"Martin"  wrote in message
>
> > Are these differences due to chance factors of evolution or are they
> > adaptions to different environmental demands made on the different
> > families?  I still do not understand why the tail needs to be
> > different.
>
There's a lot we don't understand about fish locomotion and why one shape is
better than another. Some fish do live in different conditions, for instance
deep-sea fish often have a characteristic extended lower lobe of the caudal
fin, which must be related in some way to depth. Some fish school whilst
others are solitary, some eat plankton whilst others eat large prey. Still
others live on the bottom.
Evolutionary pathways can be different, for instance a deep-sea fish that
moved to shallower waters over the course of evolution would still have it's
deep sea fin, which would have to be modified to produce a tail more suited
to shallow conditions. The most famous example of this is of course the tail
of marine mammals, which is similar to the caudal fin of fishes but oriented
in the opposite direction.

You would also get processes like sexual selection working on fin
arrangements, which might push the shape in directions not indicated by
swimming efficiency. Then evolution is not always convergent - if there are
two or three basic tail shapes which are quite efficient evolution might
have found all three.
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