Adaptation

A biological adaptation is any structural (morphological or anatomical), physiological or behavioral characteristics of an organism that make it better suited in its environment and consequently guarantee its reproductive success. Due to individual phenotypic plasticity (variability), individuals will be more or less successful. Organisms that are adapted to their environment are able to:

• secure food, water, and nutrients
• obtain air, warmth, and spaces
• cope with physical conditions such as temperature, light, and heat
• defend themselves from their natural enemies
• reproduce and rear offspring
• respond to changes around them

Adaptation occurs in response to the change in the environment, relationship to other organisms or change in life style. Environmental dynamicity, voluntary or compelled shifting of place and human activities may put organisms in a new niche or in environmental stresses or pressures. In such circumstances, the organisms must develop characteristics suitable to the new situation. Organisms that are not suitably adapted to their environment will either have to move out of the habitat or die out. The term die out in the context of adaptation means that the death rate over the entire population of species exceeds the birth rate for a long enough period for the species to disappear.

There is a great difference between adaptation and acclimation or acclimatization. Adaptation occurs over many generations; it is a population phenomenon involving genetics and is generally a slow process. Acclimation or acclimatization, on the other hands, generally occurs within a single lifetime or instantly and copes with issues that are less threatening, like drastic change in some of the environmental conditions. For example, if a human is to move to a higher altitude, respiration and physical exertion will become a problem, but after spending a period of time in high altitude conditions, one may acclimatize to the reduced pressure, the person's physiology may function normally, and the change will no longer be notice. Although acclimation and acclimatization are both the adjustment of the body physiology to the change in environmental condition, the former is in the artificially controlled environment in response to single factor, where as the latter is as it occurs in the natural environment.

Types of Adaptation

Adaptation can be structural, physiological or behavioral. Structural adaptations are special body parts of an organism that help it to survive in its natural habitat (e.g., skin color, shape, body covering). Physiological adaptations are systems present in an organism that allow it to perform certain biochemical reactions (e.g., making venom, secreting slime, being able to keep a constant body temperature). Behavioral adaptations are special ways a particular organism behaves to survive in its natural habitat (e.g., becoming active at night, taking certain posture).

Based on the habitats for which organisms develop adaptation, it can be categorized into 3 fundamental types, namely aquatic, terrestrial, and volant, each of which can be further divided into many subtypes.

Aquatic Adaptation

Aquatic Adaptation occurs in those plants and animals which live in water habitats, viz., fresh water, brackish water, and sea water. Fresh water organisms develop features to prevent the entry of excess water or processes to drain excess water regularly. In the contrary, marine organisms face scarcity of water due to hypertonic (salt concentration higher than that of body fluid) sea water. So, they have mechanisms to retain water and throw excess salts that enter in water intake. The aquatic plants may be emergent rooted plants (e.g., reeds), submersed rooted plants (e.g., Hydrilla), planktons (e.g., diatoms) or floating plants (e.g., water hyacinth). Similarly, aquatic animals may be benthic occurring at the bottom of water body or pelagic occurring in the water body itself. The animals may live partially or permanently in water. Thus they may be non–specialized to very highly specialized water dwellers. Primarily aquatic animals (e.g., fishes) show not a single terrestrial features, whereas the secondarily aquatic animals possess terrestrial respiration through lungs, and must visit land for laying eggs (e.g., turtle, whale). Partially water dwelling animals demonstrate amphibious adaptation with double features both for land and water (e.g., frogs, salamander) or mostly terrestrial features and only some basic aquatic adaptation (e.g., duck). The characteristic aquatic adaptations are:

• Body contour is spindle shaped and stream–lined. For this, head is elongated into rostrum or similar structure, neck is short, external ears (pinnae) are reduced and tail is laterally or dorso–ventrally compressed.
• Usually marine animals are excessively large (e.g., whale), because of the buoyancy of the salt water.
• Organs of locomotion and balancing vary greatly among the aquatic animals; fishes use paired and unpaired fins, whale and turtle have their limbs modified into paddles, in some others, hands and/or feet are webbed.
• Skin of most aquatic forms is rich in mucous glands to make it slippery. Fishes are equipped with dermal scales too. Aquatic mammals lack hair and skin glands (oil and sweat glands). In compensation, they have fatty layer below the skin known as bubbler. Besides insulating the body, it also helps in floatation.
• Primary aquatic animals are capable of utilizing dissolved Oxygen of water for respiration through general body surface, internal or external gills, and so forth. However, secondary aquatic forms respire atmospheric air through lungs; nostrils are located at the apex of the head.
• In case of fishes, the hollow outgrowth of alimentary canal called air bladder functions as organ of floatation and accessory respiratory organ as it is filled with air. In whale and others, extraordinarily massive lungs and closable nostrils serve for the purpose.
• Fishes have lateral line systems extending the whole length of the body. It contains neuromast organs which act as rheoreceptors (pressure receptors).

