Comments to Cladistics/Eclecticism, part 3

[Curtis Clark]

At 09:21 AM 2/8/02, Susanne Schulmeister wrote:
>The point is: the decisive moment for speciation is
>not when the populations get separated geographically
>­ the decisive moment for speciation is the moment
>these populations develop a reproductive barrier. And
>this is not the same!

The reproductive species concept is no more "cladistic" than the 
morphological, and bad population genetics, as well. What you have 
described is the "test of sympatry", which is an ad hoc method to estimate 
reproductive isolation, and reproductive isolation is an ad hoc and 
imprecise way to measure gene flow, which is the important part. Allopatry 
terminates gene flow as effectively as any other "reproductive barrier", 
and yet it is one of a small number of phenomena that were historically 
rejected by Mayr and other proponents of "biological" (reproductive) species.

>7. A species budding off another species?
>Some of you have argued that a species A from which a
>small population departs and goes “over the mountain”
>and founds a new, morphologically different species,
>remains species A if it is morphologically unchanged.
>You circumscribed this as: “Species A budds off
>species B.”
>This thinking is based on a morphological species
>concept, which sees species as morphologically
>distinguishable entities. As I said above, this
>concept is incompatible with Cladistics and, by the
>way, is completely outdated.

Sorry, your analysis is flawed in two points. First, *my* argument of 
"budding species" is not based on morphology at all, but rather population 
genetics. The genetic effects on species A of a small population forming a 
new species, and the genetic effects of the same population going extinct, 
*are identical*. I find this unsurprising, and it doesn't affect my view 
that clades are the only natural species groups.

Second, nothing precludes apomorphy from being morphological.

>  Here’s something to
>consider: There are cases in which two sister species
>look perfectly alike, which is termed “twin species”
>or “cryptic species”.

Also "sibling species" in English, which can unfortunately cause confusion 
with "sister species". (Sibling species are not constrained by theory to 
always be sister species.)

>Those of you who have argued
>that species A from above remains species A after the
>budding off of species B, simply because it is
>morphologically unchanged, now have to say that T and
>U are both species S.

The key is "genetically unchanged".

>Reproductive isolation completely suffices to give a
>group of individuals their own fate and that’s all it
>needs to make this group the founder of a new
>monophyletic group.

But this contradicts your Mars interpretation. When they left for Mars, 
that created reproductive isolation. Both Ursus arctos and Rangifer 
rangifer in Eurasia and North America are reproductively isolated, and yet 
most mammalogists regard each of them as a single species. What's missing? 
Natural selection for divergence.

>What it comes down to: Tom is right: species can be
>delimited only in relation to each other.

That's trivially true at every level, since if there were only one species, 
there would be no point in recognizing it as such.

>8. A species becoming a higher taxon?
>Tom DiBenedetto has been argueing repeatedly that a
>species, when splitting up, becomes a higher taxon.
>Even to me a cladist, this concept is new.

And useful, although it deals with species in a way that is uninformative 
with respect to population genetic phenomena.

>9. Monophyletic or polyphyletic species?
>The term monophyletic applies only to groups of
>species, not to species.

I agree totally.

>In a phylogenetic species concept, a species
>cannot “survive” its own speciation. Every speciation
>event splits a species in two species.

Inasmuch as a species that has produced a peripheral isolate cannot 
genetically "know" that it has speciated, I don't find this view useful. 
Species are the interface between tokogenesis and phylogenesis, although I 
hesitate to say they are "neither fish nor fowl" for fear of being 
misinterpreted. :-)


-- 
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu