Overview
Scientists believe that all living organisms are descendants of a single multicellular organism which inhabited the earth’s oceans approximately 3.6 million years ago. This belief is the theory of evolution, which is the theory of genetic change within species throughout numerous generations and over thousands of years. Evidence of evolution can be found throughout the modern world, in both fossils, the geography and distribution of species, and the common anatomy of species. Because of the overwhelming amount of evidence that supports it, evolution has become a principle and established scientific theory. Charles Darwin is credited with the creation of the theory of evolution and natural selection and first published these theories in 1859 in his “On the Origin of Species”. In his volume, Darwin described evolution as being “descent with modification”.
Evolution occurs as a result of changes in the allele frequency of a population. Change occurs when there is an increase of one allele or trait within a population. These changes of allele frequency are induced due to different factors which include migration, mutation, genetic drift and natural selection. There are two generally accepted varieties of evolution, microevolution and macroevolution. Microevolution is considered a small scale evolution in which a change in allele frequency occurs rapidly and over a short period of time [1]. Conversely, macroevolution is a large scale evolution. Macroevolution is the derivation of different species from a common ancestor and occurs over tens of thousands of years [2]. While scientists can view microevolution as it occurs, macroevolution differs given it occurs over much longer periods of time and therefore is more difficult to track. Instead, scientists must rely on fossils and other forms of documentation of ancient species to follow macroevolution's lineage [3].
Evolution occurs as a result of changes in the allele frequency of a population. Change occurs when there is an increase of one allele or trait within a population. These changes of allele frequency are induced due to different factors which include migration, mutation, genetic drift and natural selection. There are two generally accepted varieties of evolution, microevolution and macroevolution. Microevolution is considered a small scale evolution in which a change in allele frequency occurs rapidly and over a short period of time [1]. Conversely, macroevolution is a large scale evolution. Macroevolution is the derivation of different species from a common ancestor and occurs over tens of thousands of years [2]. While scientists can view microevolution as it occurs, macroevolution differs given it occurs over much longer periods of time and therefore is more difficult to track. Instead, scientists must rely on fossils and other forms of documentation of ancient species to follow macroevolution's lineage [3].
MutationNumerous ecologists today believe and theorize that the creation of every modern living organism is the result of a mutation, or numerous mutations, which occurred thousands of years ago. This is because mutations are the single factor of evolution which allow for an organism to form new traits [1]. The traits produced by mutations can be beneficial, ineffectual, or detrimental to an organism [1]. A mutation occurs due to a sequence of changes which begin in the DNA of an organism. Once the DNA has been in some way altered, so are the codons, the amino acids used to create proteins and the proteins themselves. This change in the proteins is what ultimately modifies the phenotype of an organism and possibly the allele frequency of that organisms population. Not all mutations though affect evolution, as a mutation may only impact evolution if it is heritable and can be passed down through a species. These mutations, heritable mutations, are known as germ-line mutations whilst the in-heritable mutations are known as somatic mutations [2]. In order for a mutation to be heritable, it must be present in the gametes or sex-cells of an organism.
Mutations occur randomly. Mutations can occur because of uv rays, x-rays, microwaves, and all other mutagens which can change the DNA sequence of cell. There are two types of mutations, point mutations and chromosomal aberrations [3]. In a point mutation, only a single base pair within the DNA sequence of an organism is changed via substitution (the substitution of a base in a DNA sequence), addition (the addition of a base to a DNA sequence), or deletion (the deletion of a base in a DNA sequence). In contrast, chromosomal aberrations occur on a much larger scale and commonly arise during the crossing over stage of meiosis or because of a transposable element (created by the repositioning of DNA within a genome) [4]. |
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MigrationMigration is defined by ecologists as the flow of genes between two independent populations of a species. Though the two may seem indistinguishable, migration differs from genetic drift as during migration the migrating species becomes relocated in a pre-inhabited area (unlike the un-inhabited area of genetic drift) and affects the allele frequency of two populations (whilst genetic drift affects the population of only one). Similarly to genetic drift though and other factors of evolution, migration often occurs at random. Migration transpires when a group of individuals is transferred over a geographical border and into a population different from the organism’s original population. Organisms can be transferred by both nature and human forces, as they can be blown over a geographical border by the force of wind or carried over a geographical border by humans and transportation vehicles. Both the way an organism is transferred, the species of the migrating organism, and the geographical border between the two populations, can influence the affect of migration on a population, the population’s allele frequency and ultimately evolution itself.
