Introduction

All species of living organisms, from bacteria to baboons to blueberries, evolved at some point from a different species. Although it may seem that living things today stay much the same, that is not the case—evolution is an ongoing process.

The theory of evolution is the unifying theory of biology, meaning it is the framework within which biologists ask questions about the living world. Its power is that it provides direction for predictions about living things that are borne out in experiment after experiment. The Ukrainian-born American geneticist Theodosius Dobzhansky famously wrote that “nothing makes sense in biology except in the light of evolution.” ^{Theodosius Dobzhansky. “Biology, Molecular and Organismic.” American Zoologist 4, no. 4 (1964): 449.} He meant that the tenet that all life has evolved and diversified from a common ancestor is the foundation from which we approach all questions in biology.