Terrestrial Adaptation

Terrestrial adaptation is shown by the plants and animals occurring in the land habitats. As there are varied types of land habitats, the adaptations shown by organisms are also of diverse kinds.

Fossorial Adaptation

This adaptation occurs in the animals leading subterranean mode of life. They are equipped with digging organs and they dig for food, protection or for shelter. Zoologically, they are primitive, defenseless, and unambitious animals. The adaptational features are:

• The body contour is cylindrical, spindle–shaped, or fusiform (e.g., earthworms, moles, badgers) so as to reduce resistance in subterranean passage.
• The head is small and tapers anteriorly to form a burrowing snout.
• Neck and pinnae are reduced to avoid obstruction in quick movement through the holes. In some, tail is also shortened.
• The eyes remain small and non–functional.
• Limbs are short and strong. Hands are broad and stout with long claws and some extra structures for digging. In Gryllotalpa (mole–cricket), fore–legs are modified into digging organs.

Cursorial Adaptation

This is running adaptation and is required by the organisms leading life in grassland, as without the hiding places, fast running is the only means of protection from the enemies there. Horses, Zebras, Deer, and so forth. show this adaptation with following modifications:

• The neck is reduced and the body is steam–lined, this will reduce the air resistance while running.
• The bones of palms (carpals, metacarpals) and soles (tarsus, metatarsus) become compact and are often fused to form canon bone.
• The fore arm bone ulna and shank bone fibula are reduced.
• Distal segments of both limbs i.e., radius, tibia, and canon bones are elongated to increase the length of the stride.
• Movement of the limbs is restricted to a fore and aft plane.

Arboreal Adaptation

This is also known as scansorial adaptation and is found in animals which are climbers on rocks, walls or trees. The features enabling them to be best suited in the habitat are:

• The chest, girdles, ribs, and limbs are strong and stout.
• Feet and hands become prehensile (catching) with opposable digits (e.g., primates, marsupials). Sometimes, the digits are grouped as 3 digits and 2 digits in the syndactyly (e.g., Chameleon). For facilitating the clinging, some have elongated claws (e.g., squirrels), while others bear rounded adhesive pads at the tip of the digits (e.g., tree frog Hyla). In wall lizard (Hemidactylus), there are double raw of lamellae in the ventral side of digits for creating vacuum to cling. This enables the animals to move even on the smooth vertical surfaces.
• Often the tail becomes prehensile too (e.g., Chameleon, monkeys).

Desert Adaptation

Desert adaptation is for the mode of life in extreme terrestrial habitat. Desert plants (xerophytes) and animals (xerocoles) show adaptations for three challenges: getting moisture, conserving moisture, and defending oneself from biotic and abiotic factors. Many of these adaptations are just physiological and behavioral:

• Different plants and animals adopt different mechanisms to procure enough water. The sand lizard (Molcoh) and horned toad (Phrynosoma) have hygroscopic skin to absorb moisture like the blotting paper even from unsaturated air. Kangaroo rat (Dipodomys) fulfills its water need from metabolic synthesis. Others satisfy their water need through the food they consume.
• Desert animals prevent water loss from their body by reducing surface area, making skin impermeable through its thickening and hardening, as well as through the presence of scales and spines (Phrynosoma, Moloch), reducing the number of sweat glands in mammals, avoiding day heat by seeking the shadows of rocks and becoming active at night (nocturnal) and excreting waste as solid dry pallet.
• Some desert animals store water in their body and use it economically; camel stores water in the tissues of over all body, whereas desert lizard (Uromastix) stores it in the large intestine.
• Because of sand and dust in the air ears, eyes, and nostrils are protected by valves, scales, fringes, eyelids or reduction.
• Jack–rabbits (Lepus), fox (Vulpes velox), and so forth. have large pinnae to function as efficient heat radiators without having to lose moisture.
• Coloration and behavior make animals harmonize with the desert surroundings. For example, sand colored and rough skinned Phrynosoma on detecting threats digs in the sand to obliterate the body contour and to harmonize in the background.
• Possession of venom (poison) is for self–defense and almost all the desert snakes, spiders are poisonous.

