Pretty straightforward eh?! Unfortunately Science is never quite a clear cut as that. Two closely related species can often breed and produce hybrids. An example of this would be a Zeedonk made from a Zebra and a Donkey. Hybrids are neither one species nor another and have characteristics of both parents. Problem is that they are usually infertile.
Makes sense really - well nearly. Remember, it is not just about animals. Many plants and fungi produce new offspring from parts of the adult. Think of spider plants with the 'baby spider plants' hanging off them. Each baby plant just has one parent! This is asexual reproduction.
Also, bacteria and many protoctists reproduce by just splitting in half - again there is only one parent.
So, basically, if there are 2 parents then you could get a hybrid - a lion and a tiger would make a Liger and if there is only one parent then interbreeding can't happen.
Did I mention ducks - no, but I will. Ducks, in particular Mallard ducks can hybridise with closely related duck species to produce fertile hybrid offspring. These hybrid offspring can then breed with other hybrids or other hybrid ducks so you end up with lots and lots of ducks with a continuous range of shapes and colours (different characteristics) but they are actually the same species.
Sticking to the bird theme we have examples where you might have groups of the same species in a population may breed with a nearby similar population which is slightly different. This population might breed with a different closely related population to it and so on. Hold on a minute, just read that last bit and it was as clear as mud. Lets think of it in a Maths way.
A B C D E F G
A can breed with B, B could interbreed with C, C could breed with D, etc, but A could not interbreed with G because they are too different. Sometimes this chain of inbreeding actually forms a ring where there is not end but species near each other can interbreed. These are called ring species.
Sea gulls form ring species and because there is a gradual change between the characteristics of the gulls it is really hard to decide if there are different species or still hybrids. Aaahhhh!!!!!!!
