Wednesday, March 17, 2010

Island Evolution

On pages 86-88 of the book, Jerry Coyne discusses the implications of evolution on landmasses separated by the ocean. In this particular case, he cites the example of the island "Más a Tierra," where Alejandro Selkirk took refuge when he was put ashore by his ship. All of the animals there were evolving in different ways from the animals on shore on the big continents, yet the reasons behind why this particular phenomenon happens is the question in this prompt. What conditions must exist for island organisms to evolve into an "endemic" status? How would these conditions affect the traits selected in these island organisms? Also, how come these separations happen in abundance on islands yet on distant continents like Africa and South America there will be similar species found?


  1. Endemic means found nowhere else in the world. The three islands of Juan Fernandez today have many rare and exotic species that are endemic. These five species of birds, 126 species of plants, a fur seal, and a handful of insects are all endemic on these islands. It is also strange because it does not have a single native species of amphibian, reptile, or mammal that are commonly found on continents all over the world. Continental drift and molecular taxonomy are two major ideas that can explain the reason why species are so diverse around the globe. The past geography of the world was very different from that of the present; there was one huge supercontinent that shifted about, joined, and separated into other pieces. DNA and protein sequences from organisms can help to explain not only evolutionary relationships among species, but also when they diverged from their common ancestors. With this information, it has been found that DNA sequences change in roughly a straight-line fashion with time. Scientists are able to use DNA sequence change and fossil ancestor information to estimate the divergence times of species that have poor fossil records. Scientists have also been able to match evolutionary relationships between species with the known movements of continents and see if the origin of species are concurrent with the origin of new continents and habitats. There are two different types of islands that help to explain how island organisms evolve into endemic species. Continental islands are those that were once connected to a continent but later separated either by rising sea levels that flooded former land bridges or by moving continental plates. Some include, the British Isles, Japan, Sri Lanka, Tasmania, and Madagascar. Oceanic islands, however, are those that were never connected to a continent. They arose from the sea floor, initially lacking any form of life, as growing volcanoes or coral reefs. Some include, the Hawaiian Islands, the Galapagos archipelago, St. Helena, and the Juan Fernandez group of islands. Oceanic islands are missing many types of native species that are seen on both continents and continental islands. Missing animals include, land mammals, reptiles, amphibians, and freshwater fish. Its not that these species would not live well on islands because through experimentation it has been found that they actually survive very well on islands, it is that they have no way of getting to these places. Native species of oceanic islands can colonize there because they have characteristics that allow them to disperse over long distances and the missing species lack these abilities. Birds are capable of flying great distances over sea, carrying with them not only their eggs but also seeds of plants that they've eaten. Parasites are found in birds feathers and small organisms stick to mud on birds feet. Just through the bird alone, many different individuals are able to travel long distances to islands completely isolated from a major land source. Plants get to islands as seeds or floating across water. Spores of ferns, fungi, and mosses can be carried huge distances by the wind. Insects too can fly to islands or be taken by winds.

  2. The species found on these islands are able to reach isolated places through wind travel, use of another organism, or sea travel. Since these species are isolated from major land sources, many of them are not coming and going from the island, which explains why they are so different from other species. They are left alone for long periods of time without interaction from other species because they are so isolated. With changes in the environment, these species evolve completely different characteristics of those species that are very similar to them on major continents. That is why species on oceanic islands become endemic. Inhabitants of oceanic islands descended from earlier species that colonized the islands, usually from nearby continents, in rare event long distant travels. Once they got there, accidental colonists were able to form many species because oceanic islands offer lots of empty habitats that lack competitors and predators. This is why speciation and natural selection are very rapid on islands. In Madagascar, the unique lemur, the most primative of the primates, was first introduced to the island 60 million years ago and radiated into more than seventy-five endemic species. "Madagascar—an island off the coast of Africa—has some of the world's most interesting animals. About 75 percent of the species found in Madagascar live nowhere else on the planet. The black lemur lives in the tropical forests of northern Madagascar. Black lemurs are notable for the differences between the sexes. Males are black while females have reddish-brown fur with a black and white face. Black lemurs feed on fruit, flowers, and young leaves." ( New Zealand has many native well-known flightless birds including, the giant moa, a thirteen-foot-tall monster hunted to extinction by about 1500, the kiwi, and the kakapo. "The ancient, flightless Kakapo is the world's rarest and strangest parrot. It the only flightless and nocturnal parrot, as well as being the heaviest in the world, weighing up to 3.5 kilograms (8 lbs). The birds live in New Zealand, an island country which had virtually no mammals living on it for millions of years. It was a place inhabited by birds and reptiles. The only types of mammal were two species of bats. The Kakapo did not learn the defense mechanisms to combat or escape mammalian predators. This made the parrot very vulnerable when new animals started showing up." ( Due to the isolation of oceanic islands, natural selection and evolution thrive rapidly in many species.

  3. Different arrays of species evolved on distant islands because they are geographically isolated from other environments of the world. As a result, Islands have remained "populated by species whose relatives had long ago been extinguished in mainland areas" (Pacific Science vol 49). The isolation of islands from other areas allowed for less competition which drove natural selection and evolution on mainland continents. With a different climate and less competitors, species on distant islands evolved to fill different ecological niches from those filled by animals with common ancestors living on the mainland. Thus, the conditions on distant islands didn't directly "affect" the traits of species that lived on that island, but rather its environment favored the selection of animals with different traits from those living in a different area with a different climate. Another effect of being on a distant island is that there wouldn't be as many original species for natural selection to act on. When there are fewer species to select, the environment is favorable for speciation because there are unoccupied niches that can be filled relatively easily with the formation of a new species. The lack of species on distant islands is shown when new species from mainland environments are introduced to an island ecosystem. Daniel Simberloff states that because species are often absent from island ecosystems, "these absences can predispose certain invaders to be especially likely to survive and to produce particular impacts"(Pacific Science vol 49). These introduced species are often extremely well-suited to the environment of distant islands and will compete with the endemic species of that island, sometimes driving species to extinction. This difference in species isn't seen to as much of a degree in separate continents because all the continents were formerly part of a single large land-mass, on which animals of the same species could move across, thus spreading the same species throughout the world. There can also be similar species found because there are similar environments and common ancestors, which promotes convergent evolution, which Coyne explains on pages 92-94. Convergent evolution may not happen on oceanic islands either because their climate is different enough to not encourage the evolution of similar species or that because oceanic islands "were never connected to a continent; they arose from the seafloor, initially bereft of life"(100). Because they didn't have a connection to a continent, they wouldn't have all the same common ancestor species. For the species that do occur naturally on oceanic islands, however, they do usually have similar species on the mainland near that island, because the mainland is where their common ancestor once migrated to an oceanic island.