I don't know about you, but I definitely see a horse as a different species from a donkey and a tiger definitely different from a lion!
So, what's the deal?
If we define species by the BSC, where "species cannot interbreed"... are these seemingly distinct species actually ONE? Are lions and tigers one species??
As with ALL science, rules are hard to make for nature!
If we rely on the biology of one "species" to differentiate it from others, there are always exceptions to the rule! In science as a whole, there are almost always exceptions to the rules!
Maybe that leaves a bad taste in your mouth. How do we know anything then? What's the point of science if it can't answer "basic" questions??
Well, yeah... How do we know anything? The answer is, we don't know anything for sure. A scientist will never tell you they are 100% sure of anything. We are humans and we are making the world around us into understandable parts. We see the diversity of life on earth and we want to name and categorize things. To do that, we use a system. Unfortunately, time does not stand still and things are always changing. The biological species concept does not take into account these kinds of things. There are other species concepts who do (evolutionary and phylogenetic species concepts, for example), but even those are flawed.
Maybe we get ligers and tigons because they are really closely related and haven't been separate species long enough. It takes TIME, lots and lots of time, for these kinds of changes to "be set in stone".
But, hey, that's one of the most amazing things about studying life on earth! There is no creation of a species. There is no "BAM!" you're a tiger and will always be a tiger.
We are trying to figure things out as we go. We are making theories and testing them. And, interestingly, we are hanging onto theories such as the biological species concept even though there are obvious exceptions.
So what are your thoughts?