A biological adaptation is any structural (morphological or anatomical), physiological, or behavioral characteristics of an organism or group of organisms (such as species) that make it better suited in its environment and consequently improves its chances of survival and reproductive success. Due to individual phenotypic plasticity (variability), individuals will be more or less successful. Some adaptations may improve reproductive success of the population, but not a particular individual, such as seen in altruistic behavior in social insects.
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 changes in the environment, life style, or relationship to other organisms. Environmental dynamicity, voluntary or compelled shifting of habitat, and human activities may put organisms in a new niche or in environmental stresses or pressures. In such circumstances, the organisms require 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 the species exceeds the birth rate for a long enough period for the species to disappear.
While adaptations provide for the individual purpose of the organism—survival, reproduction, development, maintenance—these same characteristics provide diversity and add to human fascination with, and enjoyment of, nature. Furthermore, while adaptations often are seen as a static set of suitable characteristics, in reality the process of developing adaptations is a dynamic process. Whether envisioned as the product of design or natural selection, or natural selection on the microevolutionary level and design for macroevolutionary changes, the reality is that new adaptations are needed when organisms encounter new environments, and such have arisen for millions of years.
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.
There is a great difference between adaptation and acclimation or acclimatization. The process of developing adaptations occurs over many generations; it is a population phenomenon involving genetics and is generally a slow process. Acclimation or acclimatization, on the other hand, generally occurs within a single lifetime or instantly and deals with issues that are less threatening. For example, if a human being were to move to a higher altitude, respiration and physical exertion will become a problem. However, after spending a period of time under the high altitude conditions, one may acclimatize to the reduced pressure, the person's physiology may function normally, and the change will no longer be noticed.
Types of adaptation
Adaptations 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 a certain posture).
Based on the habitats for which organisms develop adaptations, adaptations can be categorized into 3 fundamental types, namely aquatic, terrestrial, and volant (flying), each of which can be further divided into many subtypes.
Aquatic adaptations are found in those plants and animals that live in water habitats: fresh water, brackish water, and sea water. For example, fresh water organisms develop features to prevent the entry of excess water or processes to drain excess water regularly. On 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 excrete excess salts that enter in water intake. 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 a water body, or pelagic, occurring in the water body itself. The animals may live partially or permanently in water. Thus they may range from non–specialized to very highly specialized water dwellers.
Primarily aquatic animals (e.g., fishes) show not a single terrestrial feature, whereas secondarily aquatic animals (whales, dolphins) possess terrestrial respiration through lungs, and some must visit land for laying eggs (e.g., turtle). Partially water dwelling animals demonstrate amphibious adaptations with double features both for land and water (e.g., frogs, salamanders), or mostly terrestrial features and only some basic aquatic adaptations (e.g., duck).
Some characteristic aquatic adaptations are:
- Body contour is spindle shaped and streamlined. For this, the 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, whales and turtles 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 as well. Aquatic mammals have reduced or absent hair and skin glands (oil and sweat glands). In compensation, they have a fatty layer below the skin known as bubbler. Besides insulating the body, it also helps in flotation.
- Primarily aquatic animals are capable of utilizing dissolved oxygen in the water for respiration through general body surface, internal or external gills, and so forth. However, secondarily aquatic forms respire atmospheric air through lungs; nostrils are located at the apex of the head.
- In fish, the hollow outgrowth of the alimentary canal, called air bladder, functions as an organ of flotation and accessory respiratory organ as it is filled with air. In whales and other mammals, extraordinarily massive lungs and closable nostrils serve this purpose.
- Fishes have lateral line systems extending the whole length of the body. It contains neuromast organs, which act as rheoreceptors (pressure receptors).
Terrestrial adaptations are exhibited by the plants and animals living in land habitats. As there are varied types of land habitats, the adaptations shown by organisms also are of diverse kinds.
This adaptation occurs in the animals leading a subterranean mode of life. They are equipped with digging organs and they dig for food, protection, or for shelter. Zoologically, they tend to be primitive and defenseless. 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. Paws are broad and stout with long claws and some extra structures for digging. In Gryllotalpa (mole–cricket), the forelegs are modified into digging organs.
This is adaptation involving "running" and is required by those organisms living in grassland habitats, since the lack of hiding places means fast running is an important 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 streamlined, 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 forearm bone ulna and shank bone fibula are reduced.
- Distal segments of both limbs, such as 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.
This is also known as scansorial adaptation and is found in animals that live in trees or climb on rocks and walls. 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., the tree frog Hyla). In the wall lizard (Hemidactylus), there are double rows 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 as well (e.g., chameleon, monkeys).
Desert adaptations are for the mode of life in extreme terrestrial habitats. 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. The kangaroo rat (Dipodomys) fulfills its water needs from metabolic synthesis. Others satisfy their water needs 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 wastes as solid dry pellets.
- Some desert animals store water in their body and use it economically; the camel stores water in the tissues all over the body, whereas the desert lizard (Uromastix) stores it in the large intestine.
- Because of sand and dust in the air, the ears, eyes, and nostrils are protected by valves, scales, fringes, eyelids, or by being reduced in size.
- Jackrabbits (Lepus), [fox]es (Vulpes velox), others have large pinnae to function as efficient heat radiators without having to lose moisture.
