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.