Evidences Supporting Organic Evolution in Morphology and Anatomy

1. Evidences in favor of organic Evolution By G.Mallanagoud, Asst. Prof. Dept. of Zoology Veerashaiva College, Ballari, Karnataka.

2. Evidences in favor of organic Evolution Introduction: An evolution is incredibly and unimaginably a slow process, the evolutionary process cannot be seen by man during his short span of life. Hence evolution can be understood only by a comparative study of animals. So it is inevitable to give evidences for evolution. Evidences of organic evolution: There are several evidences of organic evolution. Such as follows. 1. Evidences from morphology and comparative anatomy 2. Embryological evidences 3. Physiology and biochemical evidences 4. Taxonomical evidences 5. Geographical evidences 6. Genetical evidences and so on.

3. Evidences from morphology and comparative anatomy Morphology refers to the external structures, anatomy refers to the internal structures. These structures provide ample evidences of evolution. The sources of evidences of evolution from morphology and comparative anatomy are the following. 1. Homologous structures 2. Analogous structures 3. Vestigial organs 4. Adaptive radiation 5. Parallel evolution 6. Convergent evolution 7. Atavism and 8. Connecting links.

4. HOMOLOGOUS ORGANS Homology: It refers to the traits inherited by two different organisms from a common ancestry. Definition: ”The structures which are similar in their morphology, anatomy, genetics and embryology but dissimilar in their functions are called homologous organs”. • In evolution, they are body parts of species that have similar features, indicating a common ancestor or same developmental origin. • These structures are inherited from a common ancestor. They have similar developmental pattern in the embryos. They receive the same types of bones, blood, vessels, nerves, muscles, etc. But they do dissimilar functions in different animals. The relationship between homologous structures is called homology. Homology simply refers to “similar structures with dissimilar functions”.

5. Examples of Homologous organs: 1. Forelimb of Frog and Bird: • Homology is the similarities between organs of different animals, based on the common ancestry of common embryonic origin. • Forelimbs of frog and bird are look very different but perform different functions, but exhibit the same functional plan including some bones. • The modifications including shortening and lengthening of bones in accordance with function. • The limbs are adapted for creeping in frogs and flying in birds. • In frogs, the carpels and metacarpels are fused to form carpo meata carpels. • The digits are reduced to three. • The prepetagium lies in between upper arm and trunk which forms wings. • The wing feather attached to forearms and hind arms increase wing span area, flight muscles are attached to wings cause movement of wings. • Fore limb study of frog and bird becomes evident that they have same type of internal structure, in skeleton, bones, nerves, blood vessels of muscles. This similarity is because of their descent origin, from a common ancestors and their differences are coming to their different modes of life. These evolved from common ancestors with common parts of pentadactyle limbs. • Each limb has three main divisions namely the upper arm, the fore arm and the hand. In frog and bird each of these divisions contains the same type of bones, blood vessels and muscles. • In both the frog and bird, the upper arm is supported by a single bone called humerus. The fore arm is supported by two bones, namely radius and ulna and the hand is supported by two bones, namely radius and ulna and the hand is supported by carpels, metacarpels and phalanges tipped with a claw or hoof.

7. 2. Mouth parts of cockroach and mosquito: • The mouth pars of cockroach and mosquito having similar structure , origin and development. • The mouth pars of cockroach and mosquito are the homologous organs because they exhibit homology. • Cockroaches are provided with cutting and chewing type of mouth parts and the mosquitoes are provided with piercing and sucking type of mouth parts. • In both of these organisms mouth parts perform different functions. • These mouth parts are modified according to their food habits. • In mosquito, piercing(pricking) and sucking type of mouth pars helps to penetrate the body of vertebrates and suck the blood. Because the mosquitoes are adapted for sangivorous (blood sucking) habitat. • These are the modifications of single pattern through they perform different functions. • In both cases, the mouth parts consist of basic structures like labrum, labium, mandibles, maxillae and hypo pharynx. • It is believed that cutting and chewing type of mouth parts are the ancestral characteristics from which other modified types are evolved.

8. 3. Serial homology of Crustacean appendages: • Definition: “The fundamental similarity of structures between one part of an animal and another part of the same animal is called serial homology”. Ex: Appendages of Prawn: • The appendages of crustaceans form a suitable example of serial homology. • In prawn a single individual is provided with many appendages. • All these appendages of a single individual are derived from a typical biramous appendage. • The crustacean appendages are known for linear arrangement of modified appendages with different functions and morphology in single animal. • There are 19 pairs of appendages in Prawn. • These appendages are modified for various basic patterns, these basic structures are called protoprodites, which may possess more than one segments. • In appendages, the finger nearest to medium of the body is called endoprodite and the lateral one is called exoprodite. Each one consists of a several segments. • Appendages are divided based on the location, modification and functions. The fundamental similarities of structures between one part of the animal and another part of the animal is called serial homology.

