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| − | Note: This is only a very rough draft, with notes that may be useful in developing the article. Please do not edit this article until the actual article is complete — i.e., when this notice is removed. You may add comments on what you would like to see included. [[User:Rick Swarts|Rick Swarts]] 00:05, 28 Sep 2005 (UTC)
| + | #REDIRECT [[Evidence of evolution]] |
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| − | In Darwin's comprehensive theory of evolution, there can actually be elucidated at least five major, largely independent theories. The two basic theories, and the ones which I will treat here, are: (1) the theory of evolution by common descent, and (2) the theory of modification through natural selection. The first is a kinematic theory which deals with non-causal relations between things — it deals with the pattern of evolution. The latter is a dynamic theory which deals with mechanisms and causal relationships — it deals with the process. Other theories offered by Darwin deal with (3) evolution as such (the fact of evolution), (4) the gradualness of evolution, and (5) populational speciation.
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| − | The "theory of descent with modification" essentially postulates that all organisms have descended from common ancestors by a continuous process of branching. In other words, all life evolved from one kind of organism or from a few simple kinds, and each species arose in a single geographic location, from another species that preceded it in time. Evolutionists have marshaled substantial evidence for the theory of descent with modification. That is, the "pattern of evolution" is well documented by the fossil record, the distribution patterns of existing species, methods of dating fossils, and comparison of homologous structures. Interestingly, all of the classical arguments for evolution are fundamentally arguments for imperfections that reflect history. They fit the pattern of observing that the leg of Reptile B is not the best for walking, because it evolved from Fish A. In other words, why would a rat run, a bat fly, a porpoise swim and a man type all with the same structures utilizing the same bones unless inherited from a common ancestor?
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| − | Evidence is so overwhelming for the theory of descent with modification that only religious fundamentalists have attempted to challenge this theory. Among these are the “scientific creationists.” Scientific creationists are a specific group of creationists who maintain that modern organisms did not descend from common ancestors, and that their only historical connectedness is in the mind of God. Instead, scientific creationists promulgate the view that living organisms are immutable, and were all created by God in a short time period, on a earth whose age is generally measured in 1000s of years. The substantial fossil record is dismissed in various ways, including as a trick of God and as an artifact from the Great Flood (with some organisms sinking faster than others and thus on a lower fossil plane). Although some individual presentations by scientific creationists are quite sophisticated, the overall theory of scientific creationism runs counter to an enormous body of evidence and thus is strongly criticized by most of the scientific community.
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| − | The second theory of Darwin, the "theory of modification through natural selection," is one explanation offered for how evolution might have occurred, i.e, the "process" by which evolution took place and arrived at the pattern. This theory of natural selection was the most revolutionary and controversial concept advanced by Darwin. While the theory of descent with modification was accepted soon after its introduction, the theory of natural selection took until the mid-1900s to be accepted by the scientific community. By providing a purely non-teleogical, materialistic explanation for all phenomenon of living nature, it was said it "dethroned God."
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| − | According to this theory, natural selection is the directing or creative force of evolution. Natural selection is considered far more than just a minor force for weeding out unfit organisms. Even Paley and other natural theologians accepted natural selection, albeit as a devise for removing unfit organisms, rather than as a directive force for creating new species and new designs. It is this theory of natural selection that has the three radical components mentioned earlier: (a) purposelessness (no higher purpose, just the struggle of individuals to survive and reproduce); (b) philosophical materialism (matter is seen as the ground of all existence with spirit and mind being produced by or a function of the material brain); and (c) the view that evolution is not progressive from lower to higher, but just an adaptation to local environments; it could form a man with his superior brain or a parasite, but no one could say which is higher or lower.
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| − | Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the case of artificial selection, whereby various breeds of animals and varieties of plants have been produced that are different in some respect from their ancestors, or in the often-cited, but somewhat problematic case of systematic color change in the peppered moth, Biston betularia, which was observed over a 50-year period in England. The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level. That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years. If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors. One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.
