Charles Darwin observed that the offspring of a particular parent, although they resembled the parent, tended to differ from the parent in various ways. That is, the offspring were variable. Based on his observations, Darwin hypothesized that, because of their inherent differences, some of the offspring would be better than others at doing whatever it is they needed to do to survive and reproduce. Further, he surmised that if the differences that resulted in increased survival and reproduction were heritable (that is, passed from parent to offspring), they would be passed disproportionately to the next generation, and through time, this process would lead to changes in the species.
Darwin didn't pull his ideas out of thin air. He developed his theory of evolution during the period when rapid advances were being made in a variety of fields, including geology, selective breeding in agriculture, and biogeography (the study of the locations of different species).
Not surprising, scientists have learned a fair bit more about the natural world in the hundred-plus years since Darwin proposed his theory of evolution by natural selection. What is surprising is how well most of what researchers have learned since Darwin has been in agreement with his hypothesis. As scientists have developed a more complex understanding of the details of the evolutionary process, their confidence has only increased that the mechanism Darwin first proposed is correct. This section outlines what Darwin knew and some of the things scientists have learned about the evolutionary process since Darwin.
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