[This section was written based only on class notes.] |
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Genetic DriftGenetic drift is a random change in the allele frequency of a population. The change occurs when a small and selected sampling of a species are isolated and their genes create a new allele frequency [1]. These genes are passed down for generations and this allele frequency is maintained within their population, greatly reducing the genetic variability of the population [2]. As it does not produce new adaptations nor occur frequently, genetic drift is often viewed as the most minor form of evolution.
There are two types of genetic drift, the founder effect and population bottleneck. The founder effect occurs when a small number of organisms isolate themselves from the rest of their species. If these isolated organisms have a different allele frequency then their original population, then evolution has occurred, as there has been a change in the allele frequency. The new allele frequency created in the new population is completely based on the alleles of the founders, hence the name. An example of the founder effect can be seen in the Amish population. When the Amish isolated themselves from the rest of society, the trait for six fingers was present. Over time the increased due to marriages and births occurring within the limited population of Amish. As a result the Amish community has a larger percent of people with six fingers than the general population. The allele frequency of the Amish's new population is different from that of the rest of the human world. The other form of genetic drift is population bottleneck. Population bottleneck transpires when an environmental disaster, famine, or disease occurs within an environment. This extreme and rapid environmental change causes numerous organisms of one species are killed, leaving only a small number of organisms left. This remaining population’s alleles decide and ultimately control the allele frequency of the new population and species. In both population bottleneck, the small sample size of the new population leads to the alteration of the original allele frequency [3]. An example of population bottleneck are cheetahs. After a mass hunting of cheetahs, the cheetah population was decreased to only a couple dozen cheetahs. As the sample size of these cheetahs was so small and unvaried, the new population of cheetahs had a different allele frequency then the original population of cheetahs. |
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Natural SelectionNatural selection is one of the most frequently recognized forms of evolution and is often known and referred to as “survival of the fittest”. This term is coined from the meaning of natural selection, that those best suited to an environment survive and reproduce, passing on their desirable traits whilst those poorly suited to an environment die off. In order for natural selection to occur though, numerous factors, must be present. These are, one, a variety within a trait, two, this varied trait must be heritable and finally, three, this varied and heritable trait must allow an organism to have a greater chance of survival within its environment. This final factor is especially important and enforced in environments that have reached their carrying capacity and thus have extremely limited resources. It is within these environments that only those with the most desirable traits, which often appear in the phenotype of an organisms, are able to survive and pass down their genes while other organisms are unable to survive. Thus, natural selection leads to an increase of a specific trait within an environment, which affects the allele frequency of that environment. As it occurs on a frequent basis and considerably impacts the allele frequency of an environment, natural selection is often considered the leading factor of evolution [1].
Natural selection was first observed and documented by Charles Darwin when he circumnavigated the world in 1831 [2]. A few decades after his voyage, Darwin published his records and theory in the “On the Origin of Species” [3]. When reviewing Darwin's studies and the concept of natural selection, a single example of natural selection and evolution, which was also published in Darwin’s “On the Origin of Species”, is constantly identified. This is the example of the birds of the Galapagos Islands, which Darwin studied on his voyage. Inhabiting a series of islands in the Galapagos, this unique species of bird displayed a different beak shape on each island. Darwin’s reasoning for this ecological feat was that on each island, the species of bird had adapted to a specific food source. Those birds with beaks best suited to the food source survived and thrived while the others quickly died off. Thus, in this manner, those with the best beaks, passed on their genes and beak traits to their children who likewise survived, until finally, centuries later, each island was populated with birds of only a specific beak. There are three types of natural selection, stabilizing selection, directional selection and disruptive selection. Stabilizing selection is when the environment supports the medium version of a trait. Thus the mean of a population graph affected by stabilizing would be in the center, as more organisms have the medium trait [4]. An example of stabilizing selection is insect wing size. In many environments, the ideal insect wing size is in the middle, as a medium wing size appropriately support the insects body weight and allows insects to fly at regular speeds and without attracting predator attention. Directional selection occurs when an environment supports either extreme of a trait and thus the mean of the population graph is shifted to the far right or left. An example of directional selection is giraffe necks. For giraffes, it is most beneficial to have a extremely long neck as this allows them to easily reach leaves and thus access food. In contrast, a small or medium sized neck would not be as beneficial and instead more limiting. The last type of natural selection, disruptive selection occurs when an environment supports both extremes of a trait while medium version of the trait experiences selectional preferences [5]. Disruptive selection leads the means of the population graph to be on both ends of the graph. An example of disruptive selection might be the height of a mammal. For many mammals, if an organism is tall it can reach food sources whilst intimidating predators. If an organism is short, it can move unnoticed by predators. On the other hand, if an organism is of medium height range, then it is often easily spotted by predators and is likewise non-intimidating. |
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Evidence of Evolution
Evidence supporting the theory of evolution exists throughout our world. This evidence of evolution is found in the phenotypes and genotypes of both modern and ancient animals and is observed across the continents. The first evidence of evolution is fossils. Paleontologists and other scientists constantly look for and excavate the fossils of both past and modern day species. Scientists then date the time period of the fossilized organism using relative dating, by observing the location of the fossil within sedimentary rocks, and exact data, by using radiometric dating (and isotopes). By dating the ages of these fossils, scientists can clearly understand how certain organisms have changed or evolved over thousands of years. Furthermore, fossils of extinct species provide proof that a single species can evolve into numerous different species. Likewise, by finding unicellular organism fossils, scientists can better understand evolutionary theory and how these organisms adapted and evolved to become multi-cellular.
