But, unqualified as I am (my degrees are in history not biology or genetics) I would like to address a couple of speculations put forward by edathome and someone else on another one of these hybrid threads. If a reader of this has
more experience or training in these areas, please feel free to jump in. I would and others certainly would value your insights.

Part of our problem is that the word hybrid is used in a couple of ways. The following is something I just Googled. I added the numbers before the definitions. Nothing else was changed.

http://www.biology-online.org/dictionary/Hybrid

Hybrid
(Science: biology) 1. An offspring of parents from different species or sub-species. 2. An organism that is the offspring of genetically dissimilar parents or stock; especially offspring produced by breeding plants or animals of different varieties or breeds or 3. species; a mule is a cross between a horse and a donkey. Produced by crossbreeding.The creation of offspring from two genetically dissimilar parents. 4. Two organisms are crossed with different desirable characteristics with the premise that the offspring will possess more of the desirable characteristics. See dominance and selective Breeding for related information.

Definition 1 and 3 refer to trying to cross breed two different species. 2 and 4 refer to crosses within a species. For example two dogs, perhaps a collie and a Labrador retriever are breed to one another. You may have driven past a corn field which had a sign talking about hybrid corn. Again these are where two or more strains of corn or maize or scientifically Zea mays were crossed.

An amazing number of viable crosses between very closely related species have happened in nature. Maybe even more have taken place in captivity - in aquariums or in controlled experiments in laboratories. In the discussions considering guppy x platy and guppy x Gambusia (either affinis or holbrooki) I have suggested that those who have studied livebearers would suggest that those species are not closely related and weren't likely to have effectively crossed, even though males might have courted and pursued females of the other species.

On the other hand, one or the other of those Gambusia species above were unfortunately introduced into waters in Texas containing Gambusia amistadensis. There were some crosses. Fry were devoured. That species is now considered extinct.

Introduction of those first two Gambusia and other exotics have about driven the Big Bend gambusia, Gambusia gaigei nearly to extinction. It exists in one pool.

Likewise, despite some lack of success, various Xiphophorus species have been crossed in the laboratory. The thoughtless dumping of platys (X. maculatus) in northern Mexico have resulted in the extinction of one of the genus, I think it was Xiphophorus couchianus. Where wild species overlap in territory, despite a few crosses, different habitats, different courting displays and perhaps genetic have kept the species (so far as we know) going. When people start stirring species together, species begin going extinct. And that of course is only forever. :(

Alleles are the different genes on a chromosome. They are usually paired. If the two genes are appropriate for the species but a little different from one another, they are spoken of as heterozygous. A certain amount of heterozygousity or genetic diversity may be beneficial to a species. Just the right new combinations may produce a trait which makes the organism fitter or more likely to survive.

http://www.biology-online.org/dictionary/Allele
Allele

(Science: genetics) Any one of a series of two or more different genes that occupy the same position (locus) on a chromosome.

Since autosomal chromosomes (chromosomes with genes which do the same things) are paired, each autosomal locus is represented twice. If both chromosomes have the same allele, occupying the same locus, the condition is referred to as homozygous for this allele.

If the alleles at the two loci are different, the individual or cell is referred to as heterozygous for both alleles. One of two alternate forms of a gene that can have the same locus on homologous chromosomes and are responsible for alternative traits; some alleles are dominant over others. Pertaining to the dna expression of a gene in a chromosome for a particular characteristic.

I think it was Noboru Iwasaki, in his book Guppies: Fancy Strains and How to Produce Them (TFH 1989) who noted that there had been a little genetic drift in that, under magnification, he felt that the shape of the gonopodiums in European raised domestic guppies was a little different from those on Asian raised guppies. But it didn't keep them from interbreeding. There were some alleles which were becoming a bit different.

Biologists have noticed that individuals of a species from two locations, geographically quite far apart, may not be quite as fertile as individuals from closer habitats.

That is interesting because biologists have been very anxious for some species if their genetic diversity is very limited. They see a genetic roadblock as the possible end for some species - the Cheetah for instance. And yet some small populations of endangered fishes has endured for a long time with just a few hundred individuals and maybe less than 100 breeding age adults at any one time. (One of the more celebrated cases is that of the Death Valley's Devil's Hole pupfish (Cyprinodon diabolis), which may have been isolated in that kind of population, living connected to a geothermal (92 °F), aquifer-fed pool and feeding from an algae shelf not much bigger than your living room for 10,000 years.)

If the commons and fancies were different species, then indeed I would expect infertility and possibly a shorter lifespan. That isn't the fault of our commentators but of the confusion over what a hybrid is.

The level of fertility is an interesting issue. It may be that a large common guppy could only drop 30-40 fry. The larger fancy females might drop twice that. Each guppy in the cross would have alleles from the common and fancy parents. Would their size be large or small? If one of those is the case, that gene was dominant. If the size of the guppy offspring was in between that of the parents, one could speak of incomplete dominance. That happens quite a bit. I would guess that whatever number of ripe eggs are carried by a female, almost all of them would be fertilized though since both parents are the same species.

The question of life expectancy was also raised. The guppies are the same species (Poecilia reticulatus, though at one time it was Lebistes reticulatus. The genus was changed as the result of research several decades ago. With more data, some scientists want to return guppies to  Lebistes reticulatus, though you may start seeing  Lebistes (Poecilia) reticulatus. Whatever they are scientifically called, they are the same species.

It could be that the common guppies have a life expectancy of under a year. The fancies might have an average life expectancy of a year and a half. If that is controlled by a specific allele, it would be interesting to see what would happen.

They also speak of hybrid vigor in crosses, especially those within a species. You might be familiar with dogs where a cross is often better behaved, less hyper, healthier and longer lived that the parent species. Though it wouldn't have been fertile, I was astonished by a killifish cross (Fundulopanchax walkeri x Fundulopanchax gardneri) where the male was larger than males from either species and since he combined the male coloration carried by both species, more colorful.

So it would be interesting to see which of the parent strains the offspring favored. My guess is that they would be intermediate between them and incomplete dominance seem to rule that situation. I have seen that with crossed strains. Does that mean that is what would happen this time. Time would tell. We'd have to see.