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| − | The issue has been further complicated by the fact that textbooks have persisted in presenting some proofs for evolution which are false or misleading, as pointed out by Jonathan Wells in his book Icons of Evolution. These widely-known but misleading teachings include the famous Miller –Urey experiment in which sparks are sent through a mixture of gases and yield the building blocks of amino acids, and the drawings by Ernst Haeckel of the early embryonic stages of such vertebrates as fish, chick, rabbits and humans, whereby it is exhibited that the earliest stages in all of these vertebrates are virtually identical. Wells reports that scientists have known for years both that the Miller-Urey experiments did not really approximate conditions of the early earth and that Haeckel had faked his drawings, since in reality the vertebrate embryos never look as similar as he made them look. These errors are well-known, yet textbook authors persist in using these examples. Another interesting case is the classic example of natural selection as seen in the case of the peppered moth (Biston belularia) in England, known as a case of industrial melanism, whereby a shift toward darker melanic forms is seen and attributed to an heightened predation by birds of the light-colored moths, because the lighter forms could more easily be seen on the tree trunks which have been increasingly darkened from pollution. In these cases, individuals have known that peppered moths do not normally alight on tree trunks, and there are even inverse correlations with pollution in many situations. Textbook photos are generally staged by gluing or pinning the moths to tree trunks. Some authors have responded that they knew the peppered moth case had problems, but they were good examples because they were easily grasped by the students. Use of such flawed cases has the unfortunate consequence of causing distrust of science by the students.
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| − | [[Image:HMSBeagle.jpeg|thumb|right|250px|''HMS Beagle'', from an 1841 watercolour by Owen Stanley. [[Charles Darwin]]'s work during the [[HMS Beagle|''Beagle'']] expedition let him study at first hand geology, [[fossil]]s and a multitude of living organisms as well as meeting native peoples. He methodically collected an enormous number of specimens, many new to science, which established his reputation as a naturalist and made him one of the [[ecology#The_notion_of_biocenose:_Darwin_and_Wallace|precursors of ecology]].]]
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| − | [[Image:South Djoum Chimp.jpg|right|250px|thumbnail|Genetic testing has shown that humans and [[chimpanzee]]s have most of their [[DNA]] in common. In a study of 90,000 [[base pair]]s, [[Wayne State University]]'s Morris Goodman found humans and chimpanzees share 99.4% of their DNA.[http://www.freep.com/news/nw/chimp20_20030520.htm]
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| − | [http://www.reasons.org/resources/apologetics/humans_chimps_same_genus.shtml].]]
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| − | The range of '''evidence of evolution''' show us how [[evolution]] could happen and what [[natural selection]] has to explain. [[Fossil]]s are important for estimating when various lineages developed. As fossilization is an uncommon occurrence, usually requiring hard parts (like bone) and death near a site where [[sediment]]s are being deposited, the [[fossil record]] only provides sparse and intermittent information about the evolution of life. Fossil evidence of early life is sparse before the evolution of organisms with hard body parts, such as shell, bone, and teeth, but exists in the form of ancient microfossils and the fossilization of ancient burrows and a few soft-bodied organisms.
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| − | Comparison of the genetic sequence of organisms reveals that organisms that are [[phylogeny|phylogenetically]] close have a higher degree of sequence similarity than organisms that are phylogenetically distant. Further evidence for common descent comes from genetic detritus such as [[pseudogene]]s, regions of DNA which are [[ortholog]]ous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration.
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| − | Since [[metabolism|metabolic]] processes do not leave fossils, research into the evolution of the basic cellular processes is also done largely by comparison of existing organisms. Many lineages diverged at different stages of development, so it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor.
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| − | ==Evidence from palaeontology==
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| − | [[Image:Resin with insect (aka).jpg|thumb|250px|left|'''Figure 1''': Insect trapped in resin.]]
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| − | When organisms die, they are often [[decompose|decomposed]] rapidly or [[consumption|consumed]] by [[scavenger]]s, leaving no permanent evidences of their existence. Occassionally, some organisms become preserved in some ways. The preserved remains or traces of organsims from a past [[age|geological age]] embedded in [[rock (geology)|rocks]] by natural processes are called fossils. They are extremely important as they provide direct evidence of evolution and detailed information on the [[Timeline of evolution|evolutionary history of life]] on [[Earth]]. [[Palaeontology]] is the study of past life on the Earth based on fossil records and their relations to different geological time and geological layers.
