Re: [RML] RE: Re: M. herbertaxelrodi/trifasciata

peter.unmack at ASU.Edu
Sun, 15 Dec 1996 16:22:06 -0700 (MST)

On Sun, 15 Dec 1996, Roy Hunter wrote:

Next time Rooster why don't you try and not include the original message
in your response 'cause now I have to delete it all and I can't just
highlight it all and delete like someone in windoz.

> I cant recall the name of the "line" separating species from Sahul to the
> surrounding islands.

There are two or three, but the one you are most likely referring to is
Weber's Line. Another less well accepted line is Wallace's Line.

> Would you think that rainbowfish have evolved since
> Sahul collided with the other plate in that area?

You are overlooking the fact that Weber's Line is a barrier to freshwater
fish only. If rainbowfish had evolved from a marine ancestor in the last
15 million years or so (when the Australian plate started to collide with
the Asian one) then that marine ancestor would have been able to cross
Weber's Line and you may expect them on both sides of the line.

> Or did the rainbowfish of Sahul begin evolution before the
> collision of Sahul with the surrounding land masses therefore Bedotia and
> Telmatherina would not be related.

Well, they are still related, just not sister groups. They all share a
common ancestor but there has been significant divergence within the
various lineages.

OK, lets try this out. This is purely hypothetical. 80-100 mya most of
the souther continents are joined or relatively close to one another. An
ancestral bow occurs in estuarine areas. At some point in time, perhaps
due to continents drifting and changes in marine currents this species
gets split geographically. One gets stuck off Africa and for whatever
reason it adapts to and invades freshwater on Madagascar and undoubtably
elsewhere in that region a long time ago. Thus, you have the bedotia
lineage. The other does the same in Australian waters. This fish
spreads across Australia gradually and colonises New Guinea and perhaps
some of the islands to the West of it. These things then diverge into
rainbows, blue eyes, and telmatheriniids over
long time periods and eventually all mix back into together again. By
this time, they have all significantly diverged along different paths and
have formed distinct lineages such as rainbows, blue eyes, and
telmatherinids. Through various later range expansions and isolations
you get the various genera we see today forming. One thing too worth
pointing out is that I am not sure how well it is accepted that
telmatheriniids and psuedomugils are sister groups. I think there is
some disagreement here.

> If they are related then what are the
> common traits shared between them and the Sahul rainbows??

>From Dyer and Chernoff. All Melanotaeniidae have 6 features: posterior
myodome restricted to the prootics, absence of second dorsal fin spine,
three or more pleural ribs posterior to first anal pterygiophore, greater
body depth, reduction or loss of the silvery lateral band, sexual
dimorphism in body colour and median-fin ray elongation.

Probably not quite what you were expecting huh? :-)

> wouldn't you be
> able to then isolate the marine fish that is responsible for this than just
> to say possibly "silversides"??

That assumes that their ancestor is still around today unchanged from
when rainbows split from them. Many species change slowly over time
without any isolation, this is one aspect of classic Darwinian
evolution, slow and gradual change. This means there are no branches in
the family tree. For instance, if you look at recent human evolution, the
species that existed before Homo sapiens is no longer around today. This
is because it and us are just two different points on the same line. It
changed and became us. Thus, it never really went extinct. This is
kinda difficult to explain. Hope you can understand. My point in all
this is that whatever was around when rainbows first evolved would most
likely be somewhat different today also. Fish morphology changes with time.


Peter Unmack