Sorry Mollyman. I must disagree.
I am well aware that fancy fish are easy to care for, I have a few myself, but selection for improving a particular aspect of a fish always means narrowing the gene pool. A narrowing of the gene pool can in fact lead to extinction if there happens to be something in the retained genes that is fatal or detrimental to the fish's survival. If that were always the case, the fancy fish would only be available from breeders who originally developed a strain but it does not make the genetics involved any less valid.
Please review the concerns that animal care specialists have about the basically wild animals that are on the endangered species list. They start to become concerned about the ability of an animal to reproduce and survive under ideal conditions when their numbers reach hundreds of individuals, not one or two. When the gene pool only contains a few hundred individuals, the gene pool has become so restricted that it becomes a crap shoot whether or not the species in question can recover and survive. I am sorry but a single drop of fish is a far narrower gene pool than that one they are worried about.
Back to where I started. There is nothing inherently weak about highly inbred populations of fish but when you are working with the first generation, any weakness the fish exhibit should be a sign that you need to purge the whole drop, not just the individuals that show a deficiency. After all, why would you even be tempted to reinforce "bad genes".

Nothing so absolute could be further from the truth. The oldest known, tracable, single collection, single importation of a tropical fish species is a killifish species -- Scriptoaphysemion liberiensis "calabaricus". The originals were collected and imported from an unknown locality into Germany in 1936! That importation survives in the hobby today! It has never been recollected or outcrossed. The aquarium strain was once down to a reverse trio in the 1980's. Today the strain is spread over the hobby again. I raised 100's in the 90's and the sex ratio was 50/50 and each fish developed into a spectacular and healthy specimen. You see, a livebearer or killie population already usually comes from a rather constricted and inbred gene pool. I raised a population of nezzie swords for most of the 90's and into the 2000's. Each generation was the result of carefully chosen breeders of the largeest size, best color and best form. The strain became huge with great color and vigor. The original breeder could not believe the large fish I sent him years after I obtained the originals from him. The same was true of monty swords. People could not believe the size and ease of the ones I raised based on tough culling and selection. Even if a population produces a few weak fish among large numbers of healthy fish that is not a case to destroy a strain. The biggst problem with livebearers is not too small of a gene pool but raising far too few fry from which to select breeders. Raise 300-500 fry extremely well and select the very best and you will have great stock. Its the husbandary that is to blame and run of the mill husbandary is so ingrained and accepted that hobbyists no longer realize they are poor fish keepers.

The needs of mammals is another matter but even there look how few bison were used to bring back the species in the early 20th century - or the Bald Eagle.

Hmmm.....I would have to agree with Oldman. According to our current understanding of genetics, without diversity a small closed population will experience genetic degradation. A reminder that physical health and well being aren't a clear indication of genetic strength. However, using your example of the killies, it is possible that despite some genetic inbreeding, one could breed fish which would develope a mutation to overcome the "bad" parts. But as a general rule, not a law, the larger the gene pool, the stronger the genetics of the species for preserving the original specimen.

Sorry if I butted in on a "private" discussion, but the concept of genetics in an enclosed environment fascinates me. One of the reasons I like livebearers is that I can observe this daily!

I think as hobbyists we take genetic "laws" and apply them too indiscriminately to our fish when they do not reflect what is actually happening in our tanks. I don't think a fish can evolve an adaptation to "override" 75 years of intensive inbreeding. Something else is going on. One thing is true, too few hobbyists provide the best possible conditions for their fish -- tank size, culling, diet, water changes, etc - and this average husbandary does more to weaken strains than genetics does or even has time to do. This hobbyist "weakness" explains the myth that "fancies" are hard to keep over time. It also explains why wild strains appear to be easier -- they accept less than ideal husbandary better or at least do not so obviously show the effects.

How could it be a myth when you just said....Quote:
...won`t that make them easier. Sorry it just seems contradictory. Maybe you could rephrase that for my understanding.

Personally I don`t think that genetic rules are used enough by the average hobbyist. But poor care is more rampant I think and thus more detrimental to the fish. How environment affects genetics is a WHOLE different aspect however.

Simple, if you start with small grey fish that pretty much replicate themselves then any decline in that strain is going to be less noticable than if you start with finely bred wild type or "fancy" fish. If people equate "wild" with kind of small, skinny and grey then no one will ever see the difference between what is and what should be. There is no reason to develop the "eye" it takes to see quality. They see names, collection codes, conservation and acquiring new species.

We need more genetic knowledge in the hobby (look at what the "fancies" did in that area! -- but we do not need so-called "common sense" assumptions about genetics nor the idea that a tank replicates wild conditions -- internally or externally. A little PBS is a dangerous thing.

Ok, that makes more sense! Thanx!