Patterns Of Evolution Geospiza As A Model System Biology Essay


Darwin believed that evolution existed but it was impossible to observe because it took such lengths of time that humans could not quantify it. The irony in this statement is that Darwin's observations of the finches in the Galapagos, which led to his theory of evolution through natural selection, evolve fast enough that researchers can follow multiple evolutionary changes in their lifetime. The Finches have been observed exhibiting multiple patterns of evolution all brought on by different environmental stresses. The scope of science today and our ability to watch these patterns, has led to new knowledge and understanding of natural selection as part of evolution. The ability to follow multiple patterns of evolution from one original species allows great insight on how evolution can happen on multiple levels in a short span of time.



During a circumnavigation of the globe in 1835 the Beagle expedition happened upon the Galapagos Islands, where Charles Darwin collected specimens from the individual separated islands. Darwin discovered over a dozen individual species of finches that all fall into the genus Geospiza. In 1844 Darwin published an article introducing Decent with Modification as a means of the underlying idea of natural selection, and based the article on the specimens he collected on the voyage. "Darwin finches" or the genus Geospiza are studied to this day to try to understand the process of natural selection during current environmental stresses (Campbell and Reece, 2009).

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The Galapagos Islands are a perfect place to study natural selection because the environment is controlled. Life on earth is a complex web of interactions. Every organism relies on a multitude of others to keep the environment in balance. With one slight change in an organism's environment dramatic and unforeseen change can occur. All of the islands which are part of the Galapagos share the same basic chemical composition, weather, and daylight cycles. This means that birds can be compared between the islands but also the birds can be studied over many years on one island because of the isolation. The finches on the Galapagos Islands are thought to originate from a single species (Abbott and Grant, 2008).

Each of the islands of the Galapagos are also ecologically dissimilar. Although identical climates, the vegetation on each island varies, which yields extensive room for variation (Grant and Petren, 2008). The most useful vegetative difference is the particular types of nutrition, especially seeds, and how it can impact the morphology of genus as a whole (Riley, 2011).

The combination of isolation and environmental stresses on Geospiza led to observable patterns of evolution. These patterns include directional selection, where one extreme trait, like beak shape, becomes selectively more pronounced. Stabilizing selection looks almost like a lack of selection is a population becoming more intermediate. Disruptive selection occurs when the extreme trait becomes favored and more represented. Divergent evolution occurs when traits held by a common ancestor evolve into different variations over time. An important subsection of divergent evolution is allopatric speciation, or when a population becomes separated and becomes two entirely separated subpopulations. This also includes peripatric speciation where a small population becomes isolated form a larger majority, and evolution occurs more rapidly due to the founders effects. There are also parapatric speciations, when a subpopulation remains within a habitat and acquires a new niche. Another type of evolution is parallel evolution where different species start with similar ancestors and evolve similar traits over time. All of these modes of evolution occur because of natural selection, and all have been seen in the genus Geospiza on the Galapagos Islands (Campbell and Reece, 2008).



Darwin coined the phrase "Natural Selection" by observing organisms passing unique traits to their offspring. He also observed that the organisms produce more offspring than the environment can support. From these observations he came to the conclusion that the offspring with the traits best suited for the environment survive, and pass the traits to their offspring leading to favorable traits accumulating. This can create a range of variation in a population, shifting their characteristics. In the seventeen hundreds Darwin's theory of evolution was supported by evidence but not actually reproduced experimentally. He believed that Evolution took a "very long time" therefore humans could not observe it. His hypothesis was based on Geospiza, along with other animals that are found in the Galapagos (Riley, 2011). He observed allopatric speciation in Finches based on the fact that all species on the islands seem to have some similar characteristics, but on average, favor an extreme form of a trait that is best suited for the habitat.

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Today Rosemary and Peter Grant have been studying the Galapagos finches for over thirty years and have seen proof of natural selection first hand (Riley, 2011). The theory of natural selection is becoming more concrete as researchers like the Grants show interest in learning more. Their collection of data is being analyzed and implemented by researchers and scientists all over the world. Many patterns of evolution that are true to long term speciation have been observed in the genus Geospiza during the thirty years of study.

Since the early nineteen seventies, Rosemary and Peter Grant had been studying Geospiza. In 1977 a fifth of the average rainfall fell on the Galapagos Islands, creating a major change in vegetation. This led to the highest selection intensities measured today in vertebrate animals (Boag and Grant, 1981). Small soft seeds were no longer available and large hard seeded plants became more plentiful. As an effect there was a measureable change in Geosipza morphology. As supported by the theory of natural selection finches that were better suited to this new environment survived and procreated where the birds that did not have characteristics compatible with the environmental changes died. The environmental pressures were so severe that the natural selection was quantitatively measured. It was found finches with large bodies and beaks survived best when adult mortality rates were high and food was scarce (Grant, 1993). The pattern of evolution then shifted, described by disruptive selection, because an extreme form the beak was favored instead of a small or medium sized beak being selected, the finches changed morphologically. The drought was not the only environmental change that has impacted the Geospiza, there are also many other variables that contribute to changes in patterns of evolution from natural selection.

On multiple islands, there were finches with similar ecological niches, but never on the same island. This could possibly allude to the idea that finches of similar morphology cannot coexist. When two species of Geospiza shared a habitat, they were selected to fill different ecological niches. This was observed by different beak and body size that were compatible with diets (Abbot and Grant, 1977). Parallel evolution could also explain the similarities of ecological niches between different islands. This occurs when traits in a species are selected for an single environment, but when a similar environment occurs elsewhere that species can also evolve the trait based on natural selection.

Darwin hypothesized that a species can evolve to inhabit different niches in an environment (Campbell and Reese, 2009). To avoid interspecies competition for like resources the finches could have evolved into a different niche creating a subpopulation. The change in niche is a form of parapatric evolution. Another way to look at it is the first species of finches were all in the same niche. The finches produce more offspring than the population can support, and the offspring with mutation that allow them to fill a niche that does not involve as much competition will lead to a better success rate. These flaws have to be inheritable, and can lead to a new species, because of morphological changes that adds fitness to an organism. It was also observed that the reason that multiple species of Geospiza could live compatibly was because they inherited different niches. Large bodied, large beaked finches tend to eat large hard seeds and medium sized berries and are in turn larger. On the other hand small little beaked finches eat small seeds and have a harder time with medium sized berries. This means there will not be species competition (Price and Grant, 2008).The similarities between the finches leads to the hypothesis that they had evolved from a common ancestor.

Another analysis of the relation between morphology and natural selection is the chance of survival under environmental stresses. Small Geospiza are able to face more strenuous lack in vegetation because they are smaller and do not need as much food. This means that environmental pressures will affect the two species very differently. This could have contributed to the evolution of the two species. The small Geospiza survive in tough environmental conditions by not eating, while large Geospiza have adapted to large hard seeds (Price and Grant, 1984). This leads to further speciation, including peripatric speciation. A species can become isolated based on new traits like beak size. The species no longer breeds with other species because of morphological changes that affect there mating rituals. The subpopulations then evolve more rapidly because of their small population size (Abbott and Grant, 1977).

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New evolutionary discoveries are being uncovered based on technological advances. This in turn can change our understanding of the evolution as a whole. Where Darwin could only observe the finches we can now observe quantitatively and then analyze on a molecular genetic standpoint. The morphology of the Finches beaks varies to fit form with function. The variety of beaks is a perfect example of species diversification by natural selection because of ecological niches. Instead of comparing the morphological differences in the beaks of the species of Geospiza on the Galapagos Islands to the ecological niche, it is now possible to find the various growth factors that lead to the physical change. The growth factors can then be manipulated in controlled experiments to gather more insight in evolution. After analysis in this example it was found that expression of Bmp4 coded for strong deep beaks. After further study it was confirmed that strong deep beaks in Grospiza magnirostris is indeed a mutation that can be manipulated in other birds to express similar traits (Abzhanov, 1462).


Natural selection in the species Geospiza of the Galapagos has led to speciation. There isolation has given the researchers of today the ability to study patterns of evolution in a living system. The isolation of Geospiza is directly correlated to their ability to evolve rapidly, and for obvious evolutionary patterns to arise. The current study of Geospiza gives quantitative evidence to support patterns of evolution, like allopatric speciation, disruptive selection, parapatric speciation, and paripatric speciation. This can prove that through natural selection traits of populations can shift in a multitude of ways. Researchers can then study the overlap of different modes of evolution all happening in a short period of time. The insight that is gained will add to the understanding of evolution of all life forms.