Protective Adaptation

Protection from enemies, predators, and mistakes is achieved by the use of protective devices and mechanisms like slippery surface, horns, spines, unpleasant smells (e.g., shrew), poison, hard shells, autotomy (self cutting) of tail (e.g., wall lizard) or by the use of coloration together with behavioral postures. Colorations are used for different purposes:

• Cryptic coloration or camouflage is for making the animals invisible or indistinct from the environment by assimilating with the background or by breaking up the body contour. Snow animals are white, forest animals are striped or spotted, and desert animals are sandy colored. Chameleon has several layers and varieties of chromatophores which enable it to change its colors according to the color of the surroundings.
• Resemblance Coloration together with morphological feature and behavioral posture make the animals resemble exactly the particular uninteresting objects of the environment, thus deriving protection. Some of the examples are stick insect, leaf insect (Phyllium), and others.
• Warning Coloration is meant to avoid mistake encounter of dangerous animals by general animals or of unpalatable organisms by the predators. So, the animals bear this coloration to advertise their being dangerous or unpalatable. Gila monster (Heloderma), the only known poisonous lizard, has bright black, brown yellow, and orange bands. Most poisonous snakes possess warning coloration. Bees and wasps warn others of their stings.
• Mimicry is defined as the imitation of one organism by another for the purpose of concealment, protection or other advantages. The species that imitates is called a mimic and the one which is copied a model. Depending on the purposes of mimicry, it can be protective or aggressive.
  • Protective mimicry is a protective simulation by a harmless species in form, appearance, color, and behavior of another species which is unpalatable or dangerous. For example, certain harmless flies with a pair of wings become mimetic form of four winged bees or wasps which are well known dangerous insects to derive protection. This is Batesian mimicry. If two species have same warning coloration and mutually advertise of their dangerousness or unpalatability so as to make predators learn to avoid both of them, then it is called Mullerian mimicry.
  • Aggressive mimicry is used by predators. Here, predator mimics the organism favored by its prey so as to trap the latter, for example, African lizard resembling a flower, a spider resembling an orchid flower, and so forth.

Volant Adaptation

Volant Adaptation develops in those having flying mode of life. It must include modifications which help organisms sustain and propel their body in the air. It may be for passive glide or for active true flight.

Passive gliding

These types of movement involve no propulsion other than the initial force of jumping and gravitational force. It is characterized by leaping or jumping from a high point and held up by some sustaining organs to glide to the lower level.

• The skin on either side of the body become expanded and stretched between fore and hind limbs to form what is called patagium. Patagia are sustaining organs in many animals e.g., flying squirrel (Sciuropterus), flying lemur (Galeopithecus volans). In flying lizard (Draco), the patagia are supported by 5/6 elongated ribs.
• Flying frog (Rhacophorus) possesses very large webbed feet for sustaining purpose. Its digits terminate in adhesive pad to ensure clinging on the landing surface.
• In flying fish (Exocoetus), the pectoral fins are enlarged to form gliding surface and the ventral lobe of the caudal fin is elongated to make dash on the water surface to push the animal for the gliding flight. The fish makes this flight for 200 to 300 m to escape from large fishes. Other genera of flying fishes are Dactylpterus, Pantodon, and Pegasus.

