Darwin's Finches: Islands and Evolution
Nearly 2,000 miles southwest from the Bahamas, off in the Pacific Ocean, are the islands of the Galápagos, an archipelago territory of Ecuador. In 1835, the HMS Beagle dropped anchor on the island during a surveying expedition around the world. On board was the naturalist Charles Darwin, who would use his observations of the wildlife on the island to support his groundbreaking theory of evolution by natural selection. Since publishing his theory, Darwin not only created a new paradigm for the biological sciences, but also demonstrated the powerful influence that islands have on the evolutionary history of their inhabitants.
Galápagos finches (Thraupidae) are a group of birds endemic to the islands. Darwin observed various differences that could be used to differentiate each species, particularly their beaks. Some species had large beaks, capable of crushing large nuts and other food items, while others had tiny beaks, meant for precision and collecting tiny seeds. Different species also had a tendency to live on different islands within the archipelago, resulting in a physical isolation between populations. Thus, Darwin concluded that these finches likely evolved from a single common ancestor, that possibly lived on the South American continent before colonizing the island chain. Since this discovery, islands have been viewed as natural laboratories in which biologists can observe evolution in action. |
Island Biogeography
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In 1967, ecologists Edward O. Wilson and Robert H. MacArthur published their theory of island biogeography, which explains that the number of species that live on an island is a function of the immigration of new species and extinction of current species. Furthermore, species immigration is determined by island distance from other inhabited islands or continents. Generally, islands that lie further away from continental mainlands are expected to have fewer species due to increased difficulty of immigration. In turn, the theory states that extinction rate is determined by island size, with larger islands able to support a higher biodiversity due to fewer extinctions than on small islands. For example, while there are five endemic bird species to the Bahamas, the majority of birds that inhabit the islands also have ranges in the Greater Antilles and the southeastern United States. Rock iguanas (genus Cyclura) are native lizards to the Caribbean, yet most species are highly endemic. The Bahamian rock iguana (Cyclura rileyi) is particularly restricted by only inhabiting San Salvador Island, Acklins Island, and the Exuma District of the Bahamas.
While traditionally viewed through the lens of terrestrial ecosystems, island biogeography can be applied more broadly. The concept of "insular" biogeography makes these principles inclusive to any type of isolated biome. This may include two mountain ranges separated by a valley, fragmented forests due to human intervention, or individual lakes separated by land. |
Symbiosis
In ecology, a symbiotic relationship refers to an association between two or more species. These relationships can range from one species harming another (i.e. parasitism/predation), one organism benefiting while the other receives no harm or benefit (commensalism) or where both organisms benefit (mutualism). In shallow marine coastal biomes, all three types can be found.
Corals provide an excellent example of mutualism by harboring within them an algae called Zooxanthellae. Like plants, these algae are photosynthetic, able to convert carbon dioxide into complex organic molecules and oxygen using light energy from the sun. The coral is able to use these organic molecules for its own nutrition and the oxygen for respiration. This leads to quite efficient recycling as the coral gives off carbon dioxide as a waste product, which the algae can use for more photosynthesis. The algae are able to work faster when water temperatures are higher, but there is a danger to this. When water temperatures are too high, the coral may experience toxic levels of oxygen, so it must eject its endosymbiotic algae partners. When this happens, the infamous event of "coral bleaching" takes place, causing corals to lose their coloration and enter a state of starvation until conditions recover. As ocean temperatures continue to rise, more coral bleaching events are expected to follow. In 2016, over half of the Great Barrier Reef off the eastern coast of Australia was reported to be bleached. While reefs in the Bahamas remain relatively healthy, concerns over warming waters in the Caribbean are expected to increase the frequency of bleaching. Because corals themselves are integral to the integrity of coral reef biomes, vast marine communities depend on the symbiotic relationship between coral and algae.
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From seagrass biomes, a unique three-way symbiotic relationship between seagrass, lucinid bivalves, and sulfur-oxidizing bacteria can be observed. The bacteria that live within a lucinid host are chemosynthetic, meaning they can also produce complex organic molecules from carbon dioxide, but use chemical energy as a fuel source as opposed to light energy. This energy is obtained by reduced sulfur molecules, such as hydrogen sulfide, that the bivalve takes in from the surrounding pore water. Buried just beneath seagrass roots, the lucinid benefits by having an abundant source of sulfides that the seagrass releases. In return, the seagrass is not choked out by the harmful sulfides. Because lucinids are found in over 90% of the world's seagrass beds, including those in the Bahamas, this symbiotic association is thought to be vital to the maintenance of these seagrass biomes. Check out the "Research at Mines" tab for more information on what we're doing to learn more about this relationship!
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For more information on symbiosis, be sure to visit the museum's online exhibit "Symbiosis Through Time"!
The Consequences of Climate Change
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Tropical islands can be dangerously unstable habitats in the face of rapid climate change, as we're observing today. Climate change is expected to increase the frequency and intensity of tropical storms, cause sea levels to rise, warm ocean waters to unsuitable temperatures for marine life, and compromise coastal biomes. Considering that approximately 375,000 people inhabit the Bahamas alone, many people are equally at risk to the consequences that climate change may impose.
Habitat loss has been the greatest risk to marine life along coastal biomes. While rising sea levels may appeal as a beneficial effect to marine life, rising ocean temperatures and ocean acidity stops coral growth in its tracks and scatters free-swimming organisms in a frenzy to find more suitable waters. However, studying climate change is no easy task, and involves collaboration from scientists around the world to understand its consequences to life on Earth. Greenhouse gas emissions from industrialized nations lead to rising temperatures, which cause polar glaciers to melt, and ultimately contribute to sea level rise. It's our responsibility to create solutions that protect current habitats that life depends on and alter our habitual activities to preserve current biodiveristy on islands, and around the world. Because of the role that marine life plays in building carbonate islands, the Bahamas rely on a healthy ecosystem to remain habitable for generations to come. For this reason, it is paramount that we continue to study the nature of island life in order to better understand what we can do to protect its delicate future. |
References:
Grant, P.R. 1999. Ecology and Evolution of Darwin's Finches. Princeton University Press.
Olson, S.L., Pregill, G.K., and W.b. Hilgartner. 1990. Studies on Fossil and Extant Vertebrates from San Salvador (Watling's) Island, Bahamas. Smithsonian Institution Press.
Paulay, G. 1994. Biodiversity on ocean islands: It's origin and extinction. American Zoology 34: 134-14.
Stanley, S.M. 2014. Evolutionary radiation of shallow-water Lucinidae (Bivalvia with endosymbionts) as a result of the rise of seagrasses and mangroves. Geology 42(9): 803-806.
Grant, P.R. 1999. Ecology and Evolution of Darwin's Finches. Princeton University Press.
Olson, S.L., Pregill, G.K., and W.b. Hilgartner. 1990. Studies on Fossil and Extant Vertebrates from San Salvador (Watling's) Island, Bahamas. Smithsonian Institution Press.
Paulay, G. 1994. Biodiversity on ocean islands: It's origin and extinction. American Zoology 34: 134-14.
Stanley, S.M. 2014. Evolutionary radiation of shallow-water Lucinidae (Bivalvia with endosymbionts) as a result of the rise of seagrasses and mangroves. Geology 42(9): 803-806.