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In
biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, Development ...

biology
, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

organism
s to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the population during that process. Thirdly, it is a
phenotypic trait A phenotypic trait, simply trait, or character state is a distinct variant of a phenotypic right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. Th ...
or adaptive trait, with a functional role in each individual organism, that is maintained and has
evolved Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolved
through
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
. Historically, adaptation has been described from the time of the ancient Greek philosophers such as
Empedocles Empedocles (; grc-gre, Ἐμπεδοκλῆς Empedocles (; grc-gre, wikt:Ἐμπεδοκλῆς, Ἐμπεδοκλῆς; , 444–443 BC) was a Ancient Greece, Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city ...

Empedocles
and
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

Aristotle
. In 18th and 19th century
natural theology Natural theology, once also termed physico-theology, is a type of theology Theology is the systematic study of the nature of the divine Divinity or the divine are things that are either related to, devoted to, or proceeding from a deity.
, adaptation was taken as evidence for the existence of a deity.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
proposed instead that it was explained by natural selection. Adaptation is related to
biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual selection is a form of natural selection where one sex pref ...
, which governs the rate of evolution as measured by change in
gene frequencies Allele frequency, or gene frequency, is the relative frequency of an allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...W ...
. Often, two or more species co-adapt and
co-evolve In biology, coevolution occurs when two or more species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined ...
as they develop adaptations that interlock with those of the other species, such as with
flowering plants The flowering plants, also known as Angiospermae (), or Magnoliophyta (), are the most diverse group of land plants The Embryophyta () or land plants are the most familiar group of green plants that form vegetation on earth. Embryophyta is a c ...
and
pollinating insect A pollinator is an animal that moves pollen Pollen Tube Diagram Pollen is a powdery substance consisting of pollen grains which are male microgametophytes of seed plants, which produce male gametes (sperm cells). Pollen grains have a har ...
s. In
mimicry In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...

mimicry
, species evolve to resemble other species; in
Müllerian mimicry Müllerian mimicry is a natural phenomenon in which two or more well-defended species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversit ...
this is a mutually beneficial co-evolution as each of a group of strongly defended species (such as wasps able to sting) come to advertise their defences in the same way. Features evolved for one purpose may be co-opted for a different one, as when the insulating
feather Feathers are epidermal growths that form a distinctive outer covering, or plumage Plumage ( "feather") is a layer of feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both avi ...

feather
s of
dinosaurs Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 annum, million years ago, although the exact origin and timing of the evolution o ...

dinosaurs
were co-opted for
bird flight Bird flight is the primary mode of animal locomotion, locomotion used by most bird species in which birds take off and flight, fly. Flight assists birds with feeding, Sexual reproduction, breeding, avoiding predation, predators, and Bird migratio ...
. Adaptation is a major topic in the
philosophy of biology The philosophy of biology is a subfield of philosophy of science Philosophy of science is a branch of philosophy concerned with the foundations, methodology, methods, and implications of science. The central questions of this study concern D ...
, as it concerns function and purpose (
teleology Teleology (from and )Partridge, Eric. 1977''Origins: A Short Etymological Dictionary of Modern English'' London: Routledge, p. 4187. or finalityDubray, Charles. 2020 912 Year 912 ( CMXII) was a leap year starting on Wednesday (link will dis ...
). Some biologists try to avoid terms which imply purpose in adaptation, not least because it suggests a deity's intentions, but others note that adaptation is necessarily purposeful.


History

Adaptation is an observable fact of life accepted by philosophers and natural historians from ancient times, independently of their views on
evolution Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolution
, but their explanations differed.
Empedocles Empedocles (; grc-gre, Ἐμπεδοκλῆς Empedocles (; grc-gre, wikt:Ἐμπεδοκλῆς, Ἐμπεδοκλῆς; , 444–443 BC) was a Ancient Greece, Greek pre-Socratic philosopher and a native citizen of Akragas, a Greek city ...

Empedocles
did not believe that adaptation required a
final cause The four causes or four explanations are, in Aristotelianism, Aristotelian thought, four fundamental types of answer to the question "why?", in Posterior Analytics, analysis of change or movement in nature: the Four_causes#Material, material, the ...
(a purpose), but thought that it "came about naturally, since such things survived."
Aristotle Aristotle (; grc-gre, Ἀριστοτέλης ''Aristotélēs'', ; 384–322 BC) was a Greek philosopher A philosopher is someone who practices philosophy Philosophy (from , ) is the study of general and fundamental questio ...

Aristotle
did believe in final causes, but assumed that species were fixed. In
natural theology Natural theology, once also termed physico-theology, is a type of theology Theology is the systematic study of the nature of the divine Divinity or the divine are things that are either related to, devoted to, or proceeding from a deity.
, adaptation was interpreted as the work of a deity and as evidence for the existence of God.
William Paley William Paley (July 174325 May 1805) was an English clergyman, Christian apologetics, Christian apologist, philosopher, and Utilitarianism, utilitarian. He is best known for his natural theology exposition of the teleological argument for the ex ...
believed that organisms were perfectly adapted to the lives they led, an argument that shadowed
Gottfried Wilhelm Leibniz Gottfried Wilhelm (von) Leibniz ; see inscription of the engraving depicted in the " 1666–1676" section. ( – 14 November 1716) was a German polymath A polymath ( el, πολυμαθής, ', "having learned much"; Latin Latin (, or , ...

