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What is Darwin's theory in a nutshell?
Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that increase the individual's ability to compete, survive, and reproduce.
How was Darwin's theory developed?
Unlike Lamarck, Darwin proposed common descent and a branching tree of life, meaning that two very different species could share a common ancestor. Darwin based his theory on the idea of natural selection: it synthesized a broad range of evidence from animal husbandry, biogeography, geology, morphology, and embryology.
How do sexual and asexual reproduction affect Darwin's theory?

Question: How can fossils be used to understand it
Self-contained rewrite: How can fossils be used to understand Darwin's theory?