The geography of species, the second form of evidence, shows how species can adapt to their environments. Millions of years ago, the continents were conjoined, forming a landmass called Pangea. When Pangea split, numerous species became split between continents and separated by oceans. Yet, instead of remaining identical, many of these species independently adapted to their new environments by developing new traits and characteristics. These species adaption is proof of natural selection and depicts how over time species may change and evolve. The ostrich and rhea, which respectively live in South America and Africa, are an example of this. Many scientists believe that the ostrich and rhea were once related because of the overwhelming similarities between the genotypes and phenotypes of the birds. They theorize that on Pangea, the ostrich and rhea were one bird species, yet, when Pangea separated, the ostrich and rhea were likewise separated. These birds then adapted to their new habitats, causing the differences that we see today between these two birds.
The third evidence of evolution is found in the anatomy of species. When studying the anatomies of species, scientists often look for specific traits and adaptations such as homologous structures and vestigial structures, for past occurrences such as convergent and divergent evolution, and similar DNA and embryology structures to provide evidence of evolution. Numerous modern day species share homologous structures, similar structures which are present in the species that adapted from one common ancestor. An example of a homologous structure would be the arm and hand bones of mammals including humans, bats and porpoises. This common hand bone is proof that each of these mammalian species evolved from the same ancestor and further displays the evolution of organisms throughout history. A vestigial structure is a structure which is useless to a species. An example of a vestigial structure in humans would be the appendix. Six thousand years ago when humans first evolved, they were forced to eat tough meats and plants. In order to better digest their food, humans adapted to have appendages. Also, our lack of use of appendages shows how humans have further evolved since they were first “created”. Divergent evolution occurs when a species, living int two separate and independent habitats, evolves to have different traits. Divergent evolution occurred after Pangea and can be seen throughout the world. A common example of divergent evolution is the birds of the Galapagos islands. This species of birds experienced divergent evolution as on each island because of the different food sources, the species of birds diverged and evolved to have numerous beak types, each type suited specifically for one island. Convergent evolution likewise shows how species adapt to their environment. In convergent evolution, two separate species of a separate taxonomy develop similar traits in response to the similar environment that they live in. An example of this is sharks and whales, who, though are not of the same taxonomy, share many similar characteristics such as a dorsal fin and rounded nose as they have adapted to the same ocean environment. Embryology is the study of embryos and provides further proof of evolution as it reveals that during a certain developmental stage, the embryos of numerous different and unrelated species look almost identical and both have a tail and gills. This further proves that many of the species today evolved from one common ancestor and thus continue t o share traits. Finally, molecular biology, which compares the DNA of different species shows how that because of almost identical genotypes, certain species evolved from other species. An example of this are chimps and humans, who share 98% of identical DNA as humans evolved from chimps. Molecular biology establishes the lineage of evolution between different species by using precise and collected data.
[This section was written based purely on our class notes.]
The geography of species, the second form of evidence, shows how species can adapt to their environments. Millions of years ago, the continents were conjoined, forming a landmass called Pangea. When Pangea split, numerous species became split between continents and separated by oceans. Yet, instead of remaining identical, many of these species independently adapted to their new environments by developing new traits and characteristics. These species adaption is proof of natural selection and depicts how over time species may change and evolve. The ostrich and rhea, which respectively live in South America and Africa, are an example of this. Many scientists believe that the ostrich and rhea were once related because of the overwhelming similarities between the genotypes and phenotypes of the birds. They theorize that on Pangea, the ostrich and rhea were one bird species, yet, when Pangea separated, the ostrich and rhea were likewise separated. These birds then adapted to their new habitats, causing the differences that we see today between these two birds.
