> I will take Peter up on his offer to create yet another contentious thread
Good stuff. We need some interlectual discussion on this list
occasionally, it's the only way we can get Roy to shut up! I notice
Rooster avoided answering my question relative to ssi and meta tags. :-)
> When I heard Craig talk I got the impression that there were two distinct
> "invasions" of Melanotaenia ancestors. The eacham came from the same
> invasion as the kimberley australis and then the current splendida form
> came in a later invasion. Perhaps I am oversimplifying things but it makes
> sense to me.
That is basically correct. We cannot say how many invasions there has
been except that there has been AT LEAST two. The problem is that what
happens today may overwrite some of the older history and it gets
difficult to interpret it in a testable way.
> I beleive Peter is talking about the genera Ambassis and Craterocephalus
> here as in my experience several Melanotaenia species coexist in our north.
> I was thinking about this coexistence and why as Peter points out
> > with few exceptions (usually those species that are geographically close
> >to New Guinea) they are always allopatic.
Actually, I was referring to Melanotania as well. Outside of those areas
close to New Guinea (ie Cape York, Darwin) the sympatric occurence of
rainbows is rare. The only examples are M. maccullochi/M. trifasciata/
M.s. splendida in a few small areas north of Cairns, M.
maccullochi/M.s. splendida in a few small areas around Cairns (all are also
sympatric on Cape York and elsewhere so this is no surprise).
M.exquisita/M.s. australis (Daly/Alligator rivers systems and
vicinity around Darwin) (where there is some evidence of old
hybridisation), and I guess there are four species in the Drysdale River
(M.s. australis, M. nigrans, M. gracilis, M. exquisita). Everything else
is allopatric. Ambassis is similar too and Craterocephalus is even more
striking.
> My thinking of this "coexistence" runs along the following lines. The
> original ancestors of these fishes appear to have come from warm tropical
> climates (if Sahul was warm and tropical???). As they have spread south
> they must adapt to the different conditions prevaling in their "migration"
> path. The more extreme the climatic differences from "home", the less
> "species" will survive or have a chance to adapt (survival of the fittest).
> This process would explain why we have more speciation and variation
> within species across the top end of Australia (and even greater in PNG),
> with a lessening of this variation the further south you proceed to just
> one or two species (duboulayi & fluviatilus).
Without trying to say you are wrong (especially since I don't know the
answer) let me try an alternative, though in part complimentary
explanation.
Part of what is thought to cause speciation is isolation and a diversity
of habitats. New Guinea
on average has far more diverse habitats within a given river system than
most Australian rivers do, thus there is more avialable space (ie
niches) for species to pack into. New Guinea also is far more
tectonically active. Thus a major factor in causing isolation and
subsequent speciation. Many Australian rivers don't really start in
mountains (at least not like those in NG), and many of them tend to be
short of water for much of the time (unlike many in NG). Habitats tend
to be less variable in terms of type in Australia from the headwaters to
their terminus. Obviously this is a broad generalisation but I feel it
holds in comparison between Oz and NG. Thus, Australia has less water
and less diverse habitats and less frequent longterm isolation due to
tectonics (and some other factors--I'm trying to keep this simple). This
is part of the reason why there are so few species relative to many
places in the world.
In terms of why doesn't the northern fauna go further south, part of it
is barriers, part of it is habitat. For instance, around 5 million years
ago fossil Arius catfish were present in the Lake Eyre basin (who knows
how long they persisted for? probably until ~1 million years ago as that
was when lungfish disappeared). This species must have either invaded
from the north or come in through the south when the basin drained to
the sea. However, the species died out presumably due to increasing
aridity in this part of Australia. If the climate changed and became
wetter again Arius would not reappear until a barrier is broken.
Barriers, in the shorter term are not broken very often. My point in
this is that at least some of the northern fauna was once present in
central Australia and probably more southern parts once upon a time.
Today things are too dry and the barriers have not been recently broken.
Thus anything from the north that could have invaded over the last 1-2
million years (when the sea levels have been know to regularly go down
and up) has only been able to invade the northern drainages aside from
the fact that things are still too arid in much of Australia for many
species to survive. Along the coast in the areas around
Rockhampton/Cairns etc it is likely that each time sea level dropped (up
to 100-120m) this would cause or allow a potential mixing between basins
presently isolated today. However, movement from say the Fitzroy River
at Rockhampton to say the Tully River is probably pretty limited due to
distance and that the drainages wouldn't ever connect under low sea level
conditions. Thus, my point here is that for a northern fauna to invade
(or a southern one) it would take far longer to go along the coast.
Obviously at times more northern species have occured along the coast as
we still see remnants today in species like M. trifasciata, M.
maccullochi, P. gertrudae, D. banadata etc etc. There are many possible
senarios and I could go on and on and on.....but I won't at this point in
time. Fire away!
Tootles
Peter J Unmack peter.unmack at asu.edu
PO Box 1454
Tempe AZ 85280-1454, USA
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