Some interesting definitions so far
You both suggest to define a species by reproductive isolation as it were, all organisms that can have fertile offspring together form a species. This makes sense because you would think that these will evolve independently from all other organisms and indeed that's pretty much what's traditionally now thought of as the biological species concept.
There are some deep underlying problems with this species concept, but I think I'll wait until a few more definitions come in before I comment any more on them.
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Originally Posted by kayleigh.
Why is the mule sterile? Why is a "Liger" sterile? I was never taught that and I always wondered why two animals from the same family couldn't interbreed (well they can technically) but they cannot produce fertile offspring? It's always confused me.
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That's a deceptively difficult question to answer. As you've probably guessed, it has to do with genetic stuff. You probably know that animal genes are organized into long strings of DNA and associated proteins called chromosomes. Diploid animals have one set of chromosomes from dad and one from mum. These chromosomes are copied every time a cell splits into two. It's quite simple, you get a duplicate of each chromosome and they align themselves at different sides of the cell core (nucleus) and then the whole thing splits down the middle and you get two new cells with the same two sets of chromosomes each. That's called mitosis.
However, when sperm and eggs are created, there's a slightly different process called meiosis. Instead of one cell dividing to two, one cell shall become four sperm or four eggs, each containing only half the amount of genetic material as the parent cell. They will be haploid, containing only one set of chromosomes with some from mom and some from dad. To ensure that each new egg/sperm cell gets a nice mixing of genes and chromosomes from both mom and dad in that 1 set, there's a step in meiosis where homologous chromosomes from mom and dad pair up and exchange genetic material.
When two organisms that are very dissimilar create a hybrid (f.ex horse and donkey), the diploid mule offspring's cells can have two very different sets of chromosomes. When meiosis is to take place in the hybrid, chromosomes can have a hard time pairing up with their homologues from the other dad/mum set or the mixing between them creates problems .. possibly because their homologues in the other set are so different or even because they're simply not present! Horses and Donkeys have different amount of chromosomes which would result in some chromosomes being unable to pair up with homologues. This can cause a multitude of problems, many which end in reduced ability to reproduce and even sterility.
I'm not sure if that made it understandable. Possibly, this
meiosis illustration might help a little bit.
edit :
Gurgh, had to brush up a bit on that explanation. I think it's understandable now.
edit 2 :
By the way, some "species" do hybridize quite a lot in nature, for example many birds. Hybrids are not always sterile, but may simply have lower reproductive ability which translates to lower fitness.