re: species? or variation within?

[David Peters <davidrpeters@earthlink.net>]

Thanks to Nick P. and Chris T. for these replies:

CT: If only there was a definite cut-off between species that could be
applied
to any group of species, but tough titty, I'm afraid.
Specimens are identified as the same or different species not only on
their
own merits, but of other specimens around them. Two individual specimens
may
be quite significantly different, but if other specimens exist that are
intermediate, then they are likely to be the same species (assuming that
all
specimens are roughly contemperaneous, so that evolutionary
intermediates
are not likely to be a factor). For instance, in humans, take a Russian
weightlifter and an African Pygmy - individually, they look quite
distinct,
but I'm sure you could find examples to bridge the transition quite
easily.

>>>  In this case the pygmy (= San people) skeleton would appear to be
similar to the immature skeleton of the Russian. The arms nor fingers
wouldn't be unusually long, nor the pelvis unusually tiny or large.



CT:    Probably the best way to distinguish species (assuming enough
specimens)
is to plot the measurements you're interested in of as many specimens as

possible on a graph, and see if your specimens form a single cluster or
cloud of points, or if they are clearly separate.

>>>>> Agreed. A good method.



Of course, at some point
you'll probably reach the stage where you'll have to make a guess, or as

it's usually referred to, 'formulate a hypothesis'.


>>>> As I wrote C.T. earlier, good for arguments over ale, eh?


Quoting David Peters <davidrpeters@earthlink.net>:

>There has to be a cutting off point between species, and yet there has
>to be gradual transition during phylogeny.  And when can we determine
>that distinct specimens found in separate locations represent the same

NP: Well, figuring out how/whether differing proportions correlate with
different
overall sizes would be a good place to start...


>>>>> No corelation, unfortunately. From what I've seen, and just as in
Pteranodon, the situation in Rhamphorhynchus is rather bushy and needs a
good mini-cladistic analysis involving many dozen characters, from
phalanges to pelves and let the sister taxa sort themselves out. Now, if
you want to consider size, you've got variation from a bee hummingbird
to a mallard duck with discrete size bumps here and there inbetween --
and then some _real_ morphological variation kicks in, yet all within a
Rhamphorhynchus bauplan.

The other side of the statistical argument would be to take a Carl Sagan
mind trip: Start with the sum of all Rhamphorhynchus individuals that
ever lived ?? from the last Campylognathoides to the last
Rhamphorhynchus over tens of million years. Then figure in how many
hypothetical ?discrete species steps it would take to go from 'alpha' C.
liasicus to 'omega(s)' R. muensteri and R. longiceps and R. longicaudus
-- and keep it bushy because that's how evolution works with lots of
dead ends and failed experiments -- and ultimately see how many of these
discrete steps are now known on museum shelves and how many may yet
await discovery. Just a guess, but I don't see a mere single species
coming out of this thought experiment, just based on the astronomical
first set of numbers and the morphological jump from C. liasicus to R.
muensteri, etc.


More later,

David Peters