9. ANALOGOUS ORGANS Analogy: It refers to the similarity in function of two different organisms due to convergent evolution and not common ancestry. Analogous structures: “The structures which are superficially similar but anatomically dissimilar to perform similar functions are called analogous organs”. • Similarity of structures connected with similarity of function is termed as analogy. • The analogous structures are functionally superficially similar, but anatomically they are dissimilar. • These structures are developed by unrelated animals. • Some times animals belonging to different groups live in the same habitat. Hence they lead a similar type of life. • This leads to the development of superficially similar structures to lead a similar way of life. • These animal belong to different groups, anatomically the structures are different.

10. Examples of Analogous organs: 1. Vertebrate Eye and Cephalopod Eye:- • Organs which are similar in their function but differ in structure and origin are called analogous organs. This phenomenon is called analogy. i.e Analogous organs are those which have different origin but assuming the superficial similarity and perform the same function. • Comments:- • Eye of the mammal(vertebrate) is an outer growth of the brain, while cephalopodan eye is an ectodermal in origin. • 2. Such as analogous organs do not show any evolutionary relationship. • They have originated the analogous organs during the process of evolution through adaptation to similar mode of life.

11. 2. Wings of Bird and Insects: • Organs which are similar in their function but differ in structure and origin are called analogous organs. This phenomenon is called analogy. i.e Analogous organs are those which have different origin but assuming the superficial similarity and perform the same function. • Comments:- • 1.The wings of birds and insects are the good examples for analogous organs and they exhibit analogy. • 2. The structure, origin and development of bird wings and insect wings are completely different but they perform same function. i.e flying. • 3. Wings of birds consist of feathers and supported by skeleton. It has muscles, blood vessels, nervous and other tissues. • 4. Wing of an insect is thin or membranous. Evolutionary Significance: • Analogous organs do not show their common ancestry. • They indicate the adaptation of different groups of animals to the similar mode of life.

12. Vestigial Organs Vestigial organ: “Vestigial structure is an anatomical feature or behavior that no longer seems to have a purpose in the current form of an organism.” • Features present in the modern animals that are no longer in use. • It gives hints as to the evolution of organisms. Examples: Human tailbone (Coccyx), Whale pelvis, Vermiform appendix etc.

13. Examples of Vestigial Organs 1. Vermiform Appendix: • The vermiform appendix is a good example for vestigial organs. • It is a small worm like structure present at the junction of small intestine(ileum) and large intestine. • In man, the caecum is very small and appendix is in the form of a worm-like structure called vermiform appendix. It has no useful function in man; but in other herbivorous mammals it is well developed and used for cellulose digestion. Evolutionary significance: • The presence of caecum and vermiform appendix as vestigial organ indicates that the man has evolved from ancestors who well developed and functional caecum and appendix. • The ancestors were herbivorous, in case of evolution of man has become carnivorous or omnivorous. Hence the caecum and appendix becomes functionless vestigial organ. • The vermiform appendix and reduced caecum in man form a good evidence for evolution. • Their presence in actual evidence of evolution from one evolutionary point of view, vestigial organs are the structures that were well developed and functional and necessary in our ancestors that are now in a process of disappearance.

14. 2. Coccyx in Man: • Coccyx in man is also good example for vestigial organ. • It is a tail-like structure present in the early embryo of man. • It is formed of a string of partly fused caudal vertebrae. It disappears before birth. • It is a vestigial vertebral column in the tail region. • It is composed of 3 to 5 fused caudal vertebrae. Evolutionary significance: • Sometimes children may be born with a visible tail. But the tail is reabsorbed before the adulthood is reached. This is because man has no use off this tail. For arboreal animals, the tail is essential. This indicates that man evolved from an arboreal ancestor which was having a tail. The coccyx gives a good evidence for organic evolution.

15. 3. Wisdom teeth: • Wisdom teeth (molar teeth) in man are the good example for vestigial organ. • Wisdom teeth are the last molar (posterior most) teeth. • They are 4 in numbers, 2 in the upper jaw and 2 in the lower jaw. • They erupt when we are 18 years old. Hence the name wisdom teeth. • In some persons, they fail to erupt, when they erupt they produced to be all types of dental defects. • In other mammals, these teeth erupt in time and functional. Evolutionary significance: • In man, the wisdom teeth have lost their function. It is believed that they did some functions until 2000 years ago. • The modern eating habits require less chewing and less massive jaws. Hence in the evolutionary process, the jaw has reduced and there is no space in the mouth for the wisdom teeth to grow.

16. 4. Nictitating membrane: In human eye there is a membrane called nictitating membrane it is a vestigial organ for humans but not for birds. 5. Ear muscles 6. Pelvic girdle in Whales 7. Wings of Kiwi (flightless condition) 8.Eyes in Cave animals – Proteus anguineus (salamander)

17. Differences between homologous & analogous organs Sl. No. Homologous organs Sl. No. Analogous organs 1 They have similar basic structure and embryonic origin (same basic structural design) 1 They are quite different in fundamental structure and origin (different basic structural design) 2 They perform different functions 2 They perform similar functions 3 Their appearance are different 3 They have similar appearance 4 They have inherited from common ancestry 4 Not inherited from common ancestry Ex: hands of human beings and forelimbs of horse, Forelimb of frog and bird Ex: Wing of birds and insects, Vertebrate and cephalopod eye