A major part in understanding the theory of natural selection is observing and studying scientific experiments that prove such theory correct. Many scientists have studied Geospiza fortis, a finch that lives in the Galápagos islands and that was a research subject of Charles Darwin's, who was the creator of the theory of evolution by natural selection. By following these finches, scientists have been able to gain insight into the process of natural selection and solidify all of Darwin's observations and ideas as true. G. fortis finches help the scientific community better understand the process of evolution by means of natural selection and illustrate this theory for society to comprehend.
The ecological theory of natural selection was first hypothesized by Charles Darwin as being a process in which certain individuals in nature that have certain heritable traits leave more offspring than individuals who do not possess these traits. This theory was meant to explain how adaptations arise naturally in the world (Campbell Reece, 2009). Darwin began contemplating the origin of species at a young age, after a visit to various Galápagos islands. One particular species of finch, Geospiza fortis, originally inspired Darwin's theories and forced him to question the world around him. Throughout his stay in the Galápagos, Darwin collected many types of birds, including G. fortis, and noticed that although the birds looked similar, they seemed to be different species (Campbell Reece, 2009). After many years of contemplation, Darwin concluded his work with four postulates. His first observation was that members of a population often vary in their traits. By studying many organisms, including the finches, Darwin also noticed that traits are inherited from parent to offspring. Also, all species are able to produce more offspring than the environment is able to support and survival and reproduction are not random events, but rather, set in place to have the strong survive. Many scientists, throughout the years, have put Darwin's theories to the test, commonly using G. fortis as a focus. They are determined to figure out the question of whether Darwin's theories on natural selection hold true in today's world. Through scientific study and observation of Darwin's Geospiza fortis finches, the mechanics of natural selection are revealed through genetic variation, the heritability of this variation from parent to offspring, a surplus of offspring produced, many of which do not survive, and the notion that survival and reproduction are not random events.
2a. Variation of Traits Within Species
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In the 1970's, scientists including Peter and Rosemary Grant traveled to the Galápagos islands, specifically Santa Cruz and Daphne Major, to study and test Van Valen's model. This model hypothesizes that genetic or morphological variations within species directly correlates to differences in environments or occupied niches (Grant, 1976). These scientists did this by observing G. fortis finches on the two separate islands of Santa Cruz and Daphne Major. After many years of observing the native finches, results supported the idea that differences in the environment play a major role in morphological variation within a specific species. The Grant's, along with their colleagues, observed that the finches living on Santa Cruz were exposed to many different environments where as the finches living on Daphne were exposed to few. Therefore, variation in G. fortis bill dimensions was larger on the island of Santa Cruz than on Daphne. They also found that different phenotypes of the finch lived in different areas on each island, selected food based on its size and hardness, and consumed the food in different ways, based on their specific bill size. In between the months of April and December of 1973, 220 G. fortis finches were banded and released on Daphne major. There was no evidence of natural selection between those specific months and survival was high (at a minimum value of 85%) because predation by the short-eared owl, Asio flammeus, was low. However, between the months of December and March of 1974, G. fortis began displaying selection when individuals began favoring longer bill tips. This may be explained by the larger number of smaller seeds available following the fruiting season (Grant, 1976).
Data produced from these scientific findings shows evidence that Darwin's theory of natural selection has a great influence over the level of genetic variation within a population. For example, on the island of Santa Cruz, where many different habitats coexist, the large variation of finch populations may be due in large the different environments that favor morphologically different G. fortis species (Grant, 1976). The sudden change in beak size between December and March of 1974 serves as evidence that natural selection within a population is possible. Within this period of time, G. fortis finches living in that specific population when small seeds were plentiful had to adapt to survive. Consequently, with all of these environmental adaptations happening around the Galápagos, the same species of finch gave rise to genetic variation within a population, specifically in beak size and shape.
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These scientific studies and findings help to contribute to the overall understanding of natural selection and help solidify Darwin's theories. The evidence of differences in the same species of finch on two different islands, Santa Cruz and Daphne Major, solidify the concept of natural selection and that species adapt to be best suited for the environment in which they live, in order to have the best chance to survive and reproduce. Another strengthening argument for natural selection is the observation of G. fortis beak size changing in correlation with the quantity and size of seeds available. This directly illustrates natural selection because the species had to adapt to eating smaller seeds or cease to thrive altogether.
