Science Sundays: What's in a Species?

Welcome back to Science Sundays! A place for writers to learn a little about some random science stuff, and maybe find a way to work it into their stories :) Or, ya know, have a little fun learning something random for the week!

I have gotten some awesome suggestions for future posts, but would love to hear if you have any more--just shout it out in the comments!

Today I thought I'd talk a little bit about what a species is, how we define it, and all that fun stuff. And by species, I don't mean this:

I mean, more like this:

How do we define what belongs to one species, and what belongs to another? I see this regularly used to define a new species of mutant human, or alien, in plenty of books, so I think it might be somewhat useful.

Now, most of the time, people who see different organisms don't have too much trouble organizing them into groups that look similar to one another. But, there are grey areas where this can become kind of tricky. What do I mean? Well, what if two species look really alike? Or they can interbreed? Are they separate species then?

In general, there are two ways scientists break things down at this point:

The Biological Species Concept: where species are defined as separate when they can't produce viable offspring with one another.The Ecological Species Concept: where species are kept distinct through natural selection, which selects against any possible offspring.So, how does this work? Well, according to the BSC, species are distinct from one another when they can't interbreed (ya know, have reproductively  viable offspring). So, take a horse and donkey--they can mate and have mules, but mules are sterile and can't have offspring of their own, because horses and donkeys are different species. (I know there are some random and rare exceptions to the donkey/horse thing, but in general this is the case.) This is the most common and more generally recognized method for dividing up species.
According to the ESC, then, two different species may have offspring, but their offspring aren't going to be as fit and therefore are going to be selected against. Darwin's Finches are always used to illustrate these examples, so here's one: say there are two types of finches, one with large beaks and one with small. Now the two types can interbreed no problem, but they have medium beaked offspring, and those medium sized beaks don't allow them to do well in either of the environments of their parents. So, like they don't have a big enough beak to eat the hard seeds their big-beaked parent eats, and there aren't enough smaller seeds their small-beaked parent eats to support them. So, they're the odd man out and natural selection weeds them out of the population.
Make sense? Hope it's handy!