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In the nineteenth century, biologists recognized that animals and plants possess traits that can be beneficial (e.g., increase strength) or detrimental (e.g., slower growth). Those with a beneficial trait may be more likely to survive, and those with detrimental traits may be less likely to survive. The essence of this paradigm has become known as natural selection.To read the rest, click on "Just How Random Are Mutations?"
Charles Darwin understood that sometimes the traits of various organisms can change. However, since his studies predated the field of genetics, there was yet no understanding of how these changes occurred. Instead, he attributed such changes to the effects of natural selection, as if natural selection somehow magically could cause traits to appear.
In fact, not bound by any laws of genetics, Darwin made a lot of assumptions. One key assumption was that there was “no reason to limit” how much organisms could change their traits.1 With no limits, he further assumed that such changes can dramatically transform fish into amphibians or reptiles into mammals. His presumptions provided the basic outlines of universal common descent—the idea that all life forms have arisen from a common ancestry.