Evidences of Organic Evolution

Direct and indirect proofs of existence of evolution are:

Evidences of Organic Evolution from Comparative Anatomy
Study of functional anatomy is called Tectology.

Evidences of Organic Evolution from Homology and Homologous Organs
The structures which are different in appearance and perform different functions but have similar basic structure and origin are called homologous organs. The relationship between the structures having common basic plan and similar development origin is called homology or divergent evolution. Some important examples are: Fore limbs in vertebrates like flippers (paddles) of whale or seal, wings of bat, cat's paw, front foot of horse, human hand and wings of birds. They are built on same pentadactyle plan (5- digits) but performs different functions. They show similar arrangement of structure like bones, blood vessels, muscles, nerves, etc. Function: In seal – swimming, bird and bat – flying, in cat – running and walking, in horse – running, in man – grasping.
Evidences of Organic Evolution from Analogy and Analogous Organs
The organs which are different in the basic structures and developmental origin but appear similar and perform similar function. The relationship between structures of different groups of animals due to their similar function is called analogy or convergent evolution. Example: Wings of insects and birds. They are different in basic structure and origin. Insects wing are formed from integument while the bird wing is modified fore limb. They are analogous organ, as both are flat and are adapted for flight. Fins of fishes and the flippers of whale are of similar function but structurally different. The sting of honey bees and scorpions perform similar function and look alike. They are analogous structures because the sting of honey bee is a modification of its ovipositor while that of scorpion is modified by last abdominal segment.

Evidences of Organic Evolution from Vestigial Organs
Those organs of the body which were functional in ancestors but non-functional in the descendants. Example: There are about 90-100 vestigial organs in man. Important vestigial organs in man are – Coccyx (tail bone), nictitating membrane (third eyelid), muscles of ear pinna, caecum, vermiform appendix, canine, body hair, mammary gland in male, segmental muscles on abdomen, etc. Hind limbs in Python, Pelvic girdle and pinna of whale, wings of flightless birds, etc are example of vestigial organs. Snake do not possess legs because legs are degenerated during evolution.

Evidences of Organic Evolution from Connecting Link
The living organisms having intermediate characters between two group are called connection link. Example: Virus, Euglena, Proterospongia, Peripatus, Neoplina, Balanoglossus, Dipnoi, Archaeopteryx, Prototheria. Protopterus (Lung Fish): It is a connecting link between bony fish and Amphibia. In fish paired fins, dermal scales, lateral line system and gills. In amphibians internal nares, lungs and 3 chambered heart. Ornithorhynchus (Duck billed Platypus) and Tachyglossus (spiny and eater) are egg laying mammals. They act as connecting links between mammals. Mammalian characters are hair, diaphragm, mammary glands etc. while reptilian characters are large coracoid, being oviparous, laying eggs and having cloaca.

Evidences of Organic Evolution from Living Fossils
Those animals which underwent little change during long geological periods. Some important living fossils are:

Limulus and Peripatus – Arthopoda
Nautilus and Neopilina – Mollusca
Lingula – Brachiopoda
Latimeria – Coelacanth fish
Sphenodon – Reptilia
Didelphis – Opossum

Evidences of Organic Evolution from Missing Links
Those extinct organisms which had the characters of two different groups of animals and confirm the path of evolution as lie in between these groups. Arcaeopteryx, also called lizard bird. It is a missing link between reptiles and birds.
Evidences of Organic Evolution from Reptilian Characters:
Presence of similar teeth in Jaws.
Each finger ending into a claw.
A long tail with free caudal vertebrae.
Presence of keelless sternum.

Evidences of Organic Evolution from Avian Characters:
Presence of feathers of the body. Rounded cranium. Forelimbs modified into wings, have three fingers. Presence of furcula or wish bone. Above mention characters show that the birds have evolved from reptilian ancestors. So birds are glorified reptiles (Huxley).

Evidences of Organic Evolution from Atavism or Reversion
The sudden appearance of some ancestral features in some individuals is called atavism. It means non functional organs of the ancestors become functional in descendants. Examples: Large canine in man, thick hair on body, functional nipples in male, elongation of tail bone, etc.

Organic Evolution of Life

The concept of evolution was given by Empedocle and Buffon. The trend of evolution was first explained by Lamarck. Lamarck is known as the father of evolutionary theories. Evolution means gradual change from simple to complex forms with the passage of time. Organic Evolution means evolution of living beings. Term organic evolution is given by Hebert Spencer. Organic evolution can be defined as 'Adaptations through Generations'. Inorganic evolution means evolution of non living things e.g. evolution of rivers, rocks, mountains, etc. Evolution is defined as historical and developmental race of organisms with variation. The main theme of evolution is 'Descent with modification' i.e. docterine of desires. Biological meaning of evolution is gradual changes. The process of evolution is continuous but very slow, taking place even now.

Main Postulates of Organic Evolution are:
Earlier organisms were simpler.
They gradually changed into complex organisms.
They are inter related because of common origin.
They have common shelter (earth) and environment.
Evolution is generally slow, continuous, irreversible and progressive. For evolution the most important requirement is variation. The ultimate source of organic evolution is mutation. The evolution of any species can be considered as a sum total of the species 'Adaptive changes preserved by Natural Selection'. One of the most important factors for the development of a new species is the 'Geographical Isolation'.

Theories of Organic Evolution
From time to time, various theories of organic evolution have been proposed to explain the mechanism of origin and evolution of new species. These include:

Lamarckian Theory (Lamarckism)
Environment Effect
J.B. de Lamarck (1744-1829), a French naturalist, who introduced the word biology. He proposed the theory of 'Inheritance of Acquired Characters' in his book entitled Philosophic Zoologique. He was of the view that:

Effects of Use and Disuse of Organ
Environment influences change of characters of an organisms due to changes in their needs e.g., in dry soil, the root system of plants become more extensive than in wet soil; plants grown in shade develop larger leaves than those grown in open etc. Constantly used organs become highly developed and modified in response to needs, while those disused results into degeneration (use and disuse of organs). Lamarck cited the example of evolution of Giraffe, presumably from the ancestors having equal sized fore and hind limbs and a short neck. The Giraffe lived in African desert, which have no vegetation except a few trees. Their continued efforts through many generations for getting food from high up foliage, resulted into longer fore legs and much elongated neck. Similarly, the absence of limbs in snakes and presence of vestigial organs in animals have been explained by Lamarck on the basis of their disuse. Finally, the modifications which an organism acquires during its life time, gets inherited by its offsprings.

Criticism
The observation that we make in our daily life head to a negative approach to Lamarckism. Blacksmith develops tough and powerful biceps by continued use but are not inherited by his offsprings. Constant use of eyes for reading, writing and other minute works develop impaired sight. The women in China use iron shoes to keep their feet short, but their newly born babies have normal feet. Blind, deaf and lame parents give birth to normal children.

Germplasm Theory
The greatest critic of Lamarckism was August Weismann, who in1895 proposed the Germplasm Theory. He went on cutting the tails of rats for over twenty generations but failed to get tailless offspring. He suggested that acquired characters effect the somatic cells (somaplasm) and not the germ cells (germplasm) and are thus not inherited.

Neo-Lamarckism:
The modified form of Lamarckism is called Neo-Lamarckism. Neo-Lamarckism like Gadow, Cope, Spencer, Dali, McDougall etc are of the opinion that those characters are inherited which result from the interaction between heritable factors (genes) and the environment.