The arrival of genetics in the early 20th century addressed what Darwin did not know about inheritance, but there was more to uncover: How do genes function, and where do variations come from? Trace the discovery of DNA as the carrier of genetic information and 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. In this episode, discover how the movement of individuals leads to gene flow between populations. Travel to the Galapagos Islands and neighboring Cocos Island to see how finches evolved into multiple species in the Galapagos archipelago but stayed a distinct species on isolated Cocos. Consider the implications for human evolution.

Trace the importance of geology in Darwin’s thinking and his many observations that make sense only in light of the theory of plate tectonics, which was not developed until the 1960s. Chart the breakup, movement, and reassembly of continental plates that dispersed related flora and fauna all over the planet. Also look at the Wallace Line in Indonesia, which separates Asian from Australian species.