The third evidence of evolution is found in the anatomy of species. When studying the anatomies of species, scientists often look for specific traits and adaptations such as homologous structures and vestigial structures, for past occurrences such as convergent and divergent evolution, and similar DNA and embryology structures to provide evidence of evolution. Numerous modern day species share homologous structures, similar structures which are present in the species that adapted from one common ancestor. An example of a homologous structure would be the arm and hand bones of mammals including humans, bats and porpoises. This common hand bone is proof that each of these mammalian species evolved from the same ancestor and further displays the evolution of organisms throughout history. A vestigial structure is a structure which is useless to a species. An example of a vestigial structure in humans would be the appendix. Six thousand years ago when humans first evolved, they were forced to eat tough meats and plants. In order to better digest their food, humans adapted to have appendages. Also, our lack of use of appendages shows how humans have further evolved since they were first “created”. Divergent evolution occurs when a species, living int two separate and independent habitats, evolves to have different traits. Divergent evolution occurred after Pangea and can be seen throughout the world. A common example of divergent evolution is the birds of the Galapagos islands. This species of birds experienced divergent evolution as on each island because of the different food sources, the species of birds diverged and evolved to have numerous beak types, each type suited specifically for one island. Convergent evolution likewise shows how species adapt to their environment. In convergent evolution, two separate species of a separate taxonomy develop similar traits in response to the similar environment that they live in. An example of this is sharks and whales, who, though are not of the same taxonomy, share many similar characteristics such as a dorsal fin and rounded nose as they have adapted to the same ocean environment. Embryology is the study of embryos and provides further proof of evolution as it reveals that during a certain developmental stage, the embryos of numerous different and unrelated species look almost identical and both have a tail and gills. This further proves that many of the species today evolved from one common ancestor and thus continue t o share traits. Finally, molecular biology, which compares the DNA of different species shows how that because of almost identical genotypes, certain species evolved from other species. An example of this are chimps and humans, who share 98% of identical DNA as humans evolved from chimps. Molecular biology establishes the lineage of evolution between different species by using precise and collected data.
[This section was written based purely on our class notes.]
Pangolin Adaptations
Over thousands of years, the pangolin has evolved into a complex organism with numerous adaptations which provide it protection, nutrition and ultimately allow it to prosper within its environment. The first of the pangolin's adaptations is its tough scales. These scales, which are made of keratin, are extremely dense, heavy, and sharp [17]. They cover the entire pangolins body except for its belly and are fused together in a way in which makes them easy to move and water tight [18]. Furthermore, the scales provide protection against predators, can be used as a formidable weapon, and their brown color allows pangolins to blend in with various environments. The pangolins second adaptation, its ability to curl up, is likewise an important defense mechanism. When under attack or feeling threatened, a pangolin may curl into a spherical shape. Once in this shape, only the pangolins sharp scales are displayed whilst the pangolins soft belly is protected. The pangolins abdomen muscles, which allow the pangolin to curl up, also are strong enough to prohibit a predator from prying open a curled pangolin [19].
The third of the pangolins adaptations is the pangolins ability to walk on its hind legs instead of all four limbs. By doing so, the Pangolin may avoid damaging its claws whilst moving at a relatively fast speed of up to five miles an hour [20]. Pangolins have likewise evolved to have large fore-claws. These claws are both large, long, and sharp. They allow the pangolin to quickly and effectively excavate termite and ant mounds, scale trees, pry out rocks and branches, and pull away tree bark in order to locate tree termites [21]. Without their claws, it is likely that a pangolin would be incapable of obtaining ants and termites and would thus be unable to meet its nutritional needs.
As the pangolin is a nocturnal species and thus has poor eyesight, the pangolin has advanced other methods of detecting stimuli. These include the pangolins advanced chemoreceptors and mechanoreceptors [22]. A chemoreceptor detects chemical stimuli while a mechanoreceptor detects mechanical stimuli. Because of these receptors, pangolins are constantly aware of their environment and stimuli and may appropriately act towards them. The sixth of the pangolin adaptations is the pangolins tough eyelids. When excavating ant and termite mounds in order to retrieve food, pangolins are constantly attacked by termites and ants attempting to protect themselves and their colony. These ants and termites will bite pangolins in their most vulnerable places, yet, because of its tough eyelids, pangolin eyes are protected from bites and pangolins may continue to feed unharmed [23]. Finally, the seventh and last adaptation of the pangolin is its tail. Tree pangolin tails are semi-prehensile which allows tree pangolins to easily scale and hang from trees. Likewise, when walking, the tree pangolins tail provides it both balance and support.
