sirdarksol

I've noticed that there are several questions about what hybrids are in this section, so I thought I'd give a few answers. (Much of this information is going to be pulled from other threads in this section). It also seems that there is some confusion about the difference between hybrids and genetically engineered fish.

First of all, a couple of definitions that will be used:
Species is the most specific name for something (look at the words "species" and "specific"). For example, lyretail molly is a species and a balloon molly is a species (albeit an artificial one). Species is sometimes determined by two similar animals that are capable of cross-breeding easily, but, for some reason, don't do so in the wild.
Genus is the name of a group that encompasses a number of species. For example, mollies.

A hybrid is a creature that was conceived with the sperm and egg of two creatures that are, at the very least, of separate species.
There are different types of hybrids:
Intraspecific hybrids are born of two separate sub-species. This isn't terribly uncommon, even in the wild. Most sub-species have little signs that make breeding with others of the same sub-species more likely, but if there isn't an available mate, different ones will turn to one another.
An example of an intraspecific hybrid would (I believe) be a Dalmation Molly, which is a cross between a white molly and a black molly. (Yes, this is different from information I gave before. I've done more research since then)

Interspecific hybrids are born of two separate species. As suggested above, this is unlikely to happen in the wild, though it doesn't take very much luck if the conditions are right. If two members of the same genus but different species are of the opposite sex and have no other potential mates in the area, they may mate.
An example would be the Tiger Muskie, a breed that was created for fishermen. Muskies are one of the largest carnivorous freshwater fish, but they don't put up much of a fight until they get to the boat. Northerns are smaller than Muskies, but they put up a huge fight. The idea was to create a larger fighter, and it worked.
Most interspecific hybrids are viable, which means that the hybrid creature is fertile, and can produce a population (presuming there is a member of the opposite sex around) of similar hybrids.
Some, however, are sterile. A mule, for example, is usually sterile.

Intergeneric hybrids are born of two creatures not only of different species, but of different genus'. This is very unlikely to happen in the wild, and even if two such creatures did happen to mate (perhaps between two fish that breed by spreading eggs/sperm widely), it isn't terribly likely that any offspring would result. Further, if there are offspring, there is a very strong possibility that the creature is sterile.
The only intergeneric hybrid that I can come up with is a sheep-goat hybrid. Most of these hybrids die before the embryo grows very much.

Inter-familial hybrids are even more rare. Because of the severe genetic difference between families, embryos coming from these pairings are almost never viable, and also are almost never fertile.
Apparently, there is some sort of hybrid guineafowl that is an inter-familial hybrid.

Methods of hybridization
Hybrids can happen in the wild, or they can be artificially created.
In the wild, such a pairing could be accidental, as in fish who spread eggs/sperm as a method of mating. Another possibility would be that two animals have a lack of more viable mates, or there could be a hormone gone wrong in one or both of their brains (as with the moose that fell in love with a cow).
In artificial creation, the simplest method is to remove the existence of other mates. Eventually, it is possible that two different creatures will mate in an attempt to continue some form of their bloodline. The closer the two creatures, the better the chances of this happening. Goats and sheep are very frequently kept together, and yet there are very, very few occurrences of them even attempting to mate.
Another method is artificial insemination, which is more likely to work than the above method, though it still is partly up to luck and the difference between the two creatures.
The most reliable method is in vitro fertilization. This removes any chance on the part of fertilization, but an incompatible genetic mix will still not take.

Hybridization has been happening for centuries, possibly millennia. Every domesticated dog, cat, and farm animal is a creation of hybridizing and inbreeding.

Genetic engineering is generally not natural at all. It involves cutting out a strand of DNA and replacing it with another. The most common method of doing this is with a combination of enzymes that recognize particular sequences and cut or paste them. These enzymes, as far as I know, are all artificially created.
The other method of genetic engineering (and one that I don't think has been too well tapped by human scientists, thank goodness) is probably more a matter of science fiction, but it does happen in real life. Viruses modify the DNA of a host cell, changing its function.

The glo danio is an example of a genetically engineered fish. Using a gene from a fluorescent coral and pasting it into the common zebra danio's DNA, the resulting fish is a bright color and fluoresces under black light. The original idea was to create a fish that glowed when exposed to certain levels of toxins. Release a school of glo danios in waterway, and you'll be able to tell if there's a toxic buildup.

For better or for worse, most of our food has now been touched by genetic engineering. A lot of corn has had hundreds of various traits added to it by major agricultural companies, from a chrysanthemum gene that causes the corn to produce a natural insecticide to a gene from an arctic fish that allows the corn to survive cold snaps.

Genetically engineered organisms, unlike hybrids, are very often fertile. Usually, if they aren't this trait was purposefully bred in. This fertility has its upsides and downsides.
Because they are fertile, such organisms, if released, may be able to inhabit the wild. In some cases, this may be preferable. For example, the coral reefs are dying because the water temperature is changing. These reefs could be altered to survive warmer temperatures, and then "seeded" in the original reefs.
In other cases, this can have horrific results, overwhelming natural species and drastically changing an ecosystem.
In the case of glo danios, those developing the fish did not worry about releasing them into the wild because the fish are painted like giant targets, and would almost positively be eaten before they could populate a body of water. Some people believe that even this small chance is too much to be taken (I'm not going to get into that now, and if you're going to, please bring it to the "Hot Topics" section).
However, in the case of corn, the engineering is potentially harmful. Because there's nothing stopping the wind and bees from carrying pollen from one field to another, crops separated by miles may be cross-pollinated. One engineered trait is a "suicide gene" which causes a family line to self-terminate after so many generations. In essence, the second, or third, or fourth, or whatever, generation of the organism is sterile.
Small family farms that have produced corn that was created through careful hybridization are finding their corn dying as it is exposed to corn with this suicide gene. It is very likely that even organic corn has engineered traits in it, since there's no way to keep cross-pollination from happening.

It should be noted that (I have heard at least) it is illegal to own genetically engineered critters in the EU. Again, whether or not this is a good thing is debatable, but please not here.

bhcaaron

Wow! Thanks! I love this type of info!

Butterfly

Thank you sirdarksol that was very informative.
Carol