2b. Heritability of Favorable Variations
Jeffrey Podos and colleagues traveled to El Garrapatero in the Galápagos to observe and study bimodal, or contrasting, populations of the G. fortis finch to further their knowledge on disruptive selection in these finches. This difference in two finch populations on El Garrapatero illustrates adaptive divergence, which is the diversification of a species into two different forms. These two different forms usually adapt for a specific environment (Podos, 2008). Along with this adaptive divergence, Podos and colleagues believe that reproductive isolation also led to variations in the two finch populations to arise and be passed on to offspring for many generations. They observed that the two populations of G. fortis and their offspring differ in beak size and the capacity of their bite forces, presumably due to the food to which they had access to (Podos, 2008). Along with Podos' observations, the Grant's observed similar heritable traits when looking at beak size over generations. In the finches, the relationship between beak depth of offspring and their parents directly correlate. The Grant's graphed out the parent's bill depth against the offspring's bill depth (see figure 1 below). After observing the slope of the relationship, which was the heritability of the bill depth trait, one could see that the variation was directly passed on to the offspring (Riley, 2011).
After the conclusion of these scientific experiments, researchers were convinced that Darwin's theory of natural selection was correct. Scientists, including Podos, observed two different populations of G. fortis finches that once had the same traits and now have different beak sizes and bite forces. This illustrates, once again, the theory of evolution and the idea that if variations within a species are favorable for survival, they will be passed on to offspring for many generations to come. By observing the adaptive divergence between two populations of G. fortis and the continuity of these adaptations throughout multiple generations, one can infer that these variations have been passed on to the offspring due to their success in nature. All organisms have descended with modification from one common ancestor and Darwin inferred that the individuals who inherit traits that give them a higher chance of survival and reproduction naturally leave more offspring than others with undesirable traits (Campbell Reece, 2009). Therefore, this example of two different groups of G. fortis finches with very different traits shows that once the two populations grew variation between them, they both found their specific variation favorable for the environment that they were living in. They then passed these favorable variations on to their offspring.
2c. "Survival of the Fittest" Through Surplus of Offspring
Two scientists, Peter and Rosemary Grant, studied Darwin's G. fortis finches on the Galápagos islands for years, testing Darwin's theory of natural selection. In the year of 1977, the tiny and barren island on which they were studying had only an annual total of 24mm of rain. This was a small amount, considering the average rainfall per year was 130mm (Riley, 2011). Consequently, vegetation was greatly affected by this drought and the small seeds that G. fortis finches were accustomed to eating quickly became scarce. Soon after, only large seeds were left. Naturally, the finches with the stronger beaks thrived because they were fit to eat the larger and tougher seeds that the smaller finches could not eat. Over the next few years, the Grant's made observations and noticed that the smaller finches, which were previously unable to readily consume the larger seeds, had perished. Only those finches that originally had beaks strong enough to eat the tougher seeds survived. From then on, offspring of those surviving birds tended to show characteristics of their parents, having larger and stronger beaks (Riley, 2011). After the major drought, the finches were forced to adapt to a changing environment and food source, which led to a larger population of finches who could survive off of the food supply available.
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From observing this drought in the Galápagos, the Grant's concluded that the finch that was best "fit" to survive in the given circumstances did survive, and eventually flourished. The finches that were the weakest, for example those who did not have strong enough beaks to eat the available food source, were not the strongest at that period in time. Therefore, they did not survive. These findings illustrate the idea of "survival of the fittest". This theory hypothesizes that only the best suited mutations within a certain population survive and adapt through natural selection in order for that species to be best suited for their environment (Campbell Reece, 2009). In other words, the strong variations of species survive while the weak die out. These scientific observations also illustrate Darwin's idea of "Darwinian fitness", which is the ability of an individual to survive and reproduce within its environment (Riley, 2011). As shown after the drought, those who survived and were able to eat the large and tough seeds had more fit offspring that made up a greater percentage of the population in the next generation (Riley, 2011). These findings also serve as an example of evolutionary adaptation. Natural selection works to increase the match between organisms and the specific environments in which they live in. When an environment changes, natural selection results in adaptations to new conditions, such as finches having larger beak sizes after the drought. The average beak size in the next generation of G. fortis was greater than it had been in the pre-drought population, which shows that the population had evolved by natural selection. This proves Darwin's theory correct. This scientific observation also illustrates another postulate of Darwin's. This postulate was that in every generation, more offspring are produced that can survive (Riley, 2011). A surplus of finches was produced so that when the environment changed, the entire G. fortis species did not go extinct. The strong finches with the desirable traits survived. Darwin inferenced that the unequal ability of individuals to survive and reproduce will lead to the accumulation of favorable traits in the population generations (Campbell Reece, 2009). That is exactly what happened in the case of the finches after the drought. More finches were produced than could survive so that favorable traits could accumulate over time, much like large beak sizes in Galápagos finches.
Through scientific study and observation of Darwin's Geospiza fortis finches, the mechanics of natural selection are revealed. Genetic variation, the heritability of this variation from parent to offspring, a surplus of offspring produced, and the notion that survival and reproduction are not random events can all be observed and noted when observing these finches. All scientific studies on the G. fortis species help to reveal how evolution occurs in nature and help solidify Charles Darwin's theories and findings. Without extensive scientific study in the Galápagos on Darwin's finches, the theory of evolution by natural selection may not be fully understood or accepted in today's scientific community.