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| − | For fossilization to take place, the traces and remains of organisms must be quickly buried so that weathering and decomposition did not occur. Skeletal structues or other hard parts of the organisms are the most commonly occurring form of fossilized remains. There are also some trace "fossils" showing [[mould]]s, cast or imprints of some previous organisms,
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| − | As an animal dies, the organic materials gradually decay away, such that the [[bone]]s become porous. If the animal is subsequently buried in [[mud]], [[mineral|mineral salt]]s will infiltrate into the bones and gradually fill up the pores. The bones will harden into stones and are preserved forever as fossils. This process is known as [[petrification]]. If dead animals are covered by wind-blown [[sand]], and if the sand is subsequently turned into mud by [[rain|heavy rain]] or [[flood]]s, the same process of mineral infiltration may occur. Apart from petrification, the dead bodies of organisms may be well [[preservation|preserved]] in [[oil]], in [[ice]], in hardened [[resin]] of [[coniferous|coniferous trees]] ([[amber]], Fig. 1), in tar, or in [[anaerobic]], [[acidic]] [[peat]]. Sometimes, fossilisation can be a [[trace]], an impression of a form, for example, a leaf or a footprint, which is made in layers that then harden.
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| − | ===Fossil records===
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| − | [[Image:Asaphiscuswheelerii.jpg|thumb|250px|'''Figure 2''': [[Trilobite|Fossil trilobite]]. Trilobites, hard-shelled animals, related to living [[crab]]s and [[shrimp]]s, first appeared in signifcant numbers 500 [[Mya (unit)|mya]] and finally died out 250 mya. Apparently, they changed very little over many millions of years.]]
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| − | It is possible to find out how a particular group of organisms evolved by arranging its fossil records in a geological sequence. Such a sequence can be worked out because fossils are mainly found in [[sedimentary rock]]. Sedimentary rock is formed by layers of [[silt]] or mud on top of each other. Thus the resulting rock contains a series of horizontal layers or [[stratum|strata]]. Each layer contains fossils which are typical for that time period when they were laid down. The lowest strata contain the oldest rock with the earliest fossils while the highest strata contain the youngest rock with recent fossils.
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| − | A [[succession]] of animals and plants can also be seen from fossil records. Fossil evidence supports a theory of progressive increase in complexity of organisms. By studying the number and complexity of different fossils at different [[stratigraphy|stratigraphic levels]], it shows that:
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| − | * Oldest fossil-bearing rocks contain very few types of fossilized organisms and they all have a simple structure; and
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| − | * Younger rocks contain a greater variety of fossils with increasingly complex structures.
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| − | In the past, the ages of various strata and the fossils found were roughly estimated by geologists. They did so, for instance, by estimating the time for the formation of sedimentary rock layer by layer. Today, by measuring the proportions of radioactive elements and stable elements in a given rock, the ages of fossils can be precisely dated by scientists. This technique is known as [[radiometric dating]]. Throughout the fossil record, many species which appear at an early stratigraphic level disappear at a later level. This is interpreted in evolutionary terms as indicating the times at which species originated and became extinct. Geographical regions and climatic conditions have varied throughout the Earth’s history. Since organisms are adapted to particular environments, the constantly changing conditions may have favoured a mechanism for evolutionary change.
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| − | ===Evolutionary development of modern horse===
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| − | [[Image:Horseevolution.png|450px|thumb|right|'''Figure 3''': [[Evolution of the Horse|Evolution of horse]] showing reconstruction of the fossil species obtained from successive rock strata. The foots diagrams are all front views of the left forefoot. The third [[metacarpal]] is shaded throughout. The teeth are shown in longitudinal section.]]
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| − | The [[horse]] provides one of the best examples of evolutionary history ([[phylogeny]]) based on an almost complete fossil record found in [[North America]]n sedimentary deposits from the early [[Eocene]] to the present (Fig. 3).
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| − | Horse starts with a little animal called ''Hyracotherium'' which lived in North America in Eocene age about 54 million years ago and then spread across to [[Europe]] and [[Asia]]. Fossil remains of ''Hyracotherium'' obtained from Eocene rocks in North America show it to have differed from modern horse in three important respects:
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| − | * It was a small animal (like the size of a [[fox]]) lightly built and adapted for running;
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| − | * The limbs were short and slender and the feet elongated so that the digits were almost vertical. There were 4 digits in the [[forelimb]]s and 3 digits in the [[hindlimb]]s;
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| − | * The [[incisor]]s were small and the [[molar]]s had low crowns with rounded [[cusp]]s covered in [[enamel]].
