Look up the word evolution in a dictionary, and you'll come across a definition that says something about change or maybe change through time. That's good as far as it goes. But in the context of biology, evolution refers to specific changes — genetic changes — in a group of organisms through time. That concept isn't so hard to grasp, but you may be surprised by how revolutionary the idea of evolution was in the mid-1800s, when Darwin came up with his theory explaining what could cause such changes (natural selection).
Back then, the concept that species could change over time — even the concept of vast time spans — was foreign and frightening to most people. But facts are facts, evidence has a way of piling up, and the science of evolutionary biology has progressed in the century and a half since Darwin's major insight.
This part introduces the key principles of evolution by natural selection. And because to grasp the main idea, you need to know a bit about genetics, the part includes a brief discussion of that topic, too. If it makes you feel any better (and it should), reading this short discussion of genetics puts you in the position of knowing more about genetics and heredity than Darwin himself did.
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