Which different species can interbreed?
I know a lion and a tiger can.
Donkey + Horse = Mule
This site (LINK) talks about the interbreeding of red wolves and coyotes, which was threatening the existence of the endangered red wolves up until ecologists stepped in to prevent it from reoccurring.
Oh, and it also mentions the spotted-owl crossbreeding with the barred-owl.
Lion + Tiger = Liger or Tion
From this page: http://www.sierrasafarizoo.com/animals/liger.htm
“A note about ligers and tigons: A cat born to a tiger father and a lion mother is known as a Tigon. Tigons are very similar to ligers in many ways, but are quite a bit different also. We know of at least one documented instance of a tigon being born fertile. The offspring was fathered by a tiger so it was called a ti-tigon. We have been presented with other examples of tigon/liger ofspring, including a very nice looking female tig-liger.”
Something else interesting about ligers: they’re freakin’ huge.
http://www.lairweb.org.nz/tiger/ligers.html
http://luke.enlow.net/img/liger.jpg
Zonkeys, Ligers, and Wolphins, Oh My! | Answers in Genesis (creationist site, just look at the picture)
There exists a possibility that the red wolf arose from hybrid crosses of wolves and coyotes, did so very recently in time ( and possibly as an unwitting result of human alteration of the ecosystem ), and does not represent a “stable” species at all. It’s a controversial argument, for rather obvious reasons revolving around the major conservation effort that has been put forward on their behalf.
Obviously wolves and coyotes can interbreed and produce fertile offspring and they do so in broad hybrid zones in certain parts of North America. Those crosses are largely unidirectional for behavioral reasons, with female coyotes crossing with male wolves and the resultant offspring usually being brought up in a wolf pack and backcrossing exclusively with wolves. It does happen - but not so commonly or widely as to compromise the separateness of the two species as a whole ( with the red wolf being perhaps a notable exception - it may have arisen in the context of fragmented populations and habitat, when normal wolf social interaction was compromised across a wide area of the south ). Wolves and coyotes diverged some time ago and are far more genetically distinct than, say, wolves and the domestic dog ( which most authorities now consider a single species ).
The above is exhibit A) in the list of reasons why many systematic biologists ( folks that suss out evolutionary relationships ) are no longer so keen on Ernst Mayr’s old Biological Species Concept, the most commonly taught notion that species consist of naturally interbreeding populations.
Here’s an article from the “hybrid” point of view ( more or less, it leaves the possibility open, but this is one of the main researchers that first proposed the possibility ), discussing implications for conservation:
http://www.canids.org/PUBLICAT/CNDNEWS3/2conserv.htm
Here’s a pdf of an article attempting to rebut the “red wolf is not a real species” argument:
http://www.isu.edu/~bowenanc/mejournal/nowakfederoff.pdf
- Tamerlane
It’s more common than some people might think. Animals don’t really care about our notions of species boundaries. They’ll try to hump anything that sends the right signals, and sometimes it works.
Studying birds, for instance, can be very frustrating, because it can be extremely difficult to decide if you’ve discovered a whole new species or just another hybrid. There are also groups called “ring species,” where subspecies A (on the east coast) can succesfully breed with B, and B with C, etc, etc, but A cannot succesfully breed with Z (on the west coast). But A and Z are still considered the same species, because you can get gene flow between the two populations. That’s often cited as an example of speciation in progess.
Michael Jackson + his chimp, Bubbles = (you call it) 
Camel + Alpaca
Sheep + Goat
To name a few of the larger mammals that have been crossbred.
Male donkey X female horse = Mule
Female donky X male horse = Hinny
zebra X Horse = Zorse (for some reason this seems to always be a male zebra crossed on a female horse.)
American bison X domestic cow = Beefalo (Done to produce a butcher animal with a quieter disposition than bison but leaner muscle than the cow it is quite tasty in my opinion.)
I have a little quandary in my own fishtank–a Red Velvet swordtail female, possibly humped by other swordtails in the petstore, now enthusiastically humped by a male guppy… produces a lot of pearly pink fry. Maybe their color just hasn’t come in (they’re four days old) or maybe the livebearing toothcarps can interbreed?
Most, but not all, of these animal hybrids are infertile.
Sometimes the only thing stopping interbreeding between two species is behavioural;
if the species have different mating behaviours, they will avoid each other…
this will eventually develop into speciation with physical differences as well in many cases, but in captivity the two types of creatures may still hybridise.
I have little doubt that artificial genetic hybridisation will become common place in animals, just as it has been for a very long in plants.
SF worldbuilding at
http://www.orionsarm.com/main.html
Many species can interbreed, and many of the hybrids can be completely fertile. This is the case in many species of ducks, for example.
It is a common misconception that the Biological Species Concept requires that species do not produce fertile hybrids. The actual requirement is that they do not produce such hybrids in nature in such numbers or over such wide areas that the identity of each species breaks down. In other words, the BSC is based on the behavior of populations, rather than that of individuals. As long as two populations maintain their distinctiveness and rarely interbreed in nature, they can be considered distinct species even if occasional interbreeding produces fully fertile hybrids.
As Tamerlane mentions, there are significant problems with the Biological Species Concept, but I’m not going to get into that discussion here.