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| − | The probable course of development of horses from Hyracotheium to [[Equus]] (modern horse) involved at least 12 [[genus|genera]] and several hundred [[species]]. The major trends seen in the development of horse to changing environmental conditions and may be summarized as follows:
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| − | * increase in size (from 0.4m to 1.5m);
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| − | * lengthening of limbs and feet;
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| − | * reduction of lateral digits;
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| − | * increase in length and thickness of the third digit;
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| − | * increase in width of [[incisor]]s;
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| − | * replacement of [[premolar]]s by [[molar]]s; and
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| − | * increases in tooth length, crown height of molars.
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| − | The fossils plants found in different strata show that the [[marsh|marshy]], [[wood|wooded]] [[country]] in which Hytacotherium lived was gradually replaced by a drier type. Survival now depended on the head being in an elevated position for gaining a good view of the surrounding [[countryside]], and on a high turn of [[speed]] for escape from [[predator]]s. Hence the increase in size and the replacement of the played-out foot by the hoofed foot. The drier, harder ground would make the original splayed-out foot unnecessary for support. The changes in the teeth can be explained by assuming that the diet changed from soft [[vegetation]] to [[grass]]. A dominant genus from each geological period has been selected to show the progressive development of the horse. However, it is important to note that there is no evidence that the forms illustrated are direct [[relative]]s of each other.
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| − | ===Limitations===
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| − | [[Image:Miniprotoandsketch.jpg|thumb|250px|right|'''Figure 4''': The [[mini|mini car]] first appeared in [[1962]], and changed little in external design. But, as with fossils, the external outline tells us nothing about "evolution" of the internal machinery. This has developed almost beyond recognition in the case of the car (and possibly did in Trilobites too, see above). Most fossils give us an idea about external form, but little or nothing about how the organism functioned.]]
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| − | The study of fossil records is helpful for scientists to trace the evolutionary history of organisms. However, in reality, hardly any fossils that were intermediate forms between related groups of species could be found. The lack of continuous fossils records is the major limitation in evidence for the existence of such intermediate forms of organisms. These gaps in the fossil records are called the missing links.
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| − | There is a gap of about 100 million years between the early [[Cambrian|Cambrian period]] and the later [[Ordovician period]]. The early Cambrian period was the period from which numerous fossil of [[sponge]]s, [[cnidarian]]s (e.g. [[coral]]s), [[echinoderm]]s (e.g. [[brittle star]]s), [[molluscs]] (e.g. [[snail]]s) and [[arthropod]]s (e.g. [[trilobite]]s) are found. In the later Ordovician period, the first animal that really possessed the features of a [[fish]] (a [[vertebrate]] ) appeared. In other words, no fossils of an intermediate type between [[invertebrate]]s and vertebrates have been found.
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| − | The reasons for the incompleteness of fossil records are listed below:
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| − | * In general, the chance that an organism becomes fossilized after death is very low;
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| − | * some species or groups might have never become fossils because they are soft-bodied;
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| − | * some species or groups might have never become fossils because they lived and died in conditions that were not favourable for fossilization to occur;
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| − | * most fossils express an idea about external form, but little or nothing about how the organism functioned (Fig. 4);
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| − | * many fossils have been destroyed by land movements and erosion;
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| − | * all the fossils unearthed represent only a fraction of the large number of species of organisms that lived in the past; and
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| − | * some fossil remains unearthed are complete, but most are fragmentary.
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| − | ===Living fossils===
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| − | According to fossil records, a few modern species of plants and animals are found to be almost identical to the species that lived in ancient geological ages. They are existing species of ancient lineage that have remained [[morphology|morphologically]] (and probably also [[physiology|physiologically]]) unchanged for a very long time. Consequently they are called [[living fossil]]s. Examples include the [[horseshoe crab|kingcrab]] ([[horseshoe crab]]), the [[coelacanth]], the [[ginkgo]] (Fig. 5), and the [[metasequoia]].
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| − | ==Evidence from comparative anatomy==
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| − | [[Comparative anatomy|Comparative study of the anatomy]] of groups of animals or plants reveals that certain structural features are basically similar. For example, the basic structure of all [[flower]]s consists of [[sepal]]s, [[petal]]s, [[stigma]], [[style]] and [[ovary]]; yet the [[size]], [[colour]], [[number]] of parts and specific structure are different for each individual species.
