Following is an article which I wrote for the Australia New Guinea Fishes Association (ANGFA) magazine "Fishes of Sahul", back in 1985, entitled "Preserving Species Quality of Native Fishes in the Aquarium". It was specifically aimed at Australian Rainbowfishes, but applies just as fully to any other species of animal or plant.
Evolution is possible because individual organisms of similar origin (i.e. the same species) are not all exactly alike, but exhibit slightly different characteristics. Natural selection is the process in Nature whereby those individuals of any particular species with characteristics best suited to the habitat in which they live, by having a slight advantage over other individuals with characteristics less well suited to the habitat, tend to reproduce more successfully, leaving more offspring with those favourable characteristics, until eventually, the less favourable characteristics virtually disappear from the population. If members of a species migrate to a new habitat with slightly different conditions, the individuals best adapted to that new habitat might be those with slightly different characteristics, so the population of the new habitat will, after a few generations, begin to deviate from that of the old habitat. Over a very long period of time, the two populations may become so different that, eventually they can be regarded as separate species. Evolution, therefore, can be summed up as adaptation to the environment through natural selection acting upon random variations in individuals, and is the way in which new species evolve to exploit every type of niche in the environment.
With regard to Australian native freshwater fishes, there is an interesting situation at the current point in their evolutionary development. In addition to normal family, genus and species groupings, there are certain species which consist of a number of distinct and not so distinct races, this being the result of isolation of populations in different river systems. This can be seen as evolution at work, and these races can be regarded as new species in the making. Probably the most familiar example of this is Melanotaenia splendida, one of the rainbowfishes, in which a large number of races occur in various waterways throughout much of the continent. Many of these races, (and in some cases, they have already been regarded as sub-species), show quite distinct variations which are characteristic of that race, possibly indicating that these populations have stabilized, and that the habitat is also fairly stable over a large time span. In other races, however, individuals from the same population show great variability, which may indicate that these are newer populations and are still in the process of adapting, or that the habitat is constantly changing over time.
If a species is maintained in captivity under aquarium conditions, and simply bred indiscriminately, it will begin to deviate from the original wild form after a few generations, due to the fact that natural selective forces are no longer operating upon it, and unfortunately, the resulting individuals will generally end up inferior in size, shape and colour to the wild stock. It is therefore important to take steps to preserve the original wild-stock characteristics of the species or race, particularly if they are fishes which are rare or endangered or unavailable from natural sources. Careful selective breeding, using as breeders only those individuals which best display the characteristics of the wild stock, is the way to achieve this.
The first thing to determine is just what the wild stock should look like, which can often be rather difficult. The photographs and descriptions presented in "Fishes of Sahul" are one means of determining this, since they are intended to present an accurate portrait of the species or race. There have also been a number of excellent texts on native fishes published recently.
Finally, consulting the local experts on native fishes can solve the problem.
The next thing is to ensure that the breeders selected are true to type. This often means waiting until young fishes are mature enough to show all of the significant characteristics to advantage, which means not culling the fry too soon. For example, keeping only the biggest, fastest growing fry can be a mistake, as these individuals may not necessarily show full colour when they mature, (or, as some of us have found out, they may all turn out to be males!).
When breeding from a limited stock of fishes, there is the distinct possibility that, after several generations of in-breeding from the same genetic stock, congenital defects will begin to appear in some of the offspring, due to the "surfacing" of harmful recessive genes which would not normally show up in a larger population. This is because, with such a limited gene pool, any particular gene represents a larger percentage of the total number of genes available in the population than it would in a much wider gene pool, which means that the probability of a harmful double recessive showing up is also proportionately greater. Any young fishes showing any abnormalities must be culled, and on no account should they be used as breeders, as this will only perpetuate the faults and lead to a rapid decline of the stock. In order to minimize the risk of congenital defects, the original stock should consist of the largest possible number of individuals, and whenever possible, fresh individuals from another source should be obtained and introduced into the breeding program in order to inject fresh genetic material into the population.
One point worth remembering is that inferior fishes should never be passed on to another aquarist whose aims are also to preserve the wild form, as this can only have a detrimental effect on the species in the long run. In other words, the preservation of the wild form should be regarded as a universal project, and not just as a personal one.
Occasionally, an individual fish may show up with certain characteristics which outshine the original. It is obviously very tempting to breed from such individuals, and this is how fancy varieties of many animals and plants have come about, but in keeping with the aim to preserve the wild form, this temptation must be resisted. If it is decided to develop a fancy variety from such a desirable variation, the results of such a project should only be regarded as "ornamental" fishes, and not as true representatives of the species.