what is the role of darwin in it?
He defined it 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".
how does natural selection work as a mechanism of evolution A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that results in fitness differences.
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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.