The Darwinian revolution was the first revolution in biology. University of Wisconsin--Madison’s Sean B. Carroll traces the discovery of evolution through Charles Darwin's long voyage, many discoveries, and prodigious writings. Darwin introduced the concept of the 'fittest,' but how are the fittest made? The second revolution in biology was triggered by discoveries in genetics. Genetic variation, selection, and time combine to fuel the evolutionary process. The action of selection is now visible in DNA, both in preventing injurious changes and in favoring advantageous changes in traits. From the Series:HHMI's Understanding Biomedical Research Series