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| − | ===Homologous structures and divergent (adaptive) evolution===
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| − | If widely separated groups of organisms are originated from a common ancestry, they are expected to have certain basic features in common. The degree of [[similarity|resemblance]] between two organisms should indicate how closely related they are in evolution:
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| − | * Groups with little in common are assumed to have diverged from a [[common ancestor]] much earlier in geological history than groups which have a lot in common;
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| − | * in deciding how closely related two animals are, a comparative anatomist looks for [[structure]]s which, though they may serve quite different [[function]]s in the [[adult]], are fundamentally similar, suggesting a common origin. Such structures are described as [[Homology|homologous]]; and
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| − | * in case where the structures serve different functions in adult, it may be necessary to trace their origin and embryonic development.
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| − | When a group of organism share a homologous structure which is specialized to perform a variety of functions in order to adapt different environmental conditions and modes of life are called [[adaptive radiation]]. The gradual spreading of organisms with adaptive radiation is known as [[divergent evolution]].
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| − | ====Pentadactyl limb====
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| − | [[Image:Evolution pl.png|thumb|right|500px|'''Figure 5a''': The principle of [[homology]] illustrated by the adaptive radiation of the forelimb of mammals. All conform to the basic pentadactyl pattern but are modified for different usages. The third metacarpal is shaded throughout; the shoulder is crossed-hatched.]]
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| − | The pattern of limb bones called [[pentadactyl limb]] is an example of homologous structures (Fig. 5a). It is found in all classes of [[tetrapod]]s (i.e. from [[amphibian]]s to [[mammal]]s). It can even be traced back to the [[fin]]s of certain fossil fishes from which the first amphibians are thought to have evolved. The limb has a single proximal bone ([[humerus]]), two distal bones ([[radius]] and [[ulna]]), a series of [[carpal]]s ([[wrist]] bones), followed by five series of metacarpals ([[palm]] bones) and [[phalange]]s (digits). Throughout the tetrapods, the fundamental structures of pentadactyl limbs are the same, indicating that they originated from a common ancestor. But in the course of evolution, these fundamental structures have been modified. They have become superficially different and unrelated structures to serve different functions in adaption to different environments and modes of life. This phenomenon is clearly shown in the forelimbs of mammals. For example:
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| − | * In the [[monkey]], the forelimbs are much elongated to form a grasping hand for climbing and swinging among trees.
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| − | * In the [[pig]], the first digit is lost, and the second and fifth digits are reduced. The remaining two digits are longer and stouter than the rest and bear a hoof for supporting the body.
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| − | * In the horse, the forelimbs are adapted for support and running by great elongation of the third digit bearing a hoof.
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| − | * The [[mole]] has a pair of short, spade-like forelimbs for [[burrowing]].
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| − | * The [[anteater]] uses its enlarged third digit for tearing down [[ant]] hills and [[termite]] nests.
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| − | * In the [[whale]], the forelimbs become [[flipper]]s for steering and maintaining equilibrium during swimming.
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| − | * In the [[bat]], the forelimbs have turned into [[wing]]s for flying by great elongation of four digits, and the [[hook]]-like first digit remains free for hanging from [[tree]]s.
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| − | ====Insect mouthpart====
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| − | [[Image:Evolution insect mouthparts.png|thumb|left|250px|'''Figure 5b''': [[Adaptive radiation]] of insect mouthparts: a, [[antenna]]e; c, [[compound eye]]; lb, labrium; lr, labrum; md, mandibles; mx, maxillae.]]
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| − | The basic structures are the same which include a [[labrum]] (upper lip), a pair of [[mandible]]s, a [[hypopharynx]] (flor of mouth), a pair of [[maxillae]] and a [[labium]]. These structures are enlarged and modified, others are reduced abd lost. The modifications enable the insects to exploit a variety of food materials (Fig. 5b):
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| − | (A) Primitive state — biting and chewing: e.g. [[grasshopper]]. Strong mandibles and maxillae for manipulating food.
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| − | (B) Ticking and biting: e.g. [[honey bee]]. Labium long to lap up [[nectar]]; mandibles chew [[pollen]] and mould [[wax]].
