Question

What is organic evolution? Give four evidences in the support of evolution.

Answer 1

Evidences of Organic Evolution : 

The process of evolution is very slow and its results can’t be observed in the life span of man. Study of various branches of zoology provide many evidences in favor of organic evolution. 

Evidences from Zoo-geography : 

Zoo-geographical distribution of the animals provide evidence of evolution. It has observed some times that the regions of different geographical conditions inhabit similar fauna & flora and the regions of similar climatic conditions inhabit different fauna & flora. It can be explained with help of following examples

• Lung fishes

There are three genera of lung fishes which are found in different continents- 

(a) Protopterus-Africa 

(b) Lepidosiren -South America 

(c) Neocerartodus -Australia 

In the ancient time, the Australia & Africa were connected to the Asia by Bering strait and similarly the Africa was connected to the South America. These fishes were found earlier in all these regions but as a result of geographical changes they get confined to specific regions.

• Lion and elephant are found only in Asia and Africa, not in America & Australia, and Giraffe, ostrich and Hippopotamus are found in Africa. 

• Prototherians & metatherians are found only in Australia, because the Australia continent is isolated from the rest world. In Australia the marsupials remained protected & many new species get evolved . In the rest parts of the world, the evolution of eutherians resulted in the extinction of the marsupials.

• Darwin’s Finches- Darwin studied 20-22 types of birds in the Galapagos Islands situated in Pacific ocean. They are called as Darwin’s Finches 

• The species of camel are found in the form of camel in the North America & Asia and in the form of lama in South America. 

Evidences from comparative Anatomy : 

The vertebrate animals have many similarities like vertebral column. spinal cord, pharyngeal gills slits etc. It indicates that these animals have evolved from a common ancestor. Following organs provide evidence to this fact- 

(a) Homologous organs 

• The animals of different species have many organs which are similar in origin but they differ in structure. Such as fore limb of man, fore leg of horse, wings of bat. wings of birds and paddles of whale.

• The relationship between the homologous structures is called as homology which is due to common ancestor.

Homology among (a) wing of bat, (b) Fore leg of horse, (c) Fore limb of man, (d) Flipper of whale 

Homology is of three types

1. Phylogenetic homology 

• The homology which is found in the animals of different species is called as phylogenetic homology. 

Example – Fore limbs of man. whale, horse & bat. 

2. Serial homology 

• The homology in the various organs of the animals of same species on the basis of origin. 

Example- Fore limb & hind limb of man; various appendages of prawn etc. 

3. Sexual homology 

• The homology found in the male & female of a species is called as sexual homology. 

Example- Testes & ovaries of male & female. 

(b) Analogous organs

• In many animals different structures perform similar functions & appear to be similar. Such as wings of bats, birds & butter fly 

• The wings of bats, pterodactyl, bird & butterfly are used in flight but they have different origin. It is called as Analogy. Such organs are called as analogous organs.

Analogy in (a) wing of insect, (b) wing of bird and (c) wing of bat 

(c) Vestigeal organs

• Many animals have various non-functional organs which are called as Vestigeal organs. Such organs were functional but in the course of evolution they gradually reduced & became vestigial.

• Presence of such organs is a direct evidence of evolution.

• There are many vestigial organs in the body of human beings such as

1. Vermiform appendix 

It is a well known example of vestigial organ. In other mammals, the vermiform appendix is functional & helps in the digestion of cellulose. The vermiform appendix of man is a vestigial of the functional vermiform appendix of ancestors. 

2. Ear muscles in man is an another example of vestigial organs. In many mammals these muscles are functional & are use in moving the pinnae to receive the sound waves. In human being, they are of no use, hence became vestigeal. 

3. Presence of 3 to 5 vestigeal caudal vertebrae which are vestiges of well developed tail of other mammals. 

4. Red colored skin fold in the eyes. It is the vestiges of nictitating membrane of frog, rabbit & cat. 

5. Presence of pointed canines & 3rd molar and hair on the body 

6. Mammary glands in man & clitorius in woman. 

• Other animals also have vestigial organs. Such as highly reduced pelvic girdles in snakes & whales; reduced wings in flightless birds & siplent bone in the hind legs of horse & deer etc which is vestiges of fibula.

