“It was an observation he made on the birds of the Galapagos Islands that led him to the branching theory. The Galapagos Islands are actually peaks of submarine volcanoes that have never had a land connection with South America or any other continent. All of the Galapagos fauna and flora got there by over-water (distance) colonization. Darwin knew that there was only one species of mockingbird in South America, but he found a species of mockingbird on each of three islands in the Galapagos, with each species different from the others. He concluded quite rightly that a single colonization of the South American mockingbird had given rise, by branching descent, to three different species on three different islands in the Galapagos. Then, he further reasoned that probably all mockingbirds in the world had descended from a common ancestor, because they are basically so simlar to each other. Mockingbirds and their relatives, like thrashers and catbirds, then presumably also had a common ancestor.

This chain of inferences led Darwin to the ultimate conclusion that all organisms on Earth had common ancestors and that probably all life on Earth had started with a single origin of life.”

~ Ernst Mayr, What Evolution Is, Basic Books, New York, 2001

Notes by Robert Curry concerning Dr. Mayr’s quotation:

• There are actually several species of mockingbirds in South America, at least three of which Darwin had observed in Argentina and Chile before arriving in Galápagos
• Darwin visited a total of four islands in the Galápagos (San Cristóbal, Floreana, Isabela, and Santiago). He observed and collected three of the four species of mockingbirds now recognized in the archipelago; one of these three species was the same on two of the islands … although both Darwin and John Gould were confused and incorrect about which of the islands shared a common species. The fourth endemic species (on Española) was not discovered until many years later.
• It is not necessarily true that the ancestor of today’s Galápagos mockingbirds came from South America: another possibility, supported by phylogenetic analyses (Arbogast et al. 2006, Lovette et al. 2011), is that the ancestor came from further north (western Mexico).

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Darwin’s mockingbird: the endemic mockingbirds of the Galápagos
http://robertcurrylab.com/darwins-mockingbirds/
© Robert L. Curry
Page last updated: 22 Sep 2016