Which of these naturalists synthesized a concept of natural selection independently of darwin?

The answer is Alfred Wallace. 
Darwin began formulating his theory of natural selection in the late 1830s but he went on working silently on it for twenty years. He wanted to amass a wealth of evidence before publicly presenting his idea. During those years he corresponded briefly with Wallace (right), who was exploring the wildlife of South America and Asia. Wallace supplied Darwin's help publishing his own ideas on evolution. He sent Darwin his theory in 1858, which, to Darwin's shock, nearly replicated Darwin's own.

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