The third of the pangolins adaptations is the pangolins ability to walk on its hind legs instead of all four limbs. By doing so, the Pangolin may avoid damaging its claws whilst moving at a relatively fast speed of up to five miles an hour [20]. Pangolins have likewise evolved to have large fore-claws. These claws are both large, long, and sharp. They allow the pangolin to quickly and effectively excavate termite and ant mounds, scale trees, pry out rocks and branches, and pull away tree bark in order to locate tree termites [21]. Without their claws, it is likely that a pangolin would be incapable of obtaining ants and termites and would thus be unable to meet its nutritional needs.
As the pangolin is a nocturnal species and thus has poor eyesight, the pangolin has advanced other methods of detecting stimuli. These include the pangolins advanced chemoreceptors and mechanoreceptors [22]. A chemoreceptor detects chemical stimuli while a mechanoreceptor detects mechanical stimuli. Because of these receptors, pangolins are constantly aware of their environment and stimuli and may appropriately act towards them. The sixth of the pangolin adaptations is the pangolins tough eyelids. When excavating ant and termite mounds in order to retrieve food, pangolins are constantly attacked by termites and ants attempting to protect themselves and their colony. These ants and termites will bite pangolins in their most vulnerable places, yet, because of its tough eyelids, pangolin eyes are protected from bites and pangolins may continue to feed unharmed [23]. Finally, the seventh and last adaptation of the pangolin is its tail. Tree pangolin tails are semi-prehensile which allows tree pangolins to easily scale and hang from trees. Likewise, when walking, the tree pangolins tail provides it both balance and support.
Early Pangolins
The earliest Pangolin fossils were excavated in India and date back to the Pleistocene Epoch, 2,600,00 B.C. to 11,700 B.C. [24]. Yet, an ongoing debate between scientists, ecologists, and researchers revolves around these fossils, as some scientists believe that the pangolin is related to the stegosaurus, an ancient dinosaur which lived during the jurassic age. Even though the pangolin and stegosaurus are not genetically related, some scientists claim that they shared similar physical characteristics. They argue that, like the pangolin, the stegosaurus had a long tail covered in flat and protective plates, had a similar conical skull, walked with its nose to the ground (its highest point of its body at its hips), and had an almost identical proportion of limb size [25]. Likewise, the first pangolin fossils were found not long after this dinosaur was believed to have become extinct. Could it be possible that the pangolin evolved from the stegosaurus? Could the stegosaurus have adapted to the size and shape of the current pangolin, as only the smallest of stegosauruses could survive? Other scientists, who support Darwin's theories of evolution controvert these beliefs and assert that the similarities between the pangolin and stegosaurs are due to both animals adapting to similar niches and not a common lineage between the two [26].
Taxonomy
Special Adaptations
1. Pangolins have ten to twenty-seven inch tongues which they use to catch and eat insects and termites. [1]
2. A majority of a Pangolin's body is covered with sharp scales which can cut humans and predators. The reason that when it feels threatened, the Pangolin will role into a ball, as it is protecting its "soft" parts from an attack. A form of defense which is very effective. [2]
3. Pangolins have a special gland which secretes a pungent liquid which can scare off predators. [3]
4. Tree Pangolins have semi-prehensile tails which they use to climb and hang off of trees. [4]
5. Pangolin's have long and curved claws which allow them to easily dig holes and pull bark off from trees in order to locate insects and termites. [5]
6. Instead of using teeth to grind its food (because it has no teeth), Pangolins use small pebbles in their stomach (like a bird's gizzard) to mechanically digest their food. [6]
2. A majority of a Pangolin's body is covered with sharp scales which can cut humans and predators. The reason that when it feels threatened, the Pangolin will role into a ball, as it is protecting its "soft" parts from an attack. A form of defense which is very effective. [2]
3. Pangolins have a special gland which secretes a pungent liquid which can scare off predators. [3]
4. Tree Pangolins have semi-prehensile tails which they use to climb and hang off of trees. [4]
5. Pangolin's have long and curved claws which allow them to easily dig holes and pull bark off from trees in order to locate insects and termites. [5]
6. Instead of using teeth to grind its food (because it has no teeth), Pangolins use small pebbles in their stomach (like a bird's gizzard) to mechanically digest their food. [6]