Contents
Adaptation
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Evolution
Mechanisms and processes
Adaptation
Genetic drift
Gene flow
Mutation
Natural selection
Speciation
Research and history
Introduction
Evidence
Evolutionary history of life
History
Modern synthesis
Social effect
Theory and fact
Objections / Controversy
Evolutionary biology fields
Cladistics
Ecological genetics
Evolutionary development
Evolutionary psychology
Human evolution
Molecular evolution
Phylogenetics
Population genetics
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Adaptation is the evolutionary process whereby a population becomes better suited to its habitat.[1][2] This process takes place over many generations,[3] and is one of the basic phenomena of biology.[4]
The term adaptation may also refer to a feature which is especially important for an organism's survival.[5] For example, the adaptation of horses' teeth to the grinding of grass, or their ability to run fast and escape predators. Such adaptations are produced in a variable population by the better suited forms reproducing more successfully, that is, by natural selection.
General principles
The significance of an adaptation can only be understood in relation to the total biology of the species. Julian Huxley [6]
Adaptation is, first of all, a process, rather than a physical part of a body.[7] The distinction may be seen in an internal parasite (such as a fluke), where the bodily structure is greatly simplified, but nevertheless the organism is highly adapted to its unusual 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.[8] 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 may be seen as one aspect of a two-stage process. First, there is speciation (species-splitting or cladogenesis), caused by geographical isolation or some other mechanism.[9][10] Second, there follows adaptation, driven by natural selection. Something like this must have happened with Darwin's finches, and there are many other examples. The present favourite is the evolution of cichlid fish in African lakes, where the question of reproductive isolation is much more complex.[11][12]
Another great principle is that an organism must be viable at all stages of its development and at all stages of its evolution. This is obviously true, and it follows that there are constraints on the evolution of development, behaviour and structure of organisms. The main constraint, over which there has been much debate, is the requirement that changes in the system during evolution should be relatively small changes, because the body systems are so complex and interlinked. This is a sound principle, though there may be rare exceptions: polyploidy in plants is common,[13] and the symbiosis of micro-organisms that formed the eukaryota is a more exotic example.[14]
All adaptations help organisms survive in their ecological niches.[15] These adaptative traits may be structural, behavioral or physiological. Structural adaptations are physical features of an organism (shape, body covering, defensive or offensive 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, mating, 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.
Definitions
The following definitions are mainly due to Theodosius Dobzhansky.
1. Adaptation is the evolutionary process whereby an organism becomes better able to live in its habitat or habitats.[16] 2. 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.[17] 3. An adaptive trait is an aspect of the developmental pattern of the organism which enables or enhances the probability of that organism surviving and reproducing.[18]Adaptedness and fitness
From the above definitions, it is clear that there is a relationship between adaptedness and fitness (a key population genetics concept). Fitness is an estimate and a predictor of the rate of natural selection. What natural selection does is change the relative frequencies of alternative phenotypes, insofar as they are heritable.[19] Although the two are connected, the one does not imply the other: a phenotype with high adaptedness may not have high fitness. Dobzhansky mentioned the example of the Californian redwood, which is highly adapted, but a relic species in danger of extinction.[16] Elliott Sober commented that adaptation was a retrospective concept since it implied something about the history of a trait, whereas fitness predicts a trait's future.[20]
1. Fitness. The degree of demographic difference among phenotypes. Usually a relative measure: the average contribution to a breeding population by a phenotype or a class of phenotypes. This is also known as Darwinian fitness, relative fitness, selective coefficient, and other terms. 2. Adaptedness. Usually an absolute measure: the average absolute contribution to the breeding population by a carrier of a phenotype or a class of phenotypes. Also known as absolute fitness, and as the Malthusian parameter when applied to species as a whole.[21]Brief history
Adaptation as a fact of life has been accepted by all the great thinkers who have tackled the world of living organisms. It is their explanations of how adaptation arises that separates these thinkers. A few of the most significant ideas:[22]
- Empedocles did not believe that adaptation required a final cause (~ purpose), but "came about naturally, since such things survived". Aristotle, however, did believe in final causes.
- In natural theology, adaptation was interpreted as the work of a deity, even as evidence for the existence of God.[23] William Paley believed that organisms were perfectly adapted to the lives they lead, an argument that shadowed Leibniz, who had argued that God had brought about the best of all possible worlds. Voltaire's Dr Pangloss[24] is a parody of this optimistic idea, and Hume also argued against design.[25] The Bridgewater Treatises are a product of natural theology, though some of the authors managed to present their work in a fairly neutral manner. The series was lampooned by Robert Knox, who held quasi-evolutionary views, as the Bilgewater Treatises. Darwin broke with the tradition by emphasising the flaws and limitations which occurred in the animal and plant worlds.[26]
- Lamarck. His is a proto-evolutionary theory of the inheritance of acquired traits, whose main purpose is to explain adaptations by natural means.[27] He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus "the influence of circumstances", usually expressed as use and disuse. His evolutionary ideas, and those of Geoffroy, fail because they cannot be reconciled with heredity. This was known even before Mendel by medical men interested in human races (Wells, Lawrence), and especially by Weismann.
Many other students of natural history, such as Buffon, accepted adaptation, and some also accepted evolution, without voicing their opinions as to the mechanism. This illustrates the real merit of Darwin and Wallace, and secondary figures such as Bates, for pushing forward a mechanism whose significance had only been glimpsed previously. A century later, experimental field studies and breeding experiments by such as Ford and Dobzhansky produced evidence that natural selection was not only the 'engine' behind adaptation, but was a much stronger force than had previously been thought.[28][29][30]
Types of adaptation
Adaptation is the heart and soul of evolution. Niles Eldredge [31]
Changes in habitat
Before Darwin, adaptation was seen as a fixed relationship between an organism and its habitat. It was not appreciated that as the climate changed, so did the habitat; and as the habitat changed, so did the biota. Also, habitats are subject to changes in their biota: for example, invasions of species from other areas. The relative numbers of species in a given habitat are always changing. Change is the rule, though much depends on the speed and degree of the change.