New species form in the Galápagos when populations of the same species diverge from each other on different islands, then later come together on the same island. This program reveals the barrier to interbreeding between different species of Darwin's finches. It examines why the barrier sometimes leaks and the consequences of rare gene exchange between species. The premating barrier to interbreeding is based on song learned early in life in association with parental appearance and used later in life when choosing a mate. Hybridization is rare, but can occur between species of similar size when a male learns the song of another species. On Daphne the Medium Ground Finch occasionally hybridizes with the slightly larger Cactus Finch, but never hybridizes with the much larger Large Ground Finch. Hybrids survive the dry season when there is food available that is appropriate for their intermediate beak size. They breed, not with each other but with one or the other of the parental species, and with high success. In this way genes flow from one species to another. If unchecked by natural selection hybridization can lead to the fusion of two species into one.