[Taxacom] metapopulations (excerpt from ongoing discussion)

[John Grehan]

I separted this out of the ongoing thread for ease of reference.


> " Surely vicariance is such that the distance between landmasses varies

> over time and the chances of dispersal between the landmasses decreases as

> the distance between them increases. In practice, vicariance itself may

> involve dispersal over short distances. As the distance increases, the

> fewer species are able to disperse.Stephen"

>


Yes. It's a simple exponential decay with each taxon having a different

ability to disperse and an element of serendipity thrown in. Beyond the

phylogenies and the macrobiogeographical patterns there have been plenty of

experimental studies looking at island colonization (Simberloff & Wilson

for example) that show this pattern. It is like arguing that microevolution

experiments/observations don't support evolution. Anyway, back to the cage.


Jason


Jason & Stephen – you are both right in my view - that vicariance does
involve dispersal, but not just over short distances, since some ancestral
ranges were nearly global. The experimental studies of island colonization
are part of the picture as they illustrate the process of species survival
and persistence. The islands studied by Simberloff & Wilson are
biogeographically no different from disjunct habitats within a continent. A
species may occupy multiple islands or habitats, and yet also be allopatric
with respect to sister taxa, and the allopatry may involve a tectonic
correlation. This is perhaps the key fact (it's indisputable since tectonic
correlations are observable) of biogeography (and evolutionary ecology to
boot). Heads synthesized these facts under the metapopulation concept –
that because organisms disperse (in the ecological sense of ordinary means
of dispersal that are empirical and therefore observable) they are able to
maintain their existence over long periods of time, even though individual
habitats/islands may be ephemeral. This is well illustrated by the
Galapagos where organisms that are able to disperse between islands over
geological time have been able to persist at the hotspot since Mesozoic
time. Gary Nelson pointed out a long time ago that their experiment can be
replicated in one's backyard garden by sterilizing an area of soil and
watching plants 'colonise' the exposed surface. Wonderful how one can do
studies relevant to evolutionary biogeography in one's own backyard (if one
has a backyard of course).

The bottom line in all of this is that tectonic correlations have been
documented for hundreds of taxa. This is the great lacuna in the science of
biogeography (i.e. usually treated as if it does not exist). It has to be
addressed as a reality (however 'explained'), not simply ignored (which is
most often the case). Tectonic correlation is consistent with vicariance
and ecological dispersal, not 'random', 'chance' or 'freak' dispersal that
is supposed to have almost always occurred only once in a taxon, and yet
the same tectonic correlations for taxon after taxon. Tectonic correlation
shows that biogeography should, at least alongside modern genetics, be at
the core of any evolutionary program and synthesis, not just an obscure
hobby for some.

Cheers, John Grehan