what is stated in this theory?
He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".
Natural selection
. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved". 
the process of natural selection principles and applications It is the differential survival and reproduction of individuals due to differences in phenotype. what does this selection refers to? A mechanism of evolution. what do you mean by this last term? It is change in the heritable characteristics of biological populations over successive generations. is there is any specific theory related to the previous selection process? Yes, Darwin's theory.
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Natural selection [SEP] Darwin's theory
In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved". The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them and that variation is heritable, there will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to a progressive evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.