Quoting from page 308: The Competitive Exclusion Principle. No two organisms that compete in every activity can coexist indefinitely in the same environment. To coexist in time, organisms that are potentially completely competitive must be geographically isolated from each other. Otherwise, the one that is the less efficient yields to the more efficient, no matter how slight the difference. When two competing organisms coexist in the same geographical region, close examination always shows that they are not complete competitors, that one of them draws on a resource of the environment that is not available to the other. The corollary of the principle is that where there is no geographical isolation of genetically and reproductively isolated populations, there must be as many ecological niches as there are populations. The necessary condition for geographical coexistence is ecological specialization.
Quoting page 86: The Exclusion Principle in biology plays a role similar to that of the Newtonian laws of motion in physics. It is a prime guide to the discovery of facts. We use the principle coupled with an axiom that is equally fundamental but which is almost never explicitly stated. We may call this the Inequality Axiom, and it states: If two populations are distinguishable, they are competitively unequal.
Quoting page 87: Because of the compound-interest effect, no difference between competing populations is trivial. The slightest difference--and our acceptance of the Inequality Axiom asserts that a difference always exists--will result in the eventual extinction of one population by another. Put in another way, the Exclusion Principle tells us that two distinguishable populations can coexist in the same geographical region only if they live in different ecological worlds (thus avoiding complete competition and strict coexistence).
Quoting page 88-89: Recall now the sequence of development in the process of speciation. Initially, the freshly isolated populations are nearly the same genetically; as time goes on, they diverge more and more. When they are distinguishably different, but still capable of interbreeding (if put together), we may speak of them as races. Ultimately, if the physical isolation endures long enough, they become so different from each other that interbreeding is impossible; we then say that the two populations are reproductively isolated from each other, and we speak of them as distinct species. ... What are the various possible outcomes of the speciation process, and what their relative frequencies? In the light of our assumption, it is clear that, most often, the speciation process will go no further than the formation of races before the physical isolation comes to an end and the germ plasm of the two races is melded into one by interbreeding. If, however, the speciation process continues until separate species are formed before the physical barrier breaks down, then what happens? The outcome is plainly dependent on the extent to which ecological differentiation has occurred: Do the two species occupy the same ecological niche, or not--that is, are they completely competitive? It seems probable that the degree of ecological differentiation will also increase with time spent in physical isolation. On this assumption, we would predict that, more often than not, "sister species" will be incapable of coexistence: when the physical isolation is at an end, one sister species will extinguish the other.
Quoting page 253: The example illustrates the general rule that as a species becomes increasingly "successful," its struggle for existence ceases to be one of struggle with the physical environment or with other species and come to be almost exclusively competition with its own kind. We call that species most successful that has made its own kind its worst enemy. Man enjoys this kind of success.