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. Animals that live in the desert need to stay cool, or else they could overheat and die. Animals like camels have lots of different adaptations to help them survive in an area like that. To name one is their humps on their backs. Camels' humps aren't just for flashy looks. They store fat that they can feed and drink off of, since finding food and water in the desert might be difficult. And here's another: Sand blows around everywhere in the desert 90% of the time. Animals that live there don't want to get it up their nose or in their eyes. Camels have an adaptation that helps them close their nostrils to keep sand out of their noses!
General principles[change | change source]
Adaptation is, first of all, a process, rather than a physical part of a body.
Adaptation... could no longer be considered a static condition, a product of a creative past, and became instead a continuing dynamic process. Ernst Mayr.p483
An internal parasite (such as a fluke) is a good example: it has a very simple bodily structure, but still the organism is highly adapted to its particular environment. From this we see that adaptation is not just a matter of visible traits: in such parasites critical adaptations take place in the life cycle, which is often quite complex.
However, as a practical term, adaptation is often used for the product: those features of a species which result from the process. Many aspects of an animal or plant can be correctly called adaptations, though there are always some features whose function is in doubt. By using the term adaptation for the evolutionary process, and adaptive trait for the bodily part or function (the product), the two senses of the word may be distinguished.
Adaptation is one of the two main processes that explain the diverse species we see in biology. The other is speciation (species-splitting or cladogenesis).p562 A favourite example used today to study the interplay of adaptation and speciation is the evolution of cichlid fish in African lakes.
An organism must be viable at all stages of its development and at all stages of its evolution. This places constraints on the evolution of development, behaviour and structure of organisms.
The general idea is that each genetic and phenotypic change during evolution should be relatively small, because developmental systems are so complex and interlinked. But polyploidy in plants is a fairly common large genetic change. The origin of the eukaryota by the symbiosis of micro-organisms is a more exotic example.
Ecological niches[change | change source]
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 in detail (instincts), or a tendency for learning may be inherited (see neuropsychology). Examples: searching for food, sex, vocalizations.
Physiological adaptations permit the organism to perform special functions (for instance, making venom, secreting slime, phototropism); but 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.
Suits of adaptations[change | change source]
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.
Woodpeckers[change | change source]
Woodpecker adaptations are a good example of how a whole suite of features are needed for a successful way of life.
- The bill: 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 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 decelerations, woodpeckers have evolved a number of adaptations which protect the brain. These include
- small brain size
- the position of the brain spreads the area of contact between the brain and the skull
- the short duration of contact
- 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 novelties start is an important topic.
Dogs[change | change source]
Dogs. When you hear the word, you think sweet, happy, loyal pets. But they weren't always that way. Dogs used to be wild, vicious wolves that lived in the wild. One day, somebody decided to tame these wolves and turn them into something else. So then they bred dogs! Many people wanted these new, unique animals because they could use them to hunt and they could make money off of them. Others began experimenting with them. They would have two different dogs have puppies, and those puppies were a whole different breed of dog. Soon enough, everyone were breeding their own dogs. Some sold the breeds for money, some used them to hunt, and some even fashioned dog shows to show off your breed and possibly win a prize. Nowadays, there are over 40 different breeds of dogs and can be found all over the world. But there are too many dogs in the world. Over 100 dogs get put down or pass away because there are too many, and aren't getting adopted in time. Many pass away because they were abandoned by a bad person, so do the right thing and adopt a dog today and save a life.
Peacocks[change | change source]
Camouflage to avoid detection is destroyed when vivid colours are displayed at mating time. Here the risk to life is counterbalanced by the need for reproduction. The peacock's ornamental train (grown anew in time for each mating season) is a famous adaptation. It must reduce his maneuverability and flight, and is hugely conspicuous; also, its growth costs food resources.
Darwin's explanation of its advantage was in terms of sexual selection: "it depends on the advantage which certain individuals have over other individuals of the same sex and species, in relation to reproduction". The kind of sexual selection represented by the peacock is called 'mate choice', meaning the process selects the more fit over the less fit, and so has survival value. In practice, the blue peafowl Pavo cristatus is a pretty successful species, with a big natural range in India, so the overall outcome of their mating system is quite viable.
Human birth[change | change source]
The size of the human foetal brain at birth means the brain of a newborn child is quite immature. The newborn's brain cannot be larger than about 400ccs, else it will not get through the mother's pelvis. Yet the size needed for an adult brain is about 1400ccs.
The most vital things in human life (locomotion, speech) just have to wait while the brain grows and matures. That is the result of the birth compromise. Much of the problem comes from our upright bipedal stance, without which our pelvis could be shaped more suitably for birth. Neanderthals had a similar problem.
Change of function over time[change | change source]
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. It was the term used by Julian Huxley and Ernst Mayr. The term 'pre-' does not mean any foresight, it just means the adaptation was already available, serving some older function. 'Exaptation' was Stephen J. Gould's word.
One example of preadaptation is in dinosaurs, which evolved feathers with the function of thermo-insulation and display long before they were used for flight by early birds. 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. Another example is 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. Many of the features of tetrapods (land vertebrates) evolved from features with different functions in the ancestral lobe-finned fish (Sarcopterygii).
Definitions[change | change source]
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.
Related pages[change | change source]
References[change | change source]
- The Oxford Dictionary of Science defines adaptation as "Any change in the structure or functioning of an organism that makes it better suited to its environment".
- Bowler P.J. 2003. Evolution: the history of an idea. California. p10
- Patterson C. 1999. Evolution. Natural History Museum, London. p1
- Williams, George C. 1966. Adaptation and natural selection: a critique of some current evolutionary thought. Princeton. "Evolutionary adaptation is a phenomenon of pervasive importance in biology." p5
- Ridley, Mark. 2003. Evolution. Part 3: Adaptation and natural selection. 3rd ed, Wiley-Blackwell. ISBN 978-1-4051-0345-9. Contents 
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