> be with polyploidy). Therefore, whether to regard the daughter
> semispecies as a full species is a judgment call.
Again, at what point is it not a judgement call?
If I read you correctly, you're suggesting that there is a clear "white"
zone (cases where two organisms are unambiguously different species), and a
clear "black" zone (cases where two organisms are unambiguously the same
species), but that where these two zones meet, there is a section of grey
(cases where two organisms may or may not be considered conspecific,
depending on the subjective opinion of a taxonomist, or perhaps depending on
the future destiny of the descendents of the two organisms).
This seems to be a common perception among those who feel that there is some
sort of "realness" to species.
But this trichotomy perspective implies that the white zone is homogenously
white, and the black zone is homogenously black, and only the grey zone is
"fuzzy". Is that a fair representation of evolutionary reality? In other
words, is it fair to regard the part of the white (or black) zone that is
farthest from the grey as essentially identical to the part of the white (or
black) that is immediately adjacent to the gray?
An alternative view would represent the world of living things as one great
big spectrum of progressively darker shades of grey, starting with the white
end representing the two most distantly-related organisms on the planet, and
continuing to the black end representing any given pair of identical twins
or clones. The metric for the level of grey can be something like time
since most recent common ancestor, or overall genetic divergence, or
probability that future generations of descendents will exchange genes --
whatever (that's a whole 'nother debate).
Obviously, the spectrum of grey is not perfectly smooth (i.e., the most
distantly-related living humans are a LOT closer to each other, than either
is to any given Pan or Pongo), and it is this sort of sharp discontinuity
among the extant global biome that makes it easy to think of species as
"real". But just as there are cases of extant populations that fall into
your "fuzzy" grey zone between the black and white zones, every organism
unambiguously within the white (heterospecific) zone is the descendant of a
lineage of information-flow that began in the black (conspecific) zone and
passed through the grey zone. Thus, when you view the spectrum across time,
even the things that seem black and white disjunct today are following a
path of continuous blend from black to ever paler shades.
The only solution that sits well with me is the one that I think you alluded
to in the quoted passage above. That is, the Pornography Metric: "I can't
define it, but I know it when I see it." And that ultimately distills down
to the trusty, "A species is what a taxonomist or community of taxonomists
says it is." Sounds pretty artificial to me.
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
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