Some species share the exact same territories, rely on the exact same resources, and are sufficiently closely related that they can easily interbreed. So why don't these just merge into a single population? Because they simply don't want to interbreed.
Our current theories about biodiversity have trouble explaining just how a bunch of very closely related species, such as fish, can all exist in the same small habitat - a lake, say - and yet maintain their biological distinctiveness. After all, biodiversity is primarily driven by the different adaptation required to survive in various ecological niches. But if the habitat itself becomes too uniform, then surely one species will gain an evolutionary advantage and, eventually, drive all the other similar species to extinction.
The fact that that simply isn't the case in many habitats with lots of closely related species drew the attention of researchers at the University of British Columbia. They found that sexual selection - in which females choose which males they do and don't want to mate with - could actually be enough to explain why biodiversity can endure even in such close quarters between species. As long as females are picky about who they want to mate with, the different populations and species can keep co-existing. Leithen M'Gonigle, now at UC Berkeley, explains the two basic ideas at work here:
"Our model shows that species can stably coexist in the same habitat as long as two simple conditions are met. First, the distribution of resources they use must not be uniform, so that groups of females with different mate preferences can occupy different resource hotspots. Second, females must pay a cost for being choosy, through reduced survival or fecundity."
Even areas as seemingly monotonous as lakes or grasslands will have these hotspots, so meeting the first condition isn't a problem. The second is a bit trickier, but the idea is that females typically have to expend energy either to find the mates they want or to avoid those they don't. That tends to keep females from entering the territories populated by what they see as ugly males, which collectively works to maintain the biodiversity of the habitat.
Even though the populations share the same ecological wants and needs, the drive to avoid unattractive potential mates is enough to keep them separated indefinitely. You know, no one ever said evolution was something to be proud of...