I've never heard of Bergman's Principle that I can remember and itdoes
sound too broad to apply locally. Altitude and latitude arelargely the
same thing ecologically so that would have to be takeninto account.
Perhaps it's a trend, particularly in herbivores...butnot a law?
Chances are, Bergman was thinking about mainland populations
whileIslands are often the crucible of rapid evolutionary change.
(Extremeconditions, and potentially total isolation of a breeding
population.)To be sure, New Zealand's animals were shaped by their
isolation onislands. A population isolated on an island may, over
time, drift in acouple of directions including very small (pygmy
elephants) or verylarge (Moa). There's a lot about this in layman's
language in "TheSong of the Dodo". When I reread it I kept my eyes
open for whatguiding principle was underlying such changes and, as it
turns out,what was on offer were a variety of different factors and
scenarios.
Tidbit: The Polynesians always brought chooks with them when
thecolonized new animals but when westerns met the Maori, they
didn'thave any chooks left. Why bother when you had Moa everywhere?
"Moa" isa word in many Polynesian languages for chook or "fowl".
Hopefully we've got a few biologists/bio-geographers on the list
thatcan jump in and same something informed. Just as a start I'll put
in afew points from an interested layperson. It seems common-sensical
thatin the absence of competition and predation, animals may well
affordto be able to grow big, slow, and (in the case of birds)
flightless.This isn't as obvious today as it would have been before
the greatwaves of expansion across the world's oceans
because...sadly..."big,slow and flightless" may also mean "tasty."
Throughout the Pacific,for example, mostly all of the flightless birds
are gone. Ducks,geese, ibis, and pigeons, for example, are great
dispersers. I livedin Hawaii for seven years and ever year a few stray
ducks and geeseshowed up from the mainland and ibis are sighted most
years. (Therewas even a White-Tailed Eagle frighting the helicopter
tours on Kauailast year.) In any case, before the arrival of the
Polynesians, therewere numerous sorts of ducks, ibis, and geese on the
main islands -some of them quite gigantic by out standards. This is
known largelyfrom sub-fossil remains found in sink holes. There is one
goose (theNene) left on the islands but it's scarce and fairly small.
Speaking of the Nene reminds me of my own pet theory. I haven't seenit
written down but assume there's a name for it somewhere. In anycase,
it seems like plasticity ought be a selectable trait in itself.For
example, some animals retain their forms for unbelievable periodsof
time - even hundreds of millions of years (Horseshoe Crab),
whileothers are highly plastic. For example, the Nene is now (I
believe)considered part of the Canadian Goose complex. That's a
prettycomplicated group of birds - the book outlining the variations
isabout the size of a HANZAB volume, if I remember rightly. Just
bysight, the geese in the eastern part of the US are
dramaticallydifferent in size from those in the west. I just took a
quick look inSibley and he shows a weight range that differs by a
factor of 3within the group. The point is, some populations tend to
show a highdegree of variability in form and, evidently, change their
form morereadily than other animals. (Change through selection and
inheritance,like anyone else.) It's pretty easy to see how holding
your formrigidly over the eons (Horseshoe Crab) is 'selectable' as,
ifconditions haven't changed, why mess with a great plan? It's also
easyto see how morphological plasticity is beneficial when conditions
(oryour location) change. Another good island example are the
steamerducks of far southern South America. On the mainland, there are
a fewspecies and they're able to fly whereas offshore on the
Falklands, thevery similar looking island populations are flightless.
It looks likegeese and ducks at the least, and certainly some rodent
groups arefairly plastic.
Speaking of rodents, there was a post on this list back in
Mayregarding mice on Gough Island (one of the world's last great
seabirdbreeding islands). In a very short time the ordinary House
Mouse hasevolved on Gough to double or triple the average size of mice
inEurope. Founder's effect isn't likely to have had much of anything
todo with this (only exceptionally large mice were brought?) - it's
morelikely that conditions selected (favored) larger mice on Gough so
thepopulation has drifted towards the gigantic end of the mouse
spectrum.They've also sorted out that eating tender shoots of grass
and grainjust isn't as satisfying as gnawing into a nice piece of
meat...suchas an Albatross egg. There's a write up in the Guardian
here:
http://www.guardian.co.uk/environment/2008/may/19/wildlife.endangeredspecies
Anyway, there are lots of examples of dwarfism in island
populations,which is also easy to understand some of the time. (Of
course, theproblem with retrospective evolutionary explanations is
that, withoutevidence, you can make up a plausible story but it
doesn't mean it'strue.) If you're an elephant (excellent swimmers) and
you reachSicily, you may find that there just isn't as much to eat as
in theold days back on the mainland. In such conditions, animals
better ableto survive famine/reduced nutritional seasons are going to
be favoredso you end up with tiny elephants starting from a normal
population. Isaw a pygmy elephant skeleton a few years back in
Siracusa and it wasquite striking - it really looked like an elephant
but it didn't evenreach my shoulder.
As always, corrections and additions welcomed.
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