Human Vertigeal Organs

Answer 2

Differences between Homologous & Analogous organs

Homologous organs	Analogous organs
1. The body organs which are similar in origin, but may have different function.	1. The body organs which are similar in function but have different origin.
2. Their basic structure is similar	2. The basic structure is different.
3. Their function may be similar or different	3. Their function is similar
4. Examples – Fore limb of man , horses, whale.	4. Examples – Wings of birds, bat, insects etc.

(d) Atavism or Reversion 

It is the tendency to revert to ancestral type. In biology, an atavism is an evolutionary throw back, i.e. as traits (characters) reappearing which had disappeared generation before.

Examples

1. Presence of long & dense hair on the body of some human being (Like apes) 

2. Some time, small tail is seen in human body. 

3. Presence of long & pointed canines in same human babies. 

People in the modem era reverting to the ways of thinking and acting of a former time is an example of cultural atavism 

Evidence from Paleontology : 

Preserved remains of animals & plants in the layer of the earth crust are called as fossils. The study of fossils is called as paleontology. The hard parts of the animals like bones, teeth or shells are preserved in the form of fossils. 

Sometimes dead animal before decomposition get buried in the soil and as a result of chemical reactions converts into stone gradually. The near by soil also forms strata by erosion, deposition of sediment in sea, pressure & subsequent upheavals & immersion of land under water. This process is called as petrifaction. 

Leonard da vinsci (1452 to 1519) was the first to recognize that fossils provide evidence of life in the past. Darwin showed that fossils are direct evidence in support of organic evolution. 

The study of fossils revealed that the dinosaurs were the giant reptiles and they ruled the earth during Mesozoic era. Later on, they get extincted. 

Fossils of Archaeopteryx were obtained from the rocks of Jurassic period which is a connecting link between reptiles & birds. It had some reptilian characters like teeth, claws in fore limbs & scales on the body and some features of birds such as beak, feathers on the body & wings. It shows that the birds evolved from reptile-like ancestor. 

Scientists have studied the complete ancestry and history of evolution of horse, elephant and man with the help of fossils.

Hence, the fossils provide direct evidence in support of organic evolution.

Archaeopteryx 

Evidence from comparative Embryology :

Biogenetic law and Recapitulation theory given by Haeckel provide direct evidence of organic evolution. According to it. the embryonic stages of the animals are similar to their adult ancestors. At the time of formulation of recapitulation theory. Haeckel considered following principle of embxyology given by Von Baer- 

• During embryonic development general characters appear earlier than die specific characters.

• During development variations in different animals appear slowly.

• The specific features appear in the end of the embryonic development.

• The initial stages of embryonic development are similar in different animals. 

According to Muller-Haeckel law. “ ontogeny repeats phylogeny “ i.e. embryonic development of an animal repeats its evolutionary history. 

For example, during embryonic development, the human embryo resembles with fish in die beginning, later with amphibians & reptile. In seventh month, the human embryo resembles a monkey. 

Adult frog excretes urea but it’s tadpole excretes NH₃ like fishes. It is called as biochemical recapitulation .The tadpole larva has gills like fishes & human embryo has gill- slits like fishes. It is called as structural recapitulation 

The embryonic stages like zygote, cleavage, blastula, gastrula & neurula are basically similar in all animals.

Embryonic stages of various vertebrates 

Evidence from Interrelationship among organisms : 

There are millions of species of living organisms on the earth. Morphological, they appear to be different but when we study their physiology and chemical composition, a distinct interrelationship among species is seen. All the living beings are made up of cells and every cell performs all the vital activities. The basic structures of the cells of all the species is similar. 

The vital activities like respiration, excretion, reproduction, digestion are physiologically similar. All the living beings utilize O₂ in respiration & release CO₂. 

In all the multi cellular organisms, hydra to whale, the life begins from unicellular zygote. 

The above evidences reveal a common ancestor for all the living beings .