Re: Species? or variation within?

[Christopher Taylor <ck.taylor@auckland.ac.nz>]

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.
    The point of ontogeny you mention complicates matters quite considerably
- organisms change in proportion a lot as they grow. Just compare a human
baby and adult. Again, one hopes for intermediate forms to bridge the gap,
and fortunately, with vertebrates you're more likely to get them than
otherwise (we grow slowly, rather than in jumps like in arthropods).
    And even then you're not in the clear - there's sexual dimorphism to
consider, and even maybe discrete polymorphisms in the species. I did my
masters thesis on a group of arachnids in which the males of a single
species can come in two varieties - one in which the 'pincers' are about the
length of the body, one in which they're roughly four times the length. No
intermediates, but different morphs of the same species nonetheless. A
similar thing happens in trout and salmon, where there are large males that
defend a harem when spawning, and small males that try and sneak matings
with females in large-male harems. Usually, the two forms have a clear size
gap between them, with no males of an intermediate size.
    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. 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'.

    Cheers,

        Christopher Taylor

On 26/7/04 4:40 pm, "David Peters" <davidrpeters@earthlink.net> wrote:

> Say I find in one pterosaur specimen a wing only 3/4 the length of the
> other of the same nominal species ? are these actually different species
> or variation within one species?
> 
> Say I find an ilium only half the length of the tibia, but in another it
> is nearly the same length as the tibia ? species or variation within?
> 
> Say I find a skull that has a 1:3 width to length ratio, while in
> another it is 1: 4 ? species or variation within?
> 
> Say I find one pterosaur with a tail only 2/3 the length of the other ?
> species or variation within?
> 
> I've found all of the above within the medium to large Rhamphorhynchus
> specimens. About ten years ago Chris Bennett (1995) determined that
> they're all the same species, only of different ontogenetic ages. I'm in
> a quandary.
> 
> 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
> species expressing allometric change?
> 
> Thoughts?
> 
> David Peters
> St. Louis
> 
> 
> 
> 

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