After darwin proposed his ideas about natural selection as a mechanism for evolution, it took time before evolution was generally considered a theory. why do we now refer to it as a theory?

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Enough supporting evidence has accumulated for it to be called a theory.  Darwin did not invent the idea and he did not observe and collect all of the supporting evidence on his own. However, he carried out the research necessary to document that evolution has occurred, proposed a plausible mechanism (natural selection) to explain why and how it occurs, and then published the theory and evidence in order to make it available to other scientists and to the general public. Unlike Lamarck's idea that individuals alter their traits based on the environment, Darwin proposed that variations already existed within a population and those individuals with the more favorable, or useful, traits survive to reproduce. These traits then become more common in the population, leading the population to evolve slowly over time.

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