Experiment 16: Flash Cards / Models / Specimens Showing Examples of Homologous and Analogous Organs

AIM

To study homology and analogy with the help of models/charts of animals and models/charts/specimens of plants.

PRINCIPLE

Homologous organs are functionally dissimilar but anatomically or structurally similar. Different modes of life have modified the organs to enable survival. They show divergent evolution from common ancestor with same basic structure.

Analogous organs are functionally similar but structurally dissimilar. They differ in anatomical structure and embryonic origin but perform similar function. They show convergent evolution where unrelated organisms evolve similar adaptations.

Study of forelimbs of human, cat, horse, bat, whale and dolphin shows homology with humerus, radius-ulna, carpals, metacarpals and phalanges. Wings of butterfly, bird and bat and tubers like sweet potato and potato show analogy.

REQUIREMENTS

• Models/charts of animals showing forelimbs and wings
• Models/charts/specimens of plants showing thorn and tendril, root and stem modifications
• Practical record book, pencil and eraser

PROCEDURE

1. Observe the models/charts of forelimbs of human, cat, horse, bat, whale and dolphin.
2. Identify the bones humerus, radius-ulna, carpals, metacarpals and phalanges in each forelimb and compare anatomical similarity.
3. Observe the preserved specimens/models of wings of butterfly, bird and bat and note external similarity and internal skeletal difference.
4. Observe plant specimens/models of thorn of Bougainvillea and tendril of Cucurbita and sweet potato and potato for homology and analogy.
5. Record the morphological and anatomical similarities and differences and classify organs as homologous or analogous.

OBSERVATION

a) Homologous Organs:

• The organisms which show similar anatomical structures, but differ in the morphological structures and functions. These show divergent evolution.
• By externally examining the wings of the mammal, like bat and the forelimb of a man, no similarity is found. But after examining the bones one by one, it is found that each of them has arm bone (humerus), hand bones (radius-ulna), wrist bones (carpals), palm bones (metacarpals), and fingers (phalanges).
• In terms of proportions of growth of each constituent bone, there are differences. For example, the fingers of bat are much longer while the comparative study suggests that basically the forelimbs of these two creatures are made up of the same parts, that is, they are anatomically similar.
• These organs need not perform the same function, as we see that bat uses it for flying and man uses it for handling tools. Hence, the forelimb of man and the wing of bat are homologous organs. Similarly, forearms of cat and man are homologous.

Examples: Flipper - Whale, Wing - Bat, Fore limb - Horse, Paw - Cat, Hand - Human

Thorn - Bougainvillea, Tendril - Cucurbita

b) Analogous Organ:

• The organisms which show similar morphological structures and functions, but differ in the anatomical structures. These show convergent evolution.
• The internal structure of the wings of butterfly, its preserved specimen, the shape and size are observed. It is found that it is membranous and is made up of thin cuticle.
• There are veins in the wing but there is no skeleton.
• The wings of the preserved specimen of a bat and a bird are examined. Skeletal support is found. It shows that the basic structures of wings of butterfly, bird and bat are different. In other words, they are anatomically different, although externally they look alike. Wings in these animals are used for flying.
• Such organs that differ anatomically and in embryonic mode of origin but perform similar function are said to be analogous organs.

Examples: Wings of butterfly and wings of a bird, Flippers of Penguin and Dolphin, Eye of an Octopus and mammal (retinal position), Sweet potato (root modification) and potato (stem modification).

RESULT

Forelimbs of human, cat, horse, bat, whale and dolphin show similar bone pattern and are homologous showing divergent evolution. Wings of butterfly, bird and bat and tubers of sweet potato and potato show similar function but different structure and are analogous.

CONCLUSION

Homologous organs have common ancestry, similar anatomy but different functions, indicating divergent evolution. Analogous organs have different ancestry and anatomy but similar functions, indicating convergent evolution due to similar environmental adaptations.

PRECAUTIONS

• Handle the models, charts and preserved specimens carefully to avoid damage.
• Observe the bone pattern humerus, radius-ulna, carpals, metacarpals and phalanges accurately in homologous organs.
• Differentiate between morphological similarity and anatomical similarity while identifying analogous and homologous organs.