In this article we will discuss about the types and modifications in adaptation by an organism.
Types of Adaptation:
The changes in the organism by which it suits best in the environment and leave behind offspring’s are known as adaptation. Had there been no adaptation evolution would have been impossible.
The adaptation may be of two types:
(a) Primary.
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(b) Secondary.
(a) When the adaptation remains in the original direction, it is said to be primary and the subsequent changes remain more or less in the same direction from the very appearance of the animal.
Example:
Aquatic adaptation of fish.
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(b) When the adaptive changes divert from the original direction they are called secondary adaptation. The changes are diverted along different lines due to change of the original home of the organisms and they subsequently adapt themselves to some new environment.
Example:
Aquatic adaptation of whales.
From environmental point of view following and other adaptations are found in the animals.
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1. Aquatic adaptation:
Adaptation to water medium.
Water is the densest medium and adaptations to aquatic life are necessarily directed to minimise the resistance during movement.
Modifications in Adaptations:
A. Primary aquatic adaptation:
1. Head, body and tail compressed into a spindle-like form.
2. The head is wedge-shaped, the edges of the jaws and gill covers) fit perfectly.
3. Locomotion by lateral undulation of the body aided by the fins.
4. Hydrostatical air bladder to maintain a plane of flotation.
B. Secondary aquatic adaptation:
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1. The body contour spindle-shaped, neck constriction disappears and the tail enlarges.
2. The neck becomes shorter.
3. The chest tends to become cylindrical.
4. External ears lost.
5. External nostrils shift towards the apex of the head.
6. The eyes shift higher on the face.
7. Limbs are fan-like, tail flattened like the caudal fin of a fish but horizontal in most cases.
8. Loss of hair, skin glands, etc.
9. Development of a layer of fat beneath the skin.
10. Mouth armament, jaws reduced, teeth lost.
11. Speed. Usually high, about 39-40 knot per hour in porpoises.
12. Attain a large size and exceed all other animals. Examples. Primarily adapted. All fishes; secondarily adapted. Sea-turtles, Hesperornis, Penguins, Pinnipedia, Cetacea, Sirenia.
2. Volant adaptation:
Adaptation to aerial life.
Air is a homogenous medium which offers the least possible resistance to the attainment of speed. Aerial creatures cannot live in the air for an indefinite period of time and must come to land or water or branches of trees for rest. Hence their adaptation is always a double one.
Flight is of two types. Passive or gliding flight where only the sustaining surface is present and no muscular force is applied, and true flight where, muscular power is applied for the movement of the sustaining surface. In true flight, their is sustained movement through the air.
Modifications:
1. Body contour streamlined to minimise resistance through the air.
2. Sustaining surface may be simply patagium or folds of skin and wings.
(a) Patagium:
It is a fold or series of folds of the skin between the fore and hind limbs and sometimes extend to the neck and between the hind limbs and tail.
(b) Wing:
The wing has independently evolved for three times. In the bat wing, the humerus is well developed, the radius is long and curved, the ulna is vestigeal. The pollex is always free and clawed for crawling and climbing.
In the pterodactyl wing, the radius is a little shorter than ulna, the single wing finger, presumably the fourth is huge, the wing being flexed between metacarpal and proximal phalanx. In the bird wing, the digits are reduced to three and more or less fused together, patagium is vestigeal and the sustaining surface is formed by feathers (Part II, Fig. 21).
3. Pneumatic bones. The bones are hollow and air-filled.
4. Air sacs. Sacs filled with air not only render the body light, but helps in maintaining a sort of double respiration.
5. Sternum well developed and bears a median keel. The keel is lacking in running birds.
6. In birds, shoulder girdle is rigid by the development of clavicles and the pillar-like coracoids.
7. Scapula and coracoid are united by an articulation in Carinatae, but firmly ossified in Ratitae.
8. Brain well developed with enlarged cerebrum and large optic lobes.
9. The eyes are large with so-called sclerotic plates.
Fished:
The fins are usually enlarged and some flying fishes either soar in the air or skip over water for some distance. Examples. Exocoetus, Dactylopterus, Pantodon, etc.
Amphibia:
Rhacophorus is the only Amphibia having volant adaptation. The digits bear terminal adhesive pads and are connected by web-like expansion of the skin. Rudiments of patagia present in front and behind the arms. The webbed feet sustain it in prolong leaps.
Reptilia:
In flying dragon, Draco, the patagium is supported by a number of extended ribs. In the flying gecko, Ptychozoon, the skin expands laterally along the sides of the neck, body, tail, limbs and between the toes. The extinct Pterodactyls or flying dragons were a remarkable group of reptiles.
Birds:
All flying birds exhibit volant adaptation. Running birds previously thought to be degenerated flying birds are really not so. It is suggested that during their early experiments for flight some birds took to air while a group remained on the ground and was adapted for running on land.
Mammalia:
More than thirteen separate volant adaptations are found in mammals, of which only the bats attained the power of true flight. In microchiroptera, the patagium extends from the rear of the head along the front of the arm, between the fingers to the base of the claws, between the fore and hind limb, webbing the toes as well as the fingers, and between the hind limbs and the tail.
Desert adaptation:
Adaptation to desert life.
A vast stretch of drifting sand with a little or no rains, devoid of vegetation except in the rare oases, and scarcely populated by rare animals is known as desert. The struggle for existence is most severe in the desert. Moisture getting and moisture conservation, and defence against both physical and biotic environments are most acute.
Modifications:
1. Moisture getting:
The sources of moisture are plant sap, dews collected on plants and blood of the prey. The desert lizard, Moloch, absorbs water through the skin like a blotting paper when comes in contact with it.
2. Moisture conservation:
This problem in animals is met with in two ways.
(a) Storage of water:
The rumen or the first chamber of the four-chambered stomach of the camel has developed water cells in its walls. These are small bag-like structures, the narrow mouth of which can be closed by a sphincter muscle. During drinking, the stomach and water cells are filled up with water. By relaxing the sphincter muscle water from the water cells is released to the stomach according to the need.
(b) Non-perspiring:
Perspiration has a physiologic value and helps in excretion and to maintain standard body temperature. But the loss of moisture in desert animals has greater ill effect and to conserve the scarce and precious water, the desert animals have become non-perspiring.
3. Defence:
To protect themselves against rigorous physical conditions, viz. extreme heat, cold, sand storm, scorching sun rays, etc. and also against predators the desert animals have adapted themselves in different ways.
(a) The nostrils are directed upwards in digging species. In snakes, they are protected by complicated valves or reduced to pin holes.
(b) The eyes are overhung by the head shields, or margins of the lids are broadened into plates and furnished with scales. The upper eyelid may be enlarged, the lower enlarged eyelid provided with a window.
(c) The ear openings are reduced, or protected by fringes of scales or lost.
(d) The colour of the animals generally harmonises with the colour of the sand or rock. Sometimes the same species show double colour adaptation to suit local conditions.
(e) Venom is a characteristic of the desert. Venomous spiders, insects and snakes are most common in desert.
(f) Speed. Most of the animals can move with great speed.
(g) The senses of sight, hearing and smell are well developed.
(h) Intelligence is highly developed, particularly in defenceless animals.