When people speak about adaptation, they often mean a 'feature' (a trait) which helps an animal or plant survive. An example is the adaptation of horses' teeth to grinding grass. Grass is their usual food; it wears the teeth down, but horses' teeth continue to grow during life. Horses also have adapted to run fast, which helps them to escape their predators, such as lions. These features are the product of the process of adaptation.
The illustration of bird beaks shows an obvious sign of their different ways of life. However, eating a different food also means having a different digestive system, gut, claws, wings and above all, different inherited behaviour. For the major adaptations, what changes is not a single trait, but a whole group of features.
Adaptation occurs because the better adapted animals are the most likely to survive, and to reproduce successfully. This process is known as natural selection; it is the basic cause of evolutionary change.
Role in environment
These adaptive traits may be structural, behavioural or physiological. Structural adaptations are physical features of an organism (shape, body covering, armament; and also the internal organization).
Behavioural adaptations are composed of inherited behaviour chains and/or the ability to learn: behaviours may be inherited through basic instinct, or a tendency for learning may be inherited (see neuropsychology).
Physiological adaptations permit the organism to perform special functions, for instance, making venom, secreting slime, phototropism; and also more general functions such as growth and development, temperature regulation, ionic balance and other aspects of homeostasis. Adaptation, then, affects all aspects of the life of an organism.
Important adaptations do not come singly. They come in groups, which work together to make the animal or plant successful in its particular niche or life-style.
Woodpecker adaptations are a good example of how a whole group of features are needed for a successful way of life;
- The bill with its tip is chisel-like, and self-sharpening by the pecking on wood. The bird uses it to get at grubs under the bark, to widen a hole to make a nest and to signal its territory by drumming. Many of the foraging, breeding and signalling behaviours of woodpeckers involve drumming and hammering using the bill.
- Long sticky tongues used to grab insect grubs which live under bark.
- The millisecond before contact with wood a thickened nictitating membrane closes, protecting the eye from flying debris. The nostrils are also protected; they are often slit-like and have special feathers to cover them.
- To prevent brain damage from the rapid and repeated deceleration, woodpeckers have evolved a number of adaptations which protect the brain, these include, small brain size, the unequal length of the upper and lower parts of their beaks (the lower is longer). This steers the impact force downwards, away from the brain. The woodpecker’s brain is held in a skull with uneven, spongy plates that absorb shock.
Woodpeckers have a special hyoid bone, which reaches from their beak, loops over top of the skull to completely surround their brains. This acts to keep the brain in place. It is the movement of the brain inside the skull during impact, more than the blow itself, that causes concussions. If the brain is held in place, injury risks are greatly reduced.
- Woodpeckers have zygodactyl feet. These feet have four toes, the first and the fourth face backward, and the second and third face forward. This foot arrangement is good for grasping the limbs and trunks of trees. Members of this family can walk vertically up a tree trunk. In addition to the strong claws and feet, woodpeckers have short strong legs. This is typical of birds that forage on trunks.
- The tails of woodpeckers are stiffened, and when the bird perches on vertical surfaces, the tail and feet work together to support it.
- The whole system is helped by changes in the brain, nervous system, muscles and ligaments from what was usual in their ancestors.
Ancestral woodpeckers, which switched to climbing on tree trunks, had ancestral foot and tail structure. This suggests that a change in behaviour, perhaps to get at a better food source, was one of the first things that happened in the chain of events. The way evolutionary changes start is an important topic.
Functions of adaptations
Traits with no function
Not all features of an organism are adaptations. It may need field investigations or experiments to find out whether it has a function in the life of the species.
Adaptations tend to reflect the past life of a species. If a species has recently changed its life style, a once valuable adaptation may become function-less in the course of evolution. Animals which live in dark caves often lose, over a long period, their colors and eyesight.
The reasons for this may vary. The loss of structure and function may be a positive adaptation which saves energy and materials. But it may be simply a by-product of genes selected for other functions (pleiotropy). Or the structure may be linked in development, and affected by selection for some other structure.
Adaptations with multiple functions
Many adaptations serve more than one function. This is often the reason some traits become so noticeable that they almost define the species concerned.
Man's large brain serves not only for language, but also for thinking and problem-solving.
Bird feathers are not just used to fly; they are the basis of its heat conservation, temperature regulation and signalling.
Compromise and conflict between adaptations
Adaptations are never perfect. There are always trade-offs between the various functions and structures in a body. It is the organism as a whole which lives and reproduces, therefore it is the complete set of adaptations which gets passed on to future generations.
All adaptations have a downside: horse legs are great for running on grass, but they cannot scratch their backs; mammals' hair helps temperature regulation, but offers a niche for ectoparasites. Compromise and makeshift occur widely, not perfection. Selection pressures pull in different directions, and the adaptation that results is some kind of compromise.
Change of function over time
The function of a trait can, and often does, change over time. Several terms have been used to describe this: preadaptation, exaptation, cooption. 'Preadaptation' is the most common term used when a preexisting structure or trait inherited from an ancestor evolves a different function.
Sweat glands in mammals were later transformed into mammary glands. Another example is the long journey of the mammalian ear ossicles, which started in the gill covers of ancient fish, then became part of the lower jaw of reptiles, and then became part of the inner ear of mammals.
The wings of penguins. Once used for flying, they are now used for 'flying' under water.
Change of function in organs and structures is extremely common in evolution.
The following definitions are mainly due to Theodosius Dobzhansky;
- Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.
- Adaptedness is the state of being adapted: the degree to which an organism is able to live and reproduce in a given set of habitats.
- An adaptive trait is an aspect of the developmental pattern of the organism which improves the probability of that organism surviving and reproducing.
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Adaptation Facts for Kids. Kiddle Encyclopedia.