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| − | (C) Sucking: e.g. [[butterfly]]. Labrum reduced; mandibles lost; maxillae long forming sucking tube; labrum reduced.
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| − | (D) Piercing and sucking: e.g. [[mosquito|female mosquito]]. Labrum and maxillae form tube; mandibles form piercing stylets; labrum grooved to hold other parts.
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| − | ===Analogous structures and convergent evolution===
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| − | [[Image:Evolution eye.png|thumb|right|400px|'''Figure 6''': Inverted retina of vertebrate (left) and non-inverted retina of octopus (right)]]
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| − | Under similar environmental conditions, fundamentally different structures in different groups of organisms may undergo modifications to serve similar functions. This phenomenon is called [[convergent evolution]]. Similar structures, physiological processes or mode of life in organisms apparently bearing no close phylogenetic links but showing adaptions to perform the same functions are described as [[analogy|analogous]], for example:
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| − | * Wings of [[bat]]s, [[bird]]s and [[insect]]s;
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| − | * the jointed legs of [[insect]]s and [[vertebrate]]s;
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| − | * tall [[fin]] of [[fish]], [[whale]] and [[lobster]];
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| − | * [[eye]]s of the [[vertebrate]]s and [[cephalopod]] molluscs ([[squid]] and [[octopus]]). Fig. 6 illustrates difference between an inverted and non-inverted [[retina]], the sensory cells lying beneath the [[nerve fibre]]s. This results in the sensory cells being absent where the [[optic nerve]] is attached to the eye, thus creating a [[blind spot]]. The octopus eye has a non-inverted retina in which the sensory cells lie above the nerve fibres. There is therefore no blind spot in this kind of eye. Apart from this difference the two eyes are remarkably similar, an example of convergent evolution.
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| − | ===Vestigial organs===
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| − | [[Image:Magellanic-penguin02.jpg|thumb|250px|right|'''Figure 7''': [[Magellanic penguin]] has vestigial wings, and thus does not have ability to fly.]]
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| − | A further aspect of comparative anatomy is the presence of vestigial organs. Organs that are smaller and simpler in structure than corresponding parts in the ancestral species are called vestigial organs. They are usually defenerated or underdeveloped. The existence of vestigial organs can be explained in terms of changes in the environment or modes of life of the species. Those organs are thought to be functional in the ancestral species but have now become unnecessary and non-functional. Examples are the vestigial hind limbs of whales, the balancers (vestigial hind [[wing]]s) of [[fly|flies]] and mosquitos, vestigial wings of flightless birds such as [[ostrich]]es and [[penguin]]s (Fig. 7), and the vestigial [[leaf|leaves]] of some [[xerophyte]]s (e.g. [[cactus]]) and parasitic plants (e.g. [[dodder]]).
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| − | ==Evidence from geographical distribution==
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| − | Biologists have discovered many puzzling facts about the presence of certain species on various [[continent]]s and [[island]]s ([[biogeography]]).
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| − | ====Continental distribution====
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| − | All organisms are adapted to their environment to a greater or lesser extent. If the abiotic and biotic factors within a [[habitat (ecology)|habitat]] are capable of supporting a particular species in one geographic area, then one might assume that the same species would be found in a similar habitat in a similar geographic area, e.g. in [[Africa]] and [[South America]]. This is not the case. Plant and animal species are discontinuously distributed throughout the word:
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| − | * Africa has short-tailed ([[Old World]]) monkeys, [[elephant]]s, [[lion]]s and [[giraffe]]s.
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| − | * South America has long-tailed monkeys, [[puma]]s, [[jaguar]]s and [[llama]]s.
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| − | An even greater differences can be found if [[Australia]] is taken into consideration though it occupies the same [[latitude]] as South America and Africa. Pouch mammals or [[marsupial]]s like [[kangaroo]] can be found in Australia, which are totally absent from Africa and are only represented by [[opossum]] in [[South America]] and the [[Virginia Opossum]] in [[North America]]:
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| − | * In Australia, spiny anteater and [[platypus|duckbilled platypus]], the only living representatives of primitive egg-laying mammals ([[monotreme]]s) can only be found but are totally absent in the rest of the world.
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| − | * On the other hand Australia has very few placental mammals except those that have been introduced by human.
