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| Comparative Study |
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| The common features show that an evolutionary process exists and there is a link between the different groups. |
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| A study of the comparative biochemistry shows that certain similar molecules occur in different groups of animals. This can be compared to the homologous structures. |
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| For example, structure of haemoglobin present in man, chimpanzee, gorilla and gibbon show remarkable similarities. Haemoglobin is a protein that has four polypeptide chains. Each polypeptide chain is made up of a fixed number of amino acids. The haemoglobin of chimpanzees differs from that of man in only 1 amino acid, of gorilla in 3 amino acids and of gibbon in 8 amino acids. This indicates a common ancestry between the various primate groups. |
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| Right from the early days of human civilization man has been breeding selected species that are useful to him. This has produced different varieties of plants and animals. |
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| The Result of Selective Breeding |
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| The comparative anatomical studies of various types of organisms shows structural and functional differences and similarities between various species. For example, the basic structure of all the flowers is the same with the whorls of calyx, corolla, androecium and gynoecium. However, each species is different in the shape, size and number of the individual members of the species. Similarly, the limb-bone pattern of all the animals with four limbs (tetrapods) is the same - pentadactyl limb. It shows modifications in the different species as shown in the diagram: |
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| Adaptations of the Pentdactyl Limb Shown by Mammals |
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| Such organs that have a common basic form but are present in different species are called the homologous organs. Thus the wings of bat are homologous to the limbs of man. |
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| Vestigeal Organs in Man |
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| Certain homologous organs have no function in some species. Such as the appendix in human beings which is non-functional and its homologous organ is functional in herbivorous animals. Such non-functional homologous structures are called the vestigial organs. Other examples of vestigial organs are the tail bone or coccyx in human which is homologous to the ones in the monkeys and the nictitating membrane, the reduced third eyelid that forms a transparent cover over the eyes of underwater creatures. |
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| These homologous features are important in the evolutionary process. |
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| Comparative morphology involves the study of the external features of organisms. There are two types of evidences that comparative morphology reveals. They are: |
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| Analogous Structures |
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| There appear to be many structures that are similar because of the function they carry out and in their external appearance. However, these structures differ internally. Such structures are called analogous structures. For example, wings of bat and butterfly are similar in appearance and function but they are internally very different and have different origins. Similarly thorns and spines seen in plants are also analogous structures. Both are pointed structures that are protective in function. However, thorn is modification of stem and spine is modified leaf. Another example of analogous structures in plants are the tendrils of different types carrying out similar function. |
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| Analogy in Tendrils |
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| Living Fossils |
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| The study of morphology also reveals some organisms that form links between two different groups. This is further evidence that evolution has taken place and it also shows the direction taken by the process of evolution. These organisms that show the features of two different groups are called the living fossils as they provide a link between the two groups. They are as important as fossils in providing linkages and also they may have been surviving without much variation for many years. |
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| For example, some living fossils are: |
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| Duck-billed platypus, an egg-laying mammal that forms a link between reptiles and mammals; amphibians that show link between fishes and reptiles; lungfish that shows link between amphibians and fishes. |
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| Lung Fish, Connecting Link Between Fishes and Amphibians |
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| Adaptive Radiation |
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| The principle of adaptive radiation is based on two evolutionary mechanisms - convergent and divergent evolutions. |
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| Divergent evolution involves the homologous structures. A group of organisms modify a common homologous structure to perform different functions. For example, the modifications of the pentadactyl limbs as shown below: |
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| Another example of divergent evolution is the modification of the basic mouthparts of the insects into the different types - sucking, biting, etc. |
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| Mouth Parts of Insects |
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| It is because of this adaptive ability that the insects are so widely present and are of so many varieties. |
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| Convergent evolution involves analogous structures. Analogous structures are those that show no similarity in the internal structure or anatomy. However, they carry out similar structural or physiological functions. Best examples of analogous structures are the wings of butterfly, bird and bat. All the three have different internal structures but carry out similar function. |
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