The Principle of Competitive Exclusion in Population Biology

It is often observed in nature that the struggle for existence between two similar species competing for the same limited food supply and living space nearly always ends in the complete extinction of one of the species. This phenomenon is known as the “principle of competitive exclusion,” and was first enunciated, in a slightly different form, by Darwin in 1859. In his paper, “The origin of species by natural selection,” he writes:

“As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitutions and always in structure, the struggle will generally be more severe between them, if they come into competition with each other, than between the species of distinct genera.”