Natural selection was Darwin’s key insight, the idea that truly made him famous. The idea of evolution existed before he was born, but there was no credible mechanism. Darwin, and Alfred Russell Wallace, uncovered that mechanism.
Natural selection has been defined as “the non-random survival of randomly varying replicators.” Before I explain what that means, I want to explain how Darwin came across this insight. (Before I go further, I should note that, while natural selection is the primary force driving evolution, sexual selection and genetic drift also have an impact).
On the Origin of Species is filled with what we might term comparative biology. Especially about pigeons. Artificial selection is the process by which human breeders create different breeds of an animal, such as the dozens of species of dog we now have. In the mid-19th century, pigeon breeding was fairly popular. Darwin was fascinated by all of the species of pigeon and began to wonder where they all came from. Based on their physical similarities, he discerned that all the domestic pigeon breeds must have originated from a single species of wild pigeons. The reason they look different is that different lines have been artificially selected for different traits. In the Origin, artificial selection (why pigeons look different) acts as a key metaphor for natural selection (why all birds look different).
We can think of natural selection as artificial selection without conscious forethought. It is deterministic, in that its outcomes are predictable. In other words, natural selection is not random. What are random are mutations in DNA, the “replicators” in the above quote. Or maybe not. read more »