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Re: More on the genus problem
2009/9/20 Erik Boehm <email@example.com>:
> Reproductive compatibility doesn't apply to single celled organisms...
To some of them, right. They have other ways of recombining genetic
material. But others have sex and share gene pools just like animals
and plants. This is just another problem with the belief that all
around can fit into species according to the biological species
concepts, which is based on sex and gene pools.
> If we were to kill all dogs except Great Danes and Chihuahuas, would they
> then suddenly be two different species?
I do not believe species is an useful term, but if we accept that
pertenence to a species equates to contribution to a gene pool, I
would say yes, there are now just two different gene pools.
> What if said intermediate dogs are in existence, but isolated from either
> group? Are they still 2 species?
Under the laboratory definition, no, because they both may have
interfertile descent with the intermediate dog. But in the wild (as if
Great Danes and Chihuahuas were wild animals) yes, they would be two
different, not mixing gene pools.
> If the isolation ends, do they become the same species again?
Yes, the two non-mixing gene pools would now merge.
> Did there ever exist a transitional human form that would be able to
> interbreed with humans, and with chimps? If such a form existed, would that
> make us the same species?
Yes, because both gene pools would merge into one. Except if, in
practice, the intermediate does not live in the same place of the
chimp (or human). I think this is easily understood if just talking
about the gene pool, which can easily divide and merge up with other
> Phylogenetically, each individual would be like their own branch, and the
> traditional branches are really like intertwined vines.
> And of course, there is the "web" rather than "tree" analogy to take into
> account gene flow.
True. Sometimes we consider that phylogenetic trees produced by
cladistic methodology represent (or tries to represent) the true tree
of life. However, this does not need to be so; there may be horizontal
transfer which we cannot realize in the fossil record, we will never
know. Who can ascertain two duckbilled dinosaurs with strikingly
different head ornament cannot bring fertile offspring? Will expose
myself to ridicule with a further exaggeration, but the same applies
with a ceratopsian and a duckbill. The principal reason for the use of
cladistic classification lies elsewhere. It is based on its greater
information content (it requires the less exceptions possible to
organize a classification scheme).
> Its all a big mess, as I said, I've never heard a good definition for a
> species, so I wouldn't expect a good one for a genus.
True. As there is no general or theoretically defined
"difference-o-meter" to distinguish intraspecific from interspecific
differences, there is not either one to distinguish intrageneric from
intergeneric differences. As Gould said, if something ends up being a
mess, it is possible that we are asking wrong questions and so we may
better leave the matter and think on it other way. For me, in this
case the better solution is to abandon usage of these (as well as
other) ranks and only speak of clades.