According to Darwin, the organic evolution is due to
According to Darwin, interspecific competition is a potent force in organic evolution. Interspecific competition is a form of competition in which individuals of different species compete for the same resources in an ecosystem (e.g. food or living space). The effects of interspecific competition can influence the evolution of species as they adapt to avoid competition. This evolution may result in the exclusion of a species in the habitat, niche separation, and local extinction. The changes of these species over time can also change communities as other species must adapt.
Ref. NCERT Textbook statement: When Darwin spoke of the struggle for existence and survival of the fittest in nature, he was convinced that interspecific competition is a potent force in organic evolution.