Comments to Cladistics/Eclecticism, part 3
susanne71_2000 at YAHOO.DE
Fri Feb 8 18:21:51 CST 2002
here's part 3 of my comments:
6. Speciation backwards?
We had this example with some humans flying to Mars.
If these guys come back after some time and they are
still able to interbreed with the humans on Earth, it
is obviously still the same species. If they have been
completely isolated from us, with no exchange of
genetic material, long enough, there might develop a
reproductive barrier. If the humans from Mars then
come back and they cannot have fertile offspring with
the humans on Earth, the Martians and Earthlings must
be considered different species.
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!
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
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way, is completely outdated. Heres something to
consider: There are cases in which two sister species
look perfectly alike, which is termed twin species
or cryptic species. Even though they look identical,
they do not interbreed and are hence good species. It
is quite likely that the common ancestor of these twin
species looked also perfectly identical to its
descendants. Lets call the common ancestor S and the
daughter groups T and U. 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. But, hm, T and U do not
interbreed, so how can they be the same species? Weird
Now you might argue that species must have some
difference, otherwise they would interbreed. Sure, but
this could be just a different hormone, a different
song, or simply different phenology. This would
suffice to give each of them a different fate.
Reproductive isolation completely suffices to give a
group of individuals their own fate and thats all it
needs to make this group the founder of a new
What it comes down to: Tom is right: species can be
delimited only in relation to eachother.
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. (Which is
not meant as a criticism, I just havent heard that
before. But maybe this is just ignorance on my part,
Im not really interested in species concepts.) A
species that has given rise to descendant species is
usually considered a stem species = ancestral species.
The higher taxon, the clade, CONTAINS this ancestral
species and all of its descendants. Of course, this
ancestral species does not exist anymore, today. Now,
only some of its descendants are alive. So, yes, the
stem species does not exist anymore. It is a thing of
the past. To argue whether it has become extinct or
whether it has become a higher taxon, is pretty much
playing with words, I believe. It is simply two
concepts of thinking of this history. But I do find
Toms concept rather interesting.
Consider the following analogy: a unicellular
individual E. E divides into two daughter individuals,
F and G. E was once an individual, but now it does not
exist anymore as an individual. But it does exist as
two individuals. E lives on in F and G.
Has E become extinct?
Or has E become a group of two individuals?
And: does this really matter?
9. Monophyletic or polyphyletic species?
The term monophyletic applies only to groups of
species, not to species. First, because a monophyletic
group is a species and all of its descendants.
Second, the relationships within species are different
from those between species.
Equally, the term paraphyletic also does not apply to
species. But even if someone tried to use these terms
for species, there would still be now paraphyletic
speciation, only if you use a morphological species
concept. In a phylogenetic species concept, a species
cannot survive its own speciation. Every speciation
event splits a species in two species.
There is an important rule in life, which is
unfortunately not known by many people:
You should only criticise what you understand.
Criticising something that you dont understand is
commonly called intolerance and ignorance.
If you really WANT to understand Cladistics (and not
just criticise it), theres plenty of literature. All
the issues that are being debated on this list, have
been debated and solved in the literature in the 50s,
60s and 70s of the last century. Read Hennig (1966),
for starters, then read his other papers, then maybe
Farris and Kluge, etc. etc. etc. Its all out there.
Final comment: I expect that there might be many
replies to this post. I want to tell you in advance
that I dont have the time to answer most of them if
any at all. Sorry. (The time I spent on writing this
email is already more than I can afford to spend.)
P.S. Please apologize any orthographic or grammatic
errors; as I mentioned, I am not a native speaker of
New journal: ODE - Organisms, Diversity and Evolution
by the Gesellschaft für Biologische Systematik.
Institute of Zoology and Anthropology
University of Göttingen, Germany
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