Charles Darwin's insights into the nature and mechanism of the evolutionary process were built on careful observation of the patterns of variation across the natural world. In On the Origin of Species1, Darwin brought together evidence from an incredible diversity of taxa to support his case. But one major group notably lacking from this work, the microorganisms, has since provided some of the best experimental support for the theory of evolution by natural selection2. In this Review, we examine how microorganisms, and in particular evolution experiments using microorganisms, have helped to unlock the details of the adaptive process. In particular, we highlight some of the most important insights that this approach has provided, those that we think would have excited Darwin, and we discuss the important role that this approach will continue to have in furthering the understanding of organic diversity.
When Darwin developed his theory of natural selection, he assumed that evolutionary change would usually be slow and, in general, not directly observable. As he wrote in On the Origin of Species: "We see nothing of these slow changes in progress, until the hand of time has marked the long lapses of ages, and then so imperfect is our view into the long past geological ages that we only see that the forms of life are now different from what they were."
