Maintaining the Population

The following discussion assumes that the keeper of wild-type guppies is interested in keeping a diverse colony of fish. If selective breeding is used to achieve a phenotypic goal, then the comments here will be mostly irrelevant.

There are two key concepts to understand when attempting to keep a varied population of wild-type guppies. But before they are discussed, it’s probably better to get one issue out of the way first. It is likely impossible, no matter how many tanks are employed, to maintain the full genetic diversity of a wild species indefinitely while it is held in captivity. Combine that with the fact that for most hobbyists their population is started by only a few individuals; diversity is limited to begin with. This brings us to the topic of the <i>founder’s effect</i>.

To put it simply, the founder’s effect is a term for the phenomenon of a population being formed by a few individuals that break away from a larger population, and their genetic makeup is not representative of the population they come from. This is what often happens when wild guppies are collected. The founder’s effect is a way of describing when the secondary population has a different gene pool than the original population, because the individuals who formed it had different allele (specific genes) frequencies (ratios) than the original population, including the possibility of not having some alleles at all. One way to grasp this is that one might think of any true-breeding strain of fancy guppy as the result of the founder’s effect (this is an imperfect example). In this case it is selective breeding that has done the dirty work, but none-the-less the resulting population of fancy guppies has a very different gene pool and phenotype than the original population its wild ancestors belonged to. The way this translates to keeping wild guppies is that one starts with a random assortment of fish that may very well not be representative of the overall gene pool of the original population.

Now we can combine this idea with the concept of <I>genetic drift</i>. The founder’s effect and genetic drift can wreak havoc on the diversity of guppies. In short, genetic drift is a random process in which some alleles do not pass on to the next generation simply from chance, not from any sort of selective pressure. This is most likely to happen in small populations that cannot absorb the sort of random events that prevent individuals from passing on their alleles. For instance, in the wild populations of guppies are large and widely dispersed. Sometimes there is gene flow between populations, and sometimes populations may be cut off from each other for a long time, and then brought back into contact due to floods, changes of water courses, etc. What this means is that even if accidents kill some fish, the population(s) can maintain genetic equilibrium if it is large. But consider what happens with a population of only 30. Maybe the aquarium heater fails, frying half of the population and it just so happens all of the fish with the color blue where killed. The gene pool has just been changed!

One can clearly see that if we have the founder’s effect limiting our population diversity from the start, and with genetic drift lending a helping hand we will end up with fish that may not resemble their ancestors. What can an aquarist do? First, we have to accept that there is only so much we can do. For the hobbyist, the best thing to do is probably to keep a colony breeding together, while keeping a watchful eye on the phenotypes. If there are phenotypes (usually male coloring and patterns) that seem to be disappearing, a male can be selected to mate with a virgin female to ensure the male has some offspring. Technically this means selective breeding is happening, but only to maintain the diversity of the population. If females have not been selected out for this purpose (kept as virgins) all is not lost. Research has indicated that a male which mates with a female just after she drops fry will contribute the most sperm to the next batch of fry, regardless of the fact that female guppies can store sperm for up to eight broods (Hildemann and Wagner, 1954). In fact, the new male may contribute exclusively to the parentage of the next brood and successive broods. Since strict selective breeding is not as crucial in this context, this is a good alternative to isolating females by raising them separately for breeding.