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Darwin's theory of descent with modification through natural selection is an essential theme that can be observed in almost every aspect of biology. Competition between species is one of the most significant forces in determining which species flourish and which species go extinct. The species that is able to outcompete its challengers in order to attain or maintain its ecological niche has a better chance of survival and reproduction. Specifically, within the genus Geospiza, interspecific competition on different islands of the Galápagos Archipelago has resulted in morphological and ecological adaptations and provides clear evidence of natural selection. In this paper, studies done that show precise evidence of interspecific competition within Geospiza that result in some species tapering and some thriving are analyzed and connected with Darwin's concept of natural selection.
During Charles Darwin's famous journey across the world on the Beagle, his most famous stop came at a small group of islands off the coast of South America. Darwin noticed that these islands, called the Galápagos Islands, contained species more closely related to species in the South American tropics than the temperate regions of Europe. As a result he became increasingly interested in the geographic distribution of these species and spent a long duration of time studying the atypical organisms he found there (Campbell and Reece, 2009). Of the strange species he found he was particularly interested in the finches unique to the islands. Darwin noted that in the absence of birds such as the insect eating warblers and woodpeckers on the island, various types of finches had acquired the ability to fulfill the niches normally occupied by these species (Riley, 2011). In his book The Voyage of the Beagle, Darwin discusses his findings: "Seeing this gradation and diversity of structure in one small, intimately related group of birds (Geospiza), one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends" (Darwin, 345). Darwin recognized this concept, known as adaptive radiation, is an essential part in explaining the great biodiversity present in the world. Darwin began to develop the concept of evolution, which he referred to as descent with modification, and the process by which modification arises, natural selection, in his book, On the Origin of Species by Means of Natural Selection (Campbell and Reece, 2009).
In his book, Darwin discusses in detail evidence that he gathered in order to support his two main ideas. The first idea he had was that descent with modification is accountable for the huge diversity of species currently living. Secondly, Darwin used natural selection as the means by which this diversity comes about and as an explanation of why species are so well suited to their environment (Campbell and Reece, 2009). Darwin justified natural selection using four observations that he had drawn from nature: individual organisms within a species often vary greatly, some of these variations are inherited by their offspring, in each generation more offspring are produced than their environment can support and a large number of these offspring die off, and the survival and reproduction of these offspring is not random (Campbell and Reece, 2009; Riley, 2011). From these observations Darwin formulated two inferences: 1) offspring that inherited favorable traits in a given environment are much more likely to survive and reproduce than organisms that did not inherit favorable traits, and 2) the uneven ability of these individuals to survive and reproduce will lead to the accumulation of offspring with favorable traits (Campbell and Reece, 2009).
Darwin's theory of descent with modification is essential to understanding the rich biodiversity present today. The theory of evolution through natural selection is the main unifying theme in biological sciences (Smith, 1975). The significance of this theory is exactly why it is so important to continuously observe and study this process. Using studies done on the same finches that Darwin observed over a century ago, natural selection can be observed in interspecific competition amongst species in Geospiza. This approach was most famously utilized by David Lack in his book Darwin's Finches and article "Subspecies and Sympatry in Darwin's finches", in which he used the process of interspecific competition to account for many of the morphological and distributional patterns of different Geospiza (Schluter and Grant, 1982; Lack, 1947; Lack, 1969). Lack's hypothesis was challenged by Bowman who argued that interspecific competition was not the cause of the great variability within Geospiza, but rather that the flora food supply accounted for the differences among the Geospiza (Abbot et al, 1977; Bowman 1961). In their study "Comparative Ecology of Galapagos Ground Finches (Geospiza Gould): Evaluation of the Importance of Floristic Diversity and Interspecific Competition", Ian Abbott, L. K. Abbott and P. R. Grant determined that both Bowman and Lack's hypothesizes of flora and food and interspecific competition, respectively, were important in determining different parts of Geospiza morphology and ecology. In order to present a more concrete and compelling case, I have limited my scope to natural selection in Geospiza as a result of interspecific competition, but recognize the importance flora and food play in Geospiza evolution.
In a study done by Dolph Schluter and P. R. Grant called "Ecological Character Displacement in Darwin's Finches," Schluter and Grant found ecological character displacement between two species, Geospiza fuliginosa and Geospiza fortis (Schluter and Grant, 1985). Ecological character displacement occurs when morphological differences between coexisting species are enhanced as a result of interspecific competition (Brown and Wilson, 1956; Schluter and Grant 1985). Schluter and Grant attributed these differences to natural selection, brought about through interspecific competition between the G. fuliginosa and G. fortis. In their study, Schluter and Grant measured the mean beak sizes of G. fuliginosa and G. fortis in allopatry and sympatry while controlling for any differences in food supply of the populations. The results of the experiment showed a distinct morphological difference between the average beak sizes of the species in allopatry and sympatry. When the G. fuliginosa and G. fortis inhabited the same island, the average beak size of the G. fuliginosa and G. fortis were remarkably smaller and bigger, respectively, than when the two species were in allotropy, where the average beak size for both were intermediate (Schluter and Grant, 1985). This indicates that ecological character differentiation is indeed present as a result of interspecific competition between G. fortis and G. fuliginosa.
