EO> Well, I'm thinking in terms of the survival of the fittest theory.
EO> The ones that have a ceartain survival trait survive, and the others
EO> who don't get the benefit of this trait don't.
DH> That only applies when the new species must compete with the old
DH> for the same resources. In the case of the chichlid fish, one lives
DH> in salt water, another in fresh water. Both can flourish; they do
DH> not compete with eachother. Make sense?
You just had to mention my favourite evolution subject, didn't you? <g>
The cichlids are all fresh water fish although they will live but don't generally breed in brackish water and salt water usually kills them. Their extremely close relatives the pomecentrids are marine fish, and they'll survive but not breed in brackish water as well. Practically the only morphological differences between the families is a bony shelf supporting the eye socket and the way they metabolize salt. Due to their differing environments they cannot compete with each other.
The pomecentrids such as the damoiselles and percula clowns are the "older" of the two families, IMO. They are quite similar worldwide and almost everywhere there is warm water such as around coral reefs you'll find very similar species of pomecentrids. Their environment is extremely stable and the surviving species of pomecentrids have changed very little. Some species such as the perculae have evolved ingenious methods of survival by forming symbiotic relationships with sea anemones. Evolving superior survival traits appears to be common to both the cichlids and the poms.
The cichlids are the more fascinating to me as far as evolution is concerned. They evolved in a much more unstable environment - that of fresh water in lakes and rivers where water quality is constantly changing. The variety in size, shape and colour is enormous, and it mostly depends on the environment in which they are found. They are usually quite territorial and are fierce competitors, although some species such as the angelfish and discus will form small groups. They all show very peculiar feeding and breeding behaviour, much of it brought out by that competition, I think.
One thing they do have in common is they appear to be a highly adaptable family, a survival trait that has served them well. We observe from the African Rift that when cichlid populations are isolated for an evolutionarily "brief" period of time they tend to take on different characteristics from the original populations such as colour and size. The body shape of the majority of the african and asian cichlids is usually the familiar "torpedo".
In South America we see populations that have evolved under very different conditions in the isolated rivers and lakes of the rainforests. Some have evolved with dinner plate-like body shapes such as the discus which allows them to live and feed in the reeds of slow-moving rivers. The Angelfish many aquarists keep in their community tanks are another example of this body shape, although their behaviour is quite different. Others are more similar to their African cousins in appearance, and like the North American "Jack Dempsey" are highly aggressive, even more so than the Africans.
How did the great variety of cichlids and their pomecentrid cousins come to be? If you look at their distribution and fit it in with the geological and paleontological history of the planet the answer fairly jumps out at you: massive environmental changes with periods of relative stability separating the events. In other words, punctuated equilibrium.
In the past we have looked at all these fish and have had to study their environments, their morphology and the few rare fossils we've found to determine their evolutionary history. We are on the brink of cataloging and cross-referencing them by their DNA as well, which has been rather exciting to consider. While the project may never be complete the data is beginning to appear. We need more people working on it, as is always the case.