mekent61{at}hotmail.com (Martin) wrote in
news:bs2h2r$12ba$1{at}darwin.ediacara.org: 

> [moderator's note: For the readership of r.a.f.m., please note
> that sci.bio.evolution is a moderated newsgroup, and discussion
> related to the creation/evolution argument is forbidden. I also
> frown on cross-posting, but this seems innocuous and actually
> quite relevant, so I'll allow it for the time being. - JAH]
> 
> mekent61{at}hotmail.com (Martin) wrote in message
> news:... 
>> I have noticed that some families of fish such as Cyprinidae,
>> containing such fish as Barbs, tend to have caudal fins that are
>> divided into an upper and lower lobe ie, for want of a better
>> description, they are forked. On the other hand other families such
>> as Cichlidae, containing such fish as Discus, Oscar,s and Angelfish
>> tent to have rounded, paddle like caudal fins. The body shape also
>> tends to differ, with the former tending to be more streamlined and
>> the latter having a rounded appearance when viewed from the side.
>> 
>> Are these differences due to chance factors of evolution or are they
>> adaptions to different environmental demands made on the different
>> families? I've noticed that the Cyprinidae are often found in moving
>> water whereas the Cichlidae are often to be found in still waters. If
>> habitat is the key, then I can understand why the Cyprinidae are more
>> streamlined but I still do not understand why the tail needs to be
>> different.
>> 
>> Martin
> 
> Reposting to include "evolution" group.
> 

A quick review on the internet reveals that from a hydrodynamic 
standpoint, a strongly forked tail is better for high speed swimming but 
worse for maneuvering (the details of why this is so have to do with 
fluid dynamics, which is beyond what I am prepared to explain). Of course 
there are numerous other tradeoffs. At very low speeds a number of 
species use pectoral fins for locomotion. A number of salmonids (along 
with other species) use their caudal fins to create suitable places for 
egg laying. In some cases (e.g. pike) acceleration is more important than 
sustained high speed. Body shape may also be affected by habitat and 
behavior, the flounder being a prime example. For many fish sexual 
selection is an important determinant of body shape and coloring. And in 
some cases differences may well be due to chance - one web site noted 
that populations of brook trout in eastern Canada tended to have forked 
caudal fins. This might be an adaptation to a lake environment but could 
also be due to drift.

Yours,

Bill Morse
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