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Adaptation
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Adaptation is the process, which takes place under natural selection, whereby an organism becomes better suited to its habitat. Also, the term may refer to some characteristic which stands out as being especially significant in the organism's survival. For example, the adaptation of horses' teeth to the grinding of grass, or their capacity to run fast to escape predators.
ckquote>The significance of an adaptation can only be understood in relation to the total biology of the species.
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Adaptation is the process, which takes place under natural selection, whereby an organism becomes better suited to its habitat. Also, the term may refer to some characteristic which stands out as being especially significant in the organism's survival. For example, the adaptation of horses' teeth to the grinding of grass, or their capacity to run fast to escape predators.
General principles
The significance of an adaptation can only be understood in relation to the total biology of the species. Julian Huxley
Adaptation is a process, rather than a physical part of a body. The distinction may be seen in an internal parasite (such as a fluke), where the bodily structure is greatly simplified, but the organism is highly adapted to its environment. In such cases critical adaptations take place in the life-cycle, which is often quite complex. However, as a practical term, adaptation is often used for those features of a species which result from the process. Here one does find structures which were formerly adaptive, but are so no longer. Vestigial organs, such as the human caecum (appendix), are examples.
Adaptation may be seen as one aspect of a two-stage process. First, there is species-splitting (cladogenesis), caused by geographical isolation or some other mechanism. 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 favorite is the evolution of cichlid fish in African lakes, where the question of reproductive isolation is much more complex.
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, and the symbiosis of micro-organisms that formed the eukaryota is a more exotic example.
All adaptations help organisms survive in their ecological niches. These adaptations 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 inherited behaviour chains and the ability to learn: they may be inherited in detail (instincts), or a tendency for learning may be inherited (see neuropsychology). Examples: searching for food, mating, vocalizations. Physiological adaptations are 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.
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:
- 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. William Paley believed that organisms were perfectly adapted to the lives they lead, an argument that shadowed Leibnitz, who had argued that God had brought about the best of all possible worlds. Voltaire's Dr Pangloss is a parody of this optimistic idea, and Hume also argued against design. 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.
*Lamark. His is a proto-evolutionary theory of the inheritance of acquired traits, whose main purpose is to explain adaptations. He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus "the influence of circumstances". His ideas and that 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).
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 point of view whose merit 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.
Types of adaptation
Adaptation and function are two aspects of one problem. Julian Huxley
The physical environment: 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.
When the habitat changes, three things may happen to a resident population. The population may move; it may adapt genetically; or it may become extinct, at least in that locale. It is now clear that habitats do frequently change. Therefore, it follows that the process of adaptation is never finally complete. Over time, it may happen that the environment changes little, and the species comes to fit its surroundings better and better. On the other hand, it may happen that changes in the environment occur relatively rapidly, and then the species becomes less and less well adapted. Seen like this, adaptation is a tracking process, which goes on all the time, and which affects every species in a particular ecosystem. Fitness (an organism's capacity to propagate its genes) is not a static concept.
Intimate relationships: co-adaptations
In co-evolution, where the existence of one species is tightly bound up with the life of another species, new or 'improved' adaptations which occur in one species are often followed by the appearance and spread of corresponding features in the other species. There are many examples of this; the idea emphasises that the life and death of living things is intimately connected, not just with the physical environment, but with the life of other species. These relationships are intrinsically dynamic, and may continue on a trajectory for millions of years, as has the relationship between flowering plants and insects (pollination).
The gut contents, wing structures, and mouthpart morphologies of fossilized beetles and flies suggest that they acted as early pollinators. The association between beetles and angiosperms during the early Cretaceous period led to parallel radiations of angiosperms and insects into the late Cretaceous. The evolution of nectaries in late Cretaceous flowers signals the beginning of the mutualism between hymenopterans and angiosperms.
Mimicry
Henry Walter Bates' work on Amazonian butterflies led him to develop the first scientific account of mimicry, especially the kind of mimicry which bears his name: Batesian mimicry. This is the mimicry by a palatable species of an unpalatable or noxious species. A common example seen in temperate gardens is the hover-fly, many of which – though bearing no sting – mimic the warning colouration of hymenoptera (wasps and bees). Such mimicry does not need to be perfect to improve the survival of the palatable species.
Bates, Wallace and Müller believed that Batesian and Müllerian mimicry provided evidence for the action of natural selection, a view which is now standard amongst biologists. All aspects of this situation can be, and have been, the subject of research. Field and experimental work on these ideas continues to this day; the topic connects strongly to speciation, genetics and development.
The basic machinery: internal adaptations
There are some types of adaptation which are of a general nature, to do with the overall co-ordination of the systems in the body. Such adaptations may have significant consequences. Examples, in vertebrates, would be temperature regulation, or improvements in brain function, or an effective immune system. The acquisition of such major adaptations has often served as the spark for adaptive radiation, and huge success for long periods of time for a whole group of animals or plants. Adaptation is thus one of the key aspects of evolution.