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| − | ====Explanation====
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| − | [[Image:Evolution con dis.png|thumb|right|450px|'''Figure 8''': Diagrams to the land bridge between continents in past geological time (A) and the barriers formed (B) due to the submergence of [[land bridge]]s.]]
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| − | The main groups of modern mammal arose in [[Northern Hemisphere]] and subsequently migrated to three major directions:
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| − | * to South America via the [[land bridge]] in the [[Bering Strait]] and [[Isthmus of Panama]];
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| − | *to Africa via the [[Strait of Gibraltar]]; and
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| − | *and to Australia via South East Asia to which it was at one time connected by land
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| − | The shallowness of the Bering Strait would have made the passage of animals between two northern continents a relative easy matter, and it explains the present-day similarity of the two [[fauna]]s. But once they had got right down into the southern continents, they presumably became isolated from each other by various types of barrier.
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| − | *'''The submerging of the Isthmus of Panama''': isolates the South American fauna
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| − | *'''the [[Mediterranean Sea]] and the North African desert''': partially isolate the African fauna; and
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| − | *'''the submerging of the original connection between Australia and South East Asia''': isolates the Australian fauna
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| − | Once isolated, the animal in each continent has shown adaptive radiation (Fig. 8) to evolve along their own lines.
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| − | | |
| − | ====Evidence for migration and isolation====
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| − | [[Image:Evolution_evi_mig.png|450px|thumb|right|Map of the world showing distribution of present members of camel. Solid black lines indicate possible migration routes.]]
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| − | The fossil record for the [[camel]] indicated that evolution of camels started in Noth America, from which they migrated across the Bering Strait into Asia and hence to Africa, and through the Isthumus of Panama into South America. Once isolated they evolved along their own lines, giving the modern camel in Asia and Africa and llama in South America.
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| − | | |
| − | ====Continental drift====
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| − | ====Oceanic island distribution====
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| − | | |
| − | ==Evidence from comparative embryology==
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| − | | |
| − | ==Evidence from comparative physiology and biochemistry==
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| − | ===Serological studies===
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| − | ===Evolution of widely distributed proteins===
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| − | ====Cytochrome c====
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| − | ====Haemoglobin====
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| − | ====DNA====
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| − | ==See also==
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| − | | |
| − | ==References==
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| − | *Darwin, Charles [[November 24]] [[1859]]. ''On the [[Origin of Species]] by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life''. London: John Murray, Albemarle Street. 502 pages. Reprinted: Gramercy (May 22, 1995). ISBN 0517123207
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| − | *Mayr, Ernst. ''What Evolution Is''. Basic Books (October, 2002). ISBN 0465044263
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| − | *Gigerenzer, Gerd, et al., ''The empire of chance: how probability changed science and everyday life'' (New York: Cambridge University Press, 1989).
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| − | *Williams, G.C. (1966). Adaptation and Natural Selection: A Critique of some Current Evolutionary Thought. Princeton, N.J.: Princeton University Press.
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| − | *''Biological science'', Oxford, 2002.
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| − | *CJ Clegg, 1999, ''Genetics and Evolution'', John Murray. ISBN 0-7195-7552-4
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| − | *Y.K. Ho, 2004, ''Advanced-level Biology for Hong Kong'', Manhattan Press. ISBN 962-990-635-X
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| − | | |
| − | ==External links==
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| − | * [http://nationalacademies.org/evolution/ National Academies Evolution Resources]
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| − | * [http://www.chains-of-reason.org/chains/evolution-by-natural-selection/introduction.htm Evolution by Natural Selection] — An introduction to the logic of the theory of evolution by natural selection
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| − | * [http://www.pbs.org/wgbh/evolution/index.html Evolution] — Provided by ''[[Public Broadcasting Service|PBS]]''.
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| − | * [http://science.howstuffworks.com/evolution.htm/printable Howstuffworks.com — How Evolution Works]
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| − | * [http://www.genomenewsnetwork.org/categories/index/genome/evolution.php Evolution News from Genome News Network (GNN)]
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| − | * [http://www.nap.edu/books/0309063647/html/ National Academy Press: Teaching About Evolution and the Nature of Science]
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| − | {{credit|23637369}}
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| − | [[Category:Life sciences]]
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