EVIDENCES OF ORGANIC EVOLUTION
The theories which we have discussed are imaginary. There is no practical proof for them. The various animals which we are seeing to-day were gradually evolved from simple past lived organisms. It is not possible for anybody to observe a single change, because our life span is too short. Hence scientists collected evidences from different branches of biology. They are,
1. Morphological and anatomical evidences.
MORPHOLOGICAL AND ANATOMICAL EVIDENCES
In all the living organisms, the structural and functional units are cells. If all the living organisms were created by super natural power, why he created same cells in all living organisms? If we observe the shape of Chimpanzee and man, there is a great similarity. Why? Even if we observe the fore limbs of different vertebrates, there is a similarity. Why? If we observe the brain of different vertebrates, there is a similarity in structure and function. Why? All the above questions indicate that there is a relationship between the organisms.
The organs of different individuals are classified into three types. They are homologous, analogous and vestigial organs.
1. HOMOLOGOUS ORGANS
The homologous organs may differ in function and form (shape).
A. Homologous organs-
If we observe the fore limb of a Frog, wing of a Bird , paddle of a Whale and hand of a Man, all of them have same origin and same structure. Each of the organ has same bones, same blood vessels and same nerves.
Why the above organs (fore limb,wing,paddle,hand) differ in form (shape) and function? Because the above animals are living in different habitats and the organs have to perform different functions. In Frog, the homologous organs (fore limbs} perform leaping movement. In Birds, the homologous organs(Wings) perform flying movement. In Whale, the homologous organs (Paddles) perform swimming movement. In Man, the homologous organs(Hands) perform holding of objects.
The above explanation tells us that the above animals (Frog,Bird,Whale,Man) had a common ancestrol group. Probably it
may be osteolepid fishes. They lived in devonian period. One fish migrated to land. But this was not totally adopted for land life. Now and then it visited water. So one fish was evolved into Frog. Hence fore limb is adopted for leaping movement. One more fish first entered into land and finally entered into air. This lead to the origin of Bird. Hence forelimb is modified into wing for flying. One more fish first entered into the land and once again entered into water .This lead to the origin of whale. Hence fore limb is modified into paddle for swimming. One more fish first entered into the land and then acquired bipedal life. This resulted the origin of man. Hence fore limb is modified to hold the objects.
Adoptive radiation or Divergent Evolution:
The homologous organs explain adoptive radiation or divergent When the same animals (Osteolepid fishes) of same habitat entered into different habitats (land, air,water and land), they evolved into different individuals (Frog Bird ,Whale,Man). The formation of different animals of different habitats from the same animals of same habitat is called adoptive radiation or divergent evolution.
B. Example -II:
Mouth parts of different insects (Cockroach, Housefly, and Butterfly). All of them have same labrum, mandibles, maxillae and labium. But they differ in form and function. It is because their mode of feeding habits is different.
C. Example - iii: Vertebrae of different vertebrates.
D. Example - iv: Hind limbs of different vertebrates.
E. Example - v: Thorn of Bougainvillea and tendril of Cucurbit.
F. Example - vi: The brain of different vertebrates.
G. Example - vii: The wings’ of Bat and Bird.
H. Example -viii: The appendages of prawn explain serial homology.
The organs which have common form and function are called analogous organs. But they must differ in origin and structure.
A. Analogous organs-Example-I
The wings of Insect and Bird. In both the wings perform flying.But the wing of Insect is derived from ectoderm and it is supported by chitinous nervures. Whereas the wing of Bird is derived from mesoderm and it is supported by bones.
The above explanation tells us that the Insect and Bird had different ancestors. Probably the acquatic Annelid when entered into air, it evolved into lnsect.The terrestrial Reptile when entered into air, it evolved into Bird.
It is explained by analogous organs.When the different animals (Annelid,Reptile) of different habitats (Water,Iand) entered into the same habitat (air), they evolved into different animals (lnsect,Bird). It is called Convergent evolution
B. Example -ii: Scales of Fishes and Reptiles.
Tubers of seweet potato and vegetable potato.
When different animals are living in the same habitat, they contain same shape. So, analogous organs have same shape. For example Fish and Whale contain same shape as both are living in water The shape of Earthworm and Snake is same as both live in burrows.
The organs which are non functional and reduced in an organism are called vestigial organs. But these organs were well developed and performed functions in ancestors. The organs become functionless when the animals enter into new habitat or when their function is taken up by another organ or when the habits are changed. Presence of vestigial organs is the most convincing evidence in favour of
organic evolution and is also supported by the disuse principle of Lamarck.
A. Vestigial organs of Man:
The different vestigeal organs of man are Vermiform appendix Coccyx, Nictitating membrane (plica semilunaris), Muscles of Ear pinna , Clitoris of female human being etc.
B. Vestigeal organs of Ratitee Birds: Wings
C. Vestigeal organs of Python and Whale: hind limbs
D. Vestigeal organs of Horse: splint bones of limbs.
Formation of all ready disappeared organs suddenly in an individual is called atavism. Such organs are called atavistic organs. These support organic evolution. If human baby born with tail, the tail is considered as atavistic organ.
If human baby born with many pairs of mammary glands, except one pair other pairs are considered as atavistic organs.
4. CONNECTING LINKS
The organisms having the structures of two different groups are called connecting links. These explain the path of evolution . The connecting links are
A. Peripatus: It consists of nephridia like annelids and trachea like arthropods. Hence peripatus is considered as living connecting link between annelids and arthoropods. We can also say that annelids evolved into arthropods through the peripatus.
Annelids -------------------- peripatus ---------------> Arthropods.
These lay eggs like reptiles and contain mammary glands like mammals. Hence monotremes are considered as connecting inks between reptiles and mammals. We can also say that the reptiles evolved into mammals.
Reptiles --------- Monotremes ----------> Mammals