Large and small distribution areas

[Panza, Robin]

<<<From: Jan Bosselaers [mailto:dochterland at PANDORA.BE]
I have the impression that in "megadiverse" groups...a large proportion of
the known species has rather limited distribution areas and a rather smaller
number of species has worldwide distribution. On the other hand, in some
small groups...a large proportion of the known species seem to have a
worldwide or at least very large distribution area.>>>

I can think of a couple of possible reasons that are artifacts of
"diversity".

In groups with simple morphology, we just don't recognize many species.  We
can see many similar species among spiders, each of which is found in a
relatively small area.  In slime molds, a given widespread morph may, in
fact, be many similar species, each with a small distribution.  I don't
claim this is the case, but it's certainly a possibility.

While there may be quite a few ways to "be" a spider, the number of niches
is finite.  The only way to increase diversity beyond that is to have
different species occupying similar niches in different places.  A highly
diverse group exceeds the number of niches and will have ecological
equivalents spread around the world.  A low-diversity group either has its
few species widespread or there will be many places in the world where those
niches are unfulfilled (or taken up by unrelated taxa).

Dispersal rate may also be a factor.  If a group is made of species that
disperse widely and frequently, they will have colonized similar niches
around the world, and be widespread.  With much dispersal, gene flow among
populations is great, maintaining evolutionary cohesion = few, widespread
species.  In another group, unable to disperse as readily, the occasional
colonization event is followed by little/no gene flow to the parental
population, leading to local adaptation and eventual speciation = many
species with small distributions.

Robin K Panza
Section of Birds, Carnegie Museum of Natural History
4400 Forbes Ave.
Pittsburgh  PA  15213  USA
phone:  412-622-3255
fax:  412-622-8837
panzar at carnegiemuseums.org