Retracing Darwin's path to his theory of evolution by natural selection; Alfred Russel Wallace's astonishing insight; the types of evidence, not known to Darwin, that accumulate in the century and a half since his time; extending his ideas.

Missing from "On the Origin of Species" is an account of how traits pass from one generation to the next; exploring the work on genetic inheritance by Gregor Mendel; 20th-century pioneers establish the modern synthesis of evolutionary biology.

The arrival of genetics in the early 20th century; the discovery of DNA as the carrier of genetic information; the realization that mutations and other structural changes in DNA are a source of the modifications that underlie natural selection.

Natural selection is not the only mechanism driving evolution; how the movement of individuals leads to gene flow between populations; how finches evolve into multiple species in the Galapagos archipelago; they stay a distinct species on Cocos.

Geology in Darwin's thinking; his observations that make sense only in light of the theory of plate tectonics; the breakup, movement and reassembly of continental plates that disperse related flora and fauna; the Wallace Line in Indonesia.

Exploring how population bottlenecks and the founder effect lead to random changes in the frequency of genes; Darwin has an idea of this process when he proposes that spontaneous variations impact evolution; genetic drift proves more significant.