Monday, March 8, 2010

The Geography of Life: Oceanic Islands

Oceanic islands prove to be strong evidence for evolution versus creationism from pages 100-109. Although these islands are suitable for mammals, amphibians, reptiles, and fishes, there is an obvious pattern that unless introduced by man, many of these organisms have not evolved in these areas. Coyne defines oceanic islands as islands that were "never connected to a continent... [and having] unbalanced biotas" since the organisms usually found to inhabit these islands belong to similar species such as the Gálapagos finches. How do these places differ from continental islands and support the theme of evolution? Think about the different adaptive radiations and why life on these islands is unbalanced. Also, site specific examples of these adaptive radiations and explain evolution in terms of adaptations, which allow an organism to gain selective advantages in a specific environment.


  1. Islands are great places for observing evolution. According to R.P. Filson, the development of ecosystems on islands is unpredictable, which overall leads to the "unbalanced" life on the island. Depending on factors such as climate and proximity to other land masses, different plants or animals will become the pioneer species of the island. Filson says that geographical isolation is bound to lead to decreased gene flow among populations. Because these populations are so isolated from the one's they may have left behind on the mainland, they begin to genetically grow apart due to mutation, natural selection and genetic drift. This source provides a specific example of Gallotia lizards which have been found in three different species, in seven different populations among the Canary Islands (off of the west coast of Africa). This is likely to have occured because as the lizards continue to seperate as their populations grow, mutations will arise that provide the animal with some sort of advantage. This process continues to occur until the original lizard populations are no longer able to reproduce with one another, thus giving rise to many different species. The scientists examined the DNA of all the lizards and compared them only to find that the genetic differences among them lessened the closer the populations were to each other. This was justified because the closer the populations are to one another, the more gene flow, causing them to have more similar DNA. The reason why oceanic islands are so unbalanced is because the process of the organism migrating over to the island in the first place is very difficult and unlikely. Some insects and birds are able to travel through the air, plants/animals may be attached to "rafts of natural vegetation" that carries them to the island during high tides. Therefore, the species that actually make it to the island reproduce and as the population increases they spread out, migrating to more islands. Over time, mutations and natural selection will cause them to evolve into different species.

  2. Everything mentioned above is true and accurately describes speciation on oceanic islands. However, the reason for the difference between oceanic islands and continental islands is not fully explained. According to Coyne, oceanic islands are islands that were never connected to a continent; instead, they were built by volcanic action in the oceans or by the growth of coral reefs (100). As a result, these islands originally had no life except for perhaps some oceanic plankton and other microorganisms that may have been stranded on the island as it rose out of the water. Thus other forms of life had to travel to these islands across a great distance of water rather than riding on a chunk of land that has broken away from a larger landmass. This important requirement for settlement of an oceanic island has created very specific rules.
    First, the rates of extinction and immigration on an island are linked in opposite ways to the number of species on the island. If more species immigrate to the island, it is more likely that some of these species will have the same ecological niche and therefore undergo competitive exclusion, after which one of the species becomes extinct. After a number of island species have become extinct, immigration is less likely to bring an existing species to the island, and so the number of species will rise. Second, the size of an oceanic island and its distance from the mainland help determine how many species will immigrate there. A larger island can support more species than a smaller island just based on area, and an island that is closer to the mainland will naturally experience more immigration, as the distance that a species must travel to the island is shorter. (Campbell 1216)
    The unique isolation of oceanic islands has prevented heavy land mammals and reptiles from reaching them naturally, and so most native species on oceanic islands are plants, birds, and insects- all organisms with the capability of flight (or, in the case of plants, seeds that can be carried a long way by water or air). Once these organisms reached oceanic islands, they could adapt to the specific environment on a specific island. The Galapagos finches provide a perfect example of this. The Galapagos Island chain consists of 15 major islands, each of which contains a slightly different type of finch (Wikipedia). The variations in the finches are mostly in beak size and shape, signs that each species of finch adapted to the food supply on its specific island. For example, the large cactus finch has a longer beak suited for feeding on cactus, while the large ground finch has a short, heavy beak that is suited for cracking nutshells. Overall, there are 15 species of finches in the Galapagos, and they make up a radiation. Though they all likely share a common descendant- the first finches to fly out to the Galapagos Islands- they have evolved and speciated due to natural selection selecting for different traits on each island.