- Coloration and behavior allow animals to 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 desert snakes and spiders are poisonous.
Protection from enemies, predators, and even mistakes is achieved by the use of protective devices and mechanisms, such as slippery surfaces, 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. Animals living in snowy conditions may be white, forest animals may be striped or spotted, and desert animals may be sandy colored. The chameleon has several layers and varieties of chromatophores that enable it to change its colors according to the color of the surroundings.
- Resemblance coloration, together with morphological features and behavioral postures, make the animals resemble exactly the particular uninteresting objects of the environment, thus deriving protection. Some of the examples are stick insects, leaf insects (Phyllium), and others.
- Warning coloration is meant to avoid the mistake encounter of dangerous animals in general, or the encounter of unpalatable organisms by predators. 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 that is unpalatable or dangerous. For example, certain harmless flies with a pair of wings may mimic four winged bees or wasps that are well known dangerous insects, thus deriving protection. This is Batesian mimicry. If two species have same warning coloration and mutually advertise 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, a predator mimics the organism favored by its prey so as to trap the latter. For example, the African lizard resembles a flower, or a spider may resemble the flower of an orchid, and so forth.
Volant adaptation refers to adaptations in those having a flying mode of life. Included are modifications that help organisms sustain and propel their body in the air. It may be for passive gliding or for active true flight.
These types of movements 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 being held up by some sustaining organs to glide to the lower levels.
- 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, including the flying squirrel (Sciuropterus) and flying lemur (Galeopithecus volans). In the flying lizard (Draco), the patagia are supported by 5/6 elongated ribs.
- The flying frog (Rhacophorus) possesses very large webbed feet for sustaining purposes. 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 surfaces and the ventral lobe of the caudal fin is elongated to make dashes on the water surface to push the animal for the gliding flight. The fish makes this flight for 200 to 300 meters to escape from large fish. Other genera of flying fishes are Dactylpterus, Pantodon, and Pegasus.
Active true flight
Active true flight is aerial flight with both sustaining and propulsion; it is found among living forms in insects, birds, and bats. Being widely different groups, it is held that their flight developed independently. Nonetheless, 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 modified forelimbs. The humerus is well developed and the radius is long and curved, while the 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 forelimbs, but with reduced digits. They represent the most specialized wings among the modern wings. The feathers of flight are borne on the arm and hand, forming well expanded wings.
- 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, the 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 beaks and long flexible necks.
The theories of adaptation
Jean-Baptiste Lamarck was among the first to put forth a theory of adaptation, offering a process by which such adaptations could have arisen. His theory was referred to as the inheritance of acquired characters. But it failed to explain the origin and inheritance of characters as a population phenomena. Epigenetics (Pray 2004) and Baldwinian evolution (Nortman 2003) offer analogous processes in modern evolutionary theory.
Next, Charles Darwin came up with a more concrete explanation of adaptation that fit with observations. His theory of natural selection offered a mechanism by which suitable characters for particular environments could come to gradually predominate in the polymorphic population. So popular is Darwinian theory that the term adaptation is sometimes used as a synonym for natural selection, or as part of the definition ("Adaptation is the process by which animals or plants, through natural selection, come to better fit their environment.") However, most biologists discourage this usage, which also yields circular reasoning. Nonetheless, Darwin's theory does not give reasons for the underlying polymorphism on which natural selection works, and evidence of natural selection being the directing force of changes on the macroevolutionary level, such as new designs, is limited to extrapolation from changes on the microevolutionary level (within the level of species).
Industrial melanism is often presented as the best illustrative example of evolution of adaptive modification. In this case, two forms of peppered moths (Biston betularia) exist, melanic and non-melanic forms. Field studies in England over a 50-year period suggest that melanic forms increased in proportion in polluted areas because of the phenomenon of industrial melanism. This shift toward darker melanic forms is attributed to an heightened predation by birds of the light-colored moths, because the lighter forms could more easily be seen on the tree trunks that have been increasingly darkened from pollution. However, Wells (Wells 2000) pointed out that there are flaws in the studies, including the fact that peppered moths do not normally alight on tree trunks, and there are even inverse correlations with pollution in many situations.
- Alscher, R. G. and J. R. Cumming 1991. "Stress responses in plants: Adaptation and acclimation mechanisms," The Quarterly Review of Biology 66(3) : 343-344.
- Ford, M. J. 1983. "The changing climate: Responses of the natural fauna and flora," The Journal of Ecology 71(3): 1027-1028.
- Nortman, D. The evolution of phenotypic plasticity through the Baldwin Effect. Noesis VI: Article 4, 2003. Retrieved May 20, 2007.
- Pray, L. A. 2004. Epigenetics: Genome, meet your environment. The Scientist 18(13): 14. Retrieved May 20, 2007.
- Science Aid 2006. Adaptation. Retrieved May 7, 2007.
- Settel, J. 1999. Exploding Ants: Amazing Facts About How Animals Adapt, New York: Atheneum Books for Young Readers, ISBN 0689817398
- Wells, J. 2000. Icons of Evolution: Why Much of What We Teach About Evolution Is Wrong, Washington, DC: Regnery Publishing, ISBN 0895262762
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