The competitive exclusion principle states that

This principle in applied in ecology and also referred as Gause's law. 
The competitive exclusion principle states that two species that compete for the exact same resources cannot stably coexist. One of the two competitors will always have an ever so slight advantage over the other that leads to extinction of the second competitor in the long run.
Which can result as, competing related species often evolve distinguishing characteristics in areas where they both coexist. 
This aids in mate recognition, thus maintaining each species' superiority in exploiting slightly different ecological habitat.