Gottfried Wilhelm Leibniz
, who had argued that God had brought about " the best of all possible worlds."
Voltaire François-Marie Arouet (; 21 November 169430 May 1778), known by his ''nom de plume A pen name, also called a ''nom de plume'' () or a literary double, is a pseudonym A pseudonym () or alias () (originally: ψευδώνυμος in Greek) is a ...

Voltaire
's satire Dr. Pangloss is a parody of this optimistic idea, and
David Hume David Hume (; born David Home; 7 May 1711 NS (26 April 1711 OS) – 25 August 1776) Cranston, Maurice, and Thomas Edmund Jessop. 2020 999999 or triple nine most often refers to: * 999 (emergency telephone number) 250px, A sign on a beach ...

David 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 Robert Knox (4 September 1791 – 20 December 1862) was a Scottish anatomist and ethnologist best known for his involvement the Burke and Hare murders. Born in Edinburgh, Scotland, Knox eventually partnered with anatomist and former teacher J ...

Robert Knox
, who held quasi-evolutionary views, as the ''Bilgewater Treatises''.
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
broke with the tradition by emphasising the flaws and limitations which occurred in the animal and plant worlds.
Jean-Baptiste Lamarck Jean-Baptiste Pierre Antoine de Monet, chevalier de Lamarck (1 August 1744 – 18 December 1829), often known simply as Lamarck (; ), was a French naturalist Natural history is a domain of inquiry involving organisms, including animals, fu ...

Jean-Baptiste Lamarck
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''. This second, subsidiary element of his theory is what is now called
Lamarckism Lamarckism, also known as Lamarckian inheritance or neo-Lamarckism, is the notion that an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies ...

Lamarckism
, a proto-evolutionary hypothesis of the
inheritance of acquired characteristics Inheritance is the practice of passing on private property, Title (property), titles, debts, entitlements, Privilege (law), privileges, rights, and Law of obligations, obligations upon the death of an individual. The rules of inheritance differ ...
, intended to explain adaptations by natural means. Other natural historians, such as
Buffon
Buffon
, accepted adaptation, and some also accepted evolution, without voicing their opinions as to the mechanism. This illustrates the real merit of Darwin and
Alfred Russel Wallace Alfred Russel Wallace (8 January 18237 November 1913) was a British natural history, naturalist, explorer, geographer, anthropologist, biologist and illustrator. He is best known for independently conceiving the theory of evolution throug ...
, and secondary figures such as
Henry Walter Bates Henry Walter Bates (8 February 1825, in Leicester – 16 February 1892, in London) was an English natural history, naturalist and explorer who gave the first scientific account of mimicry in animals. He was most famous for his expedition to the ...

Henry Walter Bates
, for putting forward a mechanism whose significance had only been glimpsed previously. A century later, experimental field studies and breeding experiments by people such as E. B. Ford and
Theodosius Dobzhansky Theodosius Grygorovych Dobzhansky ( uk, Теодо́сій Григо́рович Добжа́нський; russian: Феодо́сий Григо́рьевич Добржа́нский; January 25, 1900 – December 18, 1975) was a prominent Ukra ...
produced evidence that natural selection was not only the 'engine' behind adaptation, but was a much stronger force than had previously been thought.


General principles


What adaptation is

Adaptation is primarily a process rather than a physical form or part of a body. An internal
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
(such as a
liver fluke Liver fluke is a collective name of a polyphyletic group of parasitic trematodes under the phylum Platyhelminthes. They are principally parasites of the liver of various mammals, including humans. Capable of moving along the blood circulation, t ...
) can illustrate the distinction: such a parasite may have a very simple bodily structure, but nevertheless the organism is highly adapted to its specific 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 Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...
, which is often quite complex. However, as a practical term, "adaptation" often refers to a ''product'': those features of a
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

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 remains in doubt. By using the term ''adaptation'' for the evolutionary ''process'', and ''adaptive trait'' for the bodily part or function (the product), one may distinguish the two different senses of the word. Adaptation is one of the two main processes that explain the observed diversity of species, such as the different species of
Darwin's finches Darwin's finches (also known as the Galápagos finches) are a group of about 18 species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiver ...

Darwin's finches
. The other process is
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

speciation
, in which new species arise, typically through
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
. An example widely used today to study the interplay of adaptation and speciation is the evolution of
cichlid Cichlids are fish from the family (biology), family Cichlidae in the order Cichliformes. Cichlids were traditionally classed in a suborder, the Labroidei, along with the wrasses (Labridae), in the order Perciformes, but molecular studies have co ...
fish