The results of this study also clearly indicate natural selection occurring between G. fortis and G. fuliginosa as a result of interspecific competition. When G. fortis and G. fuliginosa are in allotropy, they are morphological and ecological identical (Schluter and Grant, 1985). However throughout the course of G. fortis and G. fuliginosa's inhabitation of the same island, the two species reproduced and diversified. The individual G. fortis and G. fuliginosa that were the least morphologically similar were able to avoid interspecific competition and exploit different food sources. Consequently, when they avoided competing with a larger number of organisms for food they were able to survive, reproduce, and pass on their favorable traits to their offspring. Inversely, a majority of the intermediate sized beaked G. fortis and G. fuliginosa were forced to compete amongst each other for a limited amount of food and were less likely to survive and reproduce. In accord with Darwin's second inference, the G. fortis and G. fuliginosa with favorable traits passed down those traits to their offspring and became abundant over the years. As a result, on the islands where the two species are in sympatry, the species are significantly morphologically different than on the islands where they exist alone (Schluter and Grant, 1985).
In another study done by Dolph Schluter and Peter R. Grant called "The Distribution of Geospiza Difficilis in Relation to G. Fuliginosa in the Galápagos Islands: Tests of Three Hypotheses," Schluter and Grant focus on using three different hypothesizes, one of which being Lack's hypothesis of interspecific competition, to account for the altitudinal distributional patterns in Geospiza fuliginosa and Geospiza difficilis (Lack 1947; Lack 1969), They believed that competitive exclusion by G. fuliginosa of G. difficilis is a possible reason for why G. difficilis inhabits inordinately high altitudes on islands in which it is in sympatry with G.fuliginosa (Schluter and Grant, 1982).The other two hypothesis being tested, Fixed Food Requirements Hypothesis (FFR) and Variable Food Requirements Hypothesis (VFR), were restatements of Bowman's hypothesis that food supply determined the distribution of Geospiza (Schluter and Grant, 1982; Bowman 1961). Schluter and Grant conducted the study on three islands of the Galápagos Archipelago, Pinta, Genovesa, and Marchena, during both wet and dry seasons of 1979 (Grant and Boag, 1980; Schluter and Grant, 1982). They censured the abundance, altitude, diet, and food supply of the G.fuliginosa and G. difficilis. Schluter and Grant found that when in the absence of G.fuliginosa, G. difficilis lived at a similar altitude and fulfilled a similar ecological role as G.fuliginosa. However, when the two species were in sympatry, G. difficilis lived at higher altitudes and assumed a different ecological role. Schluter and Grant determined that the two hypotheses based on food requirements did not account for the distribution of G.fuliginosa and G. difficilis. Rather, the data supported that Lack's hypothesis of interspecific competition between G.fuliginosa and G. difficilis was responsible for the altitudinal distribution to the species. They concluded that G.fuliginosa had competitively excluded G. difficilis from G. difficilis' expected niche and forced G. difficilis to assume a different ecological role (Schluter and Grant, 1982).
This study is another clear example of how natural selection occurs within Geospiza as a result of interspecific competition. In allopatry, G. fuliginosa and G. difficilis occupy similar ecological roles (Schluter and Grant, 1982). However, when in sympatry, G. fuliginosa is able to competitively exclude G. difficilis from its normal ecological role. As a result, the G. difficilis that are forced to compete with G. fuliginosa are less likely to survive and reproduce. On the other hand, the G. difficilis that possess traits that make it able to assume a niche in a higher altitude and avoid interspecific competition are more likely to survive and produce more offspring. The superior ability of the G. difficilis with the more favorable traits of survival and reproduction at high altitudes will lead to an abundance of high-altitude G. difficilis. The process of natural selection accounts for the fact that G. difficilis assumes a much different ecological role in the presence of G. fuliginosa (Schluter and Grant, 1982).
In the aforementioned studies, namely the two studies by Schluter and Grant, the interspecific competition between species within Geospiza results in morphological and ecological changes. From these changes, the species that develop favorable traits are more likely to survive and prosper. Therefore, the results of these studies provide precise evidence of the subjection of these species to Darwin's natural selection.
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