Now in addition to the biological species concept, there are other definitions of what a species is. For example, there's the morphological species concept, which defines species based on morphological features, such as how they look, what sort of shared features do they have? These can both be sort of outward appearance things and more internal physiological similarities. There's also the ecological species concept, which defines species in terms of their niche. And that basically means their little carved-out role in their environment. So the nice thing about the ecological species concept is it really works for asexual species which you may not have realized. The biological species concept doesn't work so well for because the biological species concept kind of relies on sexual reproduction instead of asexual reproduction.

Now lastly, there's the phylogenetic species concept, and that's defining species based on the smallest group of individuals sharing a common ancestor on the phylogenetic tree and that tends to be a nice way of defining species, especially when you're trying to make broad stroke inferences and looking at both sexual and asexual organisms. So, how do organisms speciate? Well, one type of speciation, probably the easiest to understand, is called allopatric speciation, and this is speciation that begins with geographic isolation. "Allopatric" actually means "different country" in Greek. So the idea is that the species resulted from one species getting split and divided by geography. So here you see that we have this initial species of tree, but over the course of a really long time, and that's an important point to make, a river divides these two populations and due to the difference in their environments as a result of this division, they will begin to diverge until eventually they become two different species. There are many examples of this that you can look up if you're interested. Now, let's turn the page and talk about a different type of speciation.