Peter wrote about some work being done by Craig and team on M. eachamensis
and splendida on the Atherton table land. This work identifies eachamensis
as more closely related to australis than splendida. The interesting point
to note is that the australis he is talking about are the australis from
the Kimberley area and Craig went to pains to point out that this was
distinct from the australis found around Darwin, Arnhem Land.
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.
Peter also points out
> It is interesting to note that with only a few exceptions (usually those
species
> that are geographically close to New Guinea) they are always allopatric
(as
> are some rainbows).
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.
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).
Regards,
Barry Meiklejohn.
----------
> From: peter.unmack at ASU.Edu
> Date: Tuesday, 4 February 1997 13:33
>
> Craig Moritz's lab worked on M.
> eachamensis and found that there are some populations on the Atherton
> Tablelands that appear to be hybrids between eachams and splendidas.
> They suggest that eachams are more closely related to M.s. australis
> genetically and that the splendidas have more recently reinvaded and
> subsequently mixed in with the eachamensis types. IMHO, I think it is
> likely that similar situations will be found in several of our fishes,
> especially the genera Ambassis and Craterocephalus. It is interesting to
> note that with only a few exceptions (usually those species that are
> geographically close to New Guinea) they are always allopatric (as are
> some rainbows), meaning that in Australia it is rare to find more than
one
> species coinhabiting. In the case of the Craterocephalus there are two
> distinct lineages, "stercusmuscarum" and "eyresii", the only place that
> any members within either lineage are sympatric is Dalhousie Springs and
> even there they are usually not strickly sympatric as most springs
contain
> either one or the other species, not both (with one exception). The
> significance of this is that over time fish species invade areas and then
> reinvade them at later dates. If they have not developed reproductive
> isolation then they hybridise. The other alternatives are that they
> coexist (which appears rare) or one goes extinct.
>
> Peter Unmack