Evolution is one of the foundational principles of all of biology. Theodosius Dobzhansky, an important early leader in the field of evolutionary genetics, once remarked, “Nothing in biology makes sense except in the light of evolution.” Indeed, evolution is an all-encompassing theory that helps to make sense of much of the natural world, from the sequences of DNA found in our cells to the types of organisms that surround us. The evidence for evolution is overwhelming. Evolution has been directly observed numerous times; for example, hundreds of different insect species evolved resistance to common pesticides introduced after World War II. Evolution is supported by the fossil record, comparative anatomy, embryology, the distributions of plants and animals (biogeography), and molecular genetics.