Active true flight

It is the aerial flight with both sustaining and propulsion and is found among living forms in insects, birds, and bats. They being widely different groups, their flight might have developed independently. However, they show many common features:

• Though the flight organs in all the groups are wings, their structure varies greatly.
  • Insect wings are made up of cuticle strengthened by thickening called veins. Typically, there are two pairs of wings developed on the dorso–lateral sides of the meso– and meta–thoracic segments. In Diptera, only meso–thoracic wings are developed.
  • Bat wings are the modified fore limbs. Humerus is well developed, radius is long and curved, while ulna is vestigial. The pollex (thumb) is free and clawed for crawling and climbing. The patagia are supported by elongated second, third, fourth, and fifth digits.
  • Bird wings are also the modification of fore limbs, but with reduced digits and represent the most specialized ones among the modern wings. The feathers of flight are borne on the arm and hand forming well expanded wing.
• Sternum (breast bone) is well developed for the attachment of flight muscles. In bird, it is keeled.
• Specifically strong flight muscles are present.
• Body is made light especially in birds due to the –
  • presence of pneumatic bones
  • reduction of internal organs, e.g., ovary and oviduct of right side, urinary bladder
  • presence of air sacs in the body
  • presence of light feathers covering the body
• Especially in birds, optic lobe of the brain is highly developed, correlating with which eyes are also large to ensure good sense of vision. To overcome sudden change in air pressure, the eyes bear characteristic sclerotic plates and also comb–like, vascular and pigmented structures called pectin. They regulate the fluid pressure within the eyes.
• The conversion of forelimbs into wings in birds is compensated by the presence of toothless horny beak and long flexible neck.

The theory of adaptation

Jean-Baptiste Lamarck

Some of the adaptational features are achieved by the gradual modifications. The theory of evolution explains the occurrence of such modifications. In this regard, we can say that Jean–Baptiste Lamarck was the first to put forth the theory of adaptation. His theory was referred to as the inheritance of acquired characters. But it failed to explain the origin and inheritance of characters as the population phenomena. However, see epigenetics (Pray 2004) and Baldwinian evolution (Nortman 2003) for analogous processes in modern evolutionary theory.

Next Charles Darwin came up with more concrete explanation of adaptation. His theory of natural selection could describe how the environment suitable characters gradually predominate in the polymorphic population. Therefore, the term adaptation is sometimes used as a synonym for natural selection, but most biologists discourage this usage. It could not give reasons for the polymorphism. The modern synthetic theory of evolution is the complete explanation of micro–evolution and is the theory of adaptation explaining the origin of at least some of the adaptational features. Industrial melanism is the best illustrative example of evolution of adaptive modification.

Summary

Often we see adaptation as already established set of suitable characteristics. In the dynamic and diverged environments, organisms encounter constantly new changed environmental conditions. Therefore, adaptation is a dynamic process coupled with evolution. Genetic diversity of gene pool maintains polymorphism in the population. This is the basis for natural selection to work upon and cause immediate adaptation process. If the natural selection of the phenotypic expression of an allelic gene brings about a change in the allelic proportion, then that is actually the micro–evolution process and the adaptational progress. However, mutation may introduce new genetic variation for natural selection to work upon. Industrial melanism is an illustration of this progress. The major adaptations are, however, not explainable by this simple micro–evolutionary process.

In some extreme conditions, it is possible for the previous adaptation to be poorly selected, the advantage it confers over generations decreasing, up to and including the adaptation becoming a hindrance to the species' long–term survival. This is known as maladaptation and can apply to both humans and animals.

References

ISBN links support NWE through referral fees

• Alscher, Ruth G. and Jonathan R. Cumming. 1991. Stress Responses in Plants: Adaptation and Acclimation Mechanisms. Author(s) of Review: Abraham D. Krikorian. The Quarterly Review of Biology, Vol. 66, No. 3 (Sep., 1991), pp. 343-344

• Ford, M.J. 1983. The Changing Climate: Responses of the Natural Fauna and Flora. Author(s) of Review: S. D. Prince. The Journal of Ecology, Vol. 71, No. 3, pp. 1027-1028

• Nortman, David. 2003. The Evolution of Phenotypic Plasticity Through the Baldwin Effect Noesis VI: Article 4. Retrieved May 20, 2007.

• Pray, Leslie A. 2004. Epigenetics: Genome, Meet Your Environment. The Scientist Volume 18 | Issue 13 | 14.Retrieved May 20, 2007.

• Science Aid. Science aid: Adaptation Explanation of adaptation with cactus and polar bear examples, aimed at high school level. Retrieved May 7, 2007.

• Settel, Joanne. 1999. Exploding Ants : Amazing Facts About How Animals Adapt. New York : Atheneum Books for Young Readers. ISBN 0689817398

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