Like the age of Earth, the theory of evolution -- first developed by biologist Charles Darwin in the mid-1800s -- is another subject that people tend to get worked up about. If you've ever seen the classic movie "Inherit the Wind," you probably already know about the infamous Scopes Monkey Trial of 1925. Famous attorney Clarence Darrow argued unsuccessfully upon behalf of a high school biology teacher named John Scopes, who was accused of violating a Tennessee statute that banned anyone from teaching that humans were descended from "a lower order of animals," and decreed that the biblical story of creation was the only acceptable explanation [source: Linder]. In recent years, it's been anti-evolutionists who've fought in court and in legislatures to require that children learn "creation science" in school, in addition to evolutionary theory [source: Raffaele].
And if there's an idea that particularly bugs anti-evolutionists, it's Darwin's central concept, which is called natural selection. It's really not a difficult idea to understand. In nature, mutations -- that is, a permanent change in the genetic blueprint of organisms, which can cause them to develop different characteristics from their ancestors -- occur randomly. But evolution, the longer-term process by which animals and plants change over multiple generations, is not up to chance. Instead, changes in organisms tend to become more common over time if the change helps the organism to better survive and reproduce.
For example, imagine that some beetles are green, but then, a mutation causes some beetles to be brown, instead. The brown beetles blend into their surroundings better than the green beetles, so not as many of them are eaten by birds. Instead, more of them will survive and reproduce, and may pass along the genetic change that will make their offspring brown. Over time, the beetle population will gradually shift to being brown in color. That, of course, is the simple version. In practice, natural selection is based upon averages, not specific individuals, and it's not quite as smooth and orderly of a process [source: UC Berkeley].