Conflict between adaptations
Adaptations serving different functions may be mutually destructive. Compromise and make-shift occur widely, not perfection. Selection pressures pull in different directions, and the adaptation that results is some kind of compromise. Consider the antlers of the Irish elk, (often supposed to be far too large; in deer antler size has an allometric relationship to body size). Obviously antlers serve positively for defence against predators, and to score victories in the annual rut. But they are costly in terms of resource. Their size during the last glacial period presumably depended on the relative gain and loss of reproductive capacity in the population of elks during that time. Another example: camouflage to avoid detection is destroyed when vivid colors are displayed at mating time. Here the risk to life is counterbalanced by the necessity for reproduction. The peacock's ornamental train (grown anew in time for each mating season) must reduce his maneuverability and flight, and is hugely conspicuous; also, its growth costs food resources. Darwin's explanation 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 exclusive relation to reproduction." The kind of sexual selection represented by the peacock is called 'mate choice', with an implication that the process selects the more fit over the less fit, and so has survival value. The existence of sexual selection was for a long time in abeyance, but has been rehabilitated.
The conflict between the size of the human foetal brain at birth, (which cannot be larger than about 400ccs, else it will not get through the mother's pelvis) and the size needed for an adult brain (about 1400ccs), means the brain of a newborn child is quite immature. 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.
Shifts in function
Pre-adaptations
This occurs when a species or population has characteristics that are ideally suited for conditions which have not yet arisen. For example, the polyploid rice-grass Spartina townsendii is better adapted than either of its parent species to their own habitat of saline marsh and mud-flats. White Leghorn fowl are markedly more resistant to vitamin B deficiency than other breeds. On a plentiful diet there is no difference, but on a restricted diet this preadaptation could be decisive.
Pre-adaptation may occur because a natural population carries a huge quantity of genetic variability. In diploid eukaryotes, this is a consequence of the system of sexual reproduction, where mutant alleles get partially shielded, for example, by the selective advantage of heterozygotes. Micro-organisms, with their huge populations, also carry a great deal of genetic variability.
The first experimental evidence of the pre-adaptive nature of genetic variants in micro-organisms was provided by Salvador Luria and Max Delbrück who developed fluctuation analysis, a method to show the random fluctuation of pre-existing genetic changes that conferred resistance to phage in the bacterium Escherichia coli.
Co-option of existing traits: exaptation
The classic example is the ear ossicles of mammals, which we know from palaeontological and embrological studies originated in the upper and lower jaws and the hyoid of their Synapsid ancestors, and further back still were part of the gill arches of early fish. We owe this esoteric knowledge to the comparative anatomists, who, a century ago, were at the cutting edge of evolutionary studies. The word exaptation was coined to cover these shifts in function, which are surprisingly common in evolutionary history. The origin of wings from feathers that were originally used for
temperature regulation is a more recent discovery (see feathered dinosaurs).
Related issues
Non-adaptive traits
Some traits appear to be not adaptive, that is, selectively neutral. There may be various causes: the utility of a trait is lost and does not now appear adaptive; the utility of a trait is unknown; the trait is a consequence of another trait that is adaptive (the Spandrel idea). Of course, a trait may have been adaptive at some point in an organism's evolutionary history, but habitats change, leading to adaptations becoming redundant or even a hindrance (maladaptations). Such adaptations are termed vestigial. The utility of adaptations will ebb and flow.
Fitness landscapes; drift
Sewall Wright's explanation for evolutionary stasis was that organisms come to occupy adaptive peaks. In order to evolve to another, higher peak, the species would first have to pass through a valley of maladaptive intermediate stages. This could happen by genetic drift if the population were small enough. This was Wright's shifting balance theory of evolution. There has been much skepticism among evolutionary biologists as to whether these rather delicate conditions hold often in natural populations. Ronald Fisher felt that most populations in nature were too large for these effects of genetic drift to be important.
Extinction
Populations that are not suitably adapted to their environment may move out of the habitat or die out. Extinction occurs when the death rate over the entire species (population, gene pool ...) exceeds the birth rate for a long enough period for the species to disappear. Populations may differ greatly in their phenotypic plasticity, which is the ability of an organism with a given genotype to change its phenotype in response to changes in its habitat, or to its move to a different habitat. Population growth takes place when the birth rate of those carrying the adaptive trait exceeds over time the birth rate of those that do not carry the adaptive trait.
Co-extinction
Just as we have co-adaptation, there is also co-extinction. Co-extinction refers to the loss of a species due to the extinction of another; for example, the extinction of parasitic insects following the loss of their hosts. Co-extinction can also occur when a flowering plant loses its pollinator, or through the disruption of a food chain. "Species co-extinction is a manifestation of the interconnectedness of organisms in complex ecosystems ... While co-extinction may not be the most important cause of species extinctions, it is certainly an insidious one".
Adaptation vs. acclimatization
There is a difference between adaptation and acclimatization. Adaptation occurs over many generations; it is a gradual process caused by natural selection. Acclimatization occurs within a single lifetime. For example, if a human were to move to a higher altitude, respiration and physical exertion would become a problem, but after spending time in high altitude conditions one may acclimatize to the pressure by increasing production of red blood corpuscles. The ability to acclimatize is an adaptation, but not the acclimatization itself.
See also
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