THE GIANT TREE CACTI of the Galapagos islands struck Charles Darwin as unusual when he first saw them on The voyage of the H.M.S. Beagle in Today, these strange plants provide evidence for 1) Darwin’s theory of origin of new species, and 2) how natural selection produces adaptations to the environment over many generations.

Evolution is the foundation of modern biology. In this series of lessons, “The Case of the Disappearing Cacti”, you will 1.Discover how biogeography supports Darwin’s theory of how new species form. 2.See how natural selection produces adaptations over many generations 3.Investigate the ecology and population biology of Galapagos cacti to search for causes of the decline of an endangered species.

When Darwin visited the Galapagos, he was a special creationist* *SPECIAL CREATIONISM *SPECIAL CREATIONISM holds that each species was created separately in its modern form and placed in its current location by God. “It will be very interesting to find from future comparison to what district or "centre of creation" the organized beings of this archipelago must be attached.” - Darwin’s diary Sept. 26 th

Darwin saw that the Galapagos Archipelago was volcanic and appeared to have been thrust up from the sea bed by volcanoes. La Cumbre, Fernandina 2009 (AP Photo/Galapagos National Park) Evidence of past vulcanism  Evidence of continuing vulcanism 

Darwin put together: “..we are led to believe that within a period geologically recent the unbroken ocean was here spread out.” The strange species he saw must have come into existence afterwards. “Hence, both in space and time, we seem to be brought somewhat near to that great fact, � that mystery of mysteries, � the first appearance of new beings on this earth.” -Voyage of the Beagle

Darwin reasoned that the organisms that colonized the islands would be isolated from others of their species, and might change independently in their new homes. HYPOTHESIS: If new species arise by evolution in isolated populations, then we should find many examples of isolated islands with endemic species (species found there and no where else).

Darwin found several examples of this pattern of endemism : In the Galapagos compared to South America and Polynesia, and On the individual islands in the Galapagos.

4 species of mockingbirds, but only one per island. Natural Selection and Adaptation Mockingbird Species Espanola Floreana Galapagos San Cristobal

Darwin was told that the tortoises on each island had different shaped shells. But he did not keep records of the islands. He and his comrades ate and tossed the evidence! Tortoises were killed for food and for the water in their bladders (slightly bitter but not salty). Remains of tortoise slaughter

14 species of tortoises (3 extinct) are now known, one per island, except for Isabela, which had 5, one for each somewhat isolated volcano. Climate – El Ninos

The finches of the Galapagos are a more complicated story because each island has several species on it. The14 species probably evolved in 4 steps: 1.Isolation of finch ancestors on different islands 2.Evolution of a new species on each island 3.Re-colonization of islands by those new species 4.Adaptive radiation- the species adapted to different niches when they were together.

ASSIGNMENT On the worksheet provided, color the distribution of tree cactus species on the islands of the Galapagos. Do they fit the pattern predicted by Darwin’s theory? Be ready to explain your answer!

Extra info

What Darwin and scientists did not know at that time was that there had been many more islands in the archipelago that had formed, eroded and dropped below the sea. The Galapagos ride a tectonic plate over a hotspot in the Earth’s mantle. Volcanoes and new islands are produced as the tectonic plate travels over it.

A big oversight by Darwin! Darwin forgot to record which island he collected finch specimens from. He realize the significance of his collection only when he got back to London and consulted ornithologist, John Gould. As Darwin remarked in the second edition of his Journal of Researches, “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends” (1845:380).