In this essay we will discuss about the adaptations by living organisms to environment, explained with the help of suitable examples.
Definition of Adaptation:
The adaptations are the useful morphological, physiological or behavioural changes developed in a living organism over long period of time to adjust itself according to a particular environment.
Significance of Adaptation:
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Adaptations make the organisms more fit to its environment and increase their chances of survival and reproduction for the continuation of race. These are must as environment is not constant, so the living organisms must have an inherent ability to change.
Adaptations also form the basic cause of evolution. It is so as the useful variations increase the chances of survival of organisms and their natural selection which increase their adaptability to the environment. These useful variations are inherited generation after generation so there is a gradual increase in their frequency. Accumulation of such useful variability leads to speciation.
Examples of Adaptation:
A. Adaptations for Thermoregulation:
1. Thermal Regulation in Animals (Fig. 13.19):
(i) Some animals bring changes in their morphology or anatomy for thermoregulation as evident from following rules:
a. Bergaman’s Rule:
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The birds and mammals (warm-blooded animals) of cold regions are large sized than those of warmer region e.g., polar bear is much larger in size than the black bear of temperate area; and Antarctic penguin is about one-metre long while the penguin of equatorial Galapagos islands is only half a metre long. It is so as a large sized body will have less surface area per unit of weight so loses less heat.
b. Allen’s Rule:
The tail, snout, ears and legs of mammals are relatively shorter in colder part than in the warmer parts (Fig. 13.19). It was noticed in foxes and gazells. The pinnae of the desert fox (Megalotis zerda) are largest sized while those of red fox (Vulpes vulpes) are small sized and those of arctic fox (Alopex lagopus) are smallest in size. This is so as small structures lose less heat. Similarly, legs, ears and pinnae of Gazella picticanda of Himalayas are shorter than those of G. benetli of plains.
c. Jorden’s Rule:
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The fishes found in waters of low temperature tend to have more vertebrae than those of warmer waters e.g., 58 vertebrae in a cod hatching at 4-8°C and 54 vertebrae in a cod hatching at 10-11°C.
d. Gloger’s Rule:
The mammals, birds and insects of tropical region (warmer region) are more darker in colour and heavily pigmented than those of arctic region (cold region). But Hyla (tree frog) and Phrynosoma (Horned toad) are dark skinned in low temperatures.
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e. Rensch’s Rule:
The birds of colder regions tend to have relatively narrow wings than the birds found in warmer areas.
(ii) Polar bears and polar sea animals like seals have a thick layer of fat, called blubber, in their subcutaneous zone of skin. The blubber acts as an insulator and reduces the loss of body’s heat.
(iii) Small aquatic animals like Cladocera, Daphnia etc. show change in their body shape with seasonal temperature changes, called cyclomorphosis. In Daphnia, size of helment (carapace) on head increases with increase in environmental temperature.
2. Thermal Regulation in Plants:
Plants protect themselves from extremes of temperatures by development of thick cuticle, thick corky layer, dense hair coat, mucilage, tannins, etc. Pentosan mucilage has high moisture – retaining capacity to protect themselves from high temperatures and desiccation.
Dry seeds have low bound water and can withstand very low temperature (even of-190°C) for about three weeks. Similarly, some perennial plants also can withstand low temperatures. The leaves of such plants store fats which lower the freezing point. This is called cold resistance.
B. Adaptations for Osmoregulations:
Desert animals and plants face the problem of scarcity of water. These desert organisms develop different methods for either moisture getting or moisture conservation.
1. Osmoregulation in Desert Animals (Fig. 13.20):
The desert animals face the problem of deficiency of water by developing different methods to achieve the following goals for osmoregulation:
(i) Adaptations for Moisture Getting:
(a) Spiny lizard of Australia (Moloch horridus-Fig. 13.20A) has hygroscopic glands in the skin which can absorb water from the moist sand.
(b) Desert rabbit. Jerboas, Kangaroo rat (Dipodomys) and pocket mouse (Perognathiis) feed on dry seeds and vegetation. These animals depend only on metabolic water. Kangaroo rats and gerbils seal their burrows during day to retain the moisture.
(c) Carnivorous animals like Grasshopper mice and marsupial mulgara eat upon the insects and small rodents to draw food and water.
(d) Certain animals like desert rabbit and wood rat (Neotoma) derive water by eating succulent plants.
(ii) Adaptations for Moisture Conservation:
(a) Phrynosoma (Horned toad) and Moloch horridus have spiny and highly carnified skin to reduce evaporation of water.
(b) Camel (Fig. 13.20B) has water cells in the wall of its stomach to store metabolic water formed during the biological breakdown of fats stored in its hump.
(c) In desert mammals, the number of sweat glands is greatly reduced or absent.
(d) Desert insects have a waxy water-proofing coat on their body surface.
(e) Uromastix (desert lizard) stores water in large intestine.
(f) Small sized desert animals are generally fossorial and remain in burrows during the day time and come outside during night when the percentage of moisture in their burrows and outside is equal.
(g) Camel can lose upto 25% of its body weight by losing water from its body tissues and not from the blood without any ill-effect.
(iii) Self-Defence against Xeric Climatic Conditions:
(a) Nostrils:
In fossorial desert animals, the nostrils are generally directed upward while in snakes the nostrils are reduced to pinholes which can be closed by valves. In camel, nostrils can be closed like the eyelids.
(b) Eyes:
The eyes of burrowing desert snakes (e.g. Typhlops, sand boas), are covered by a large sized head shield, while the eyes of camel are covered by abundant eye lashes.
(c) Ears:
The ear openings in fossorial animals of desert are reduced or absent while in camels, these are protected by dense growth of hair.
(d) Desert snakes (e.g., Rattle snake), spiders and scorpions are generally venomous. So the presence of venom is another desert adaptation.
C. Phenotypic Adaptations:
Phenotypic adaptations are those adaptations which enable to organism to respond quickly to a stressful situation e.g. many natives in the Andes (e.g., Peruvians) and the Himalayas live at an altitude above 13000 feet. In these persons, oxygen – carrying capacity of their blood is higher than in those of the natives at the sea level.
It is due to the presence of greater amount of haemoglobin in their blood as these have high RBC count. It is an example of phenotypic adaptation, the main aim of which is to protect themselves from the atmospheric hypoxia (i.e. presence of less amount of oxygen in the atmospheric air at high altitudes). In India, the areas with low atmospheric oxygen are Rohtang pass near Manali and Mansarovar in Tibet.
When the persons living in the plains had ever gone to any of these areas, they start suffering from altitude sickness characterized by nausea, fatigue, heart palpitation, etc. due to less availability of oxygen. But if they live there for some time, they develop phenotypic adaptations, get acclimatised and stop experiencing altitude sickness.
D. Biochemical Adaptations:
These are those adaptations in the biochemical reactions and the enzymes controlling such reactions which enable them to live in the extreme environments e.g:
(i) Thermal Tolerance:
In most of organisms, the physiological functions are performed optimally in a narrow temperature range e.g., it is about 37°C for human beings and generally ranges between 25 to 40°C. But many types of microbes (e.g., Archaebacteria) flourish even in hot springs and deep sea hypothermal vents having temperature even more than 100°C. Similarly, many fish thrive in Antarctic waters with subzero temperatures.
(ii) Pressure Tolerance:
A great variety of marine invertebrates and fish flourish at great depths in the ocean where the pressure is even more than 100 times than the normal atmospheric pressure.
E. Behavioural Adaptations:
These are those adaptations which are shown by certain organisms in their behaviour to protect themselves from a stressful situation e.g:
(i) Poikilothermal or ectodermal animals like frogs and snakes hibernate during winter months and aestivate during summer months.
(ii) Desert lizards and crocodiles bask in the sun to absorb heat radiations and increase their body temperature in cold conditions.
(iii) Short distance and temporary migration of certain animals.
(iv) Ticks locate their warm-blooded hosts; and rattle snake detect their prey by their body heat as such animals have the thermotactic adaptation (i.e., ability to orient towards heat).
(v) Certain animals of desert become fossorial and remain inside the burrows during the day time and come outside during night when the percentage of moisture in their burrows and outside is equal.