Homologous Analogous And Vestigial Organs:
These are the organs of similar structure and origin but dissimilar in function and form. This is due to common ancestry.
Examples of Homologous Organs:
- The forelimbs of a frog, the wings of a bird, leg of a horse, the hand of a man, and the flipper of a whale are homologous organs because all of them have a similar pattern of the basic plan (pentadactyl) i.e. same number of bones, muscles, nerves and blood vessels, etc. but they do the different functions such as hopping (frog), flying (bird), running (horse), grasping (man) and swimming (whale).
- Phylloclade of Opuntia and cladode of Ruscus are homologous organs as both are modified stems. Similarly, a thorn of Bougainvillea and a tendril of Cucurbita are homologous as both arise in axillary position.
The organs that perform the same function but differ in their origin and structure, are called as analogous organs.
Examples of Analogous Organs:
- The wings of an insect are analogous to those of birds and bats because they perform the same function but have dissimilar structure and origin. The wings of an insect have modified outgrowth of the body wall whereas wings of birds and bats are modified forelimbs.
- Potato and sweet potato are analogous organs as both perform the same function of storage of food but they differ in their structure. Potato is an underground-modified stem whereas sweet potato is a modified adventitious root.
- The fins of fishes and flippers of whales are analogous organs because both perform the function of swimming but the flippers of whale are pentadactyl and the fins of fishes are not pentadactyl.
- Stings of honey bee and scorpion are analogous structures both perform the same function. The sting of the honey bee is a modified ovipositor whereas, in the scorpion, it is modified last abdominal segment.
- The eye of an octopus and the eye of a mammal differ in their retinal position but both perform the same function. Similarly, the flippers of penguin (bird) and dolphin (mammal), that perform similar functions in these aquatic animals have originated from different structures of two different lineages.
These are the reduced and functionless organs which are of no use to the possessor but they still persist generation after generation in reduced form in an individual. They were complete and functional in their ancestors.
Examples of Vestigial Organs:
- Vermiform appendix in man is considered as the remnant of the large intestine (caecum) but it is considered to be a storage organ for cellulose digestion in herbivorous mammals.
- Anterior, superior, and inferior are three patches of muscles attached to the external ear or pinna. They are used in moving the pinna in different directions for gathering sound waves. The requirement has ceased in human beings due to erect posture and ability of the head to rotate in different directions. Therefore, ear muscles have become nonfunctional in humans.
The vestigial organs reveal strong evidence for evolution. They are the remnants of organs which used to perform a normal function in the ancestor but during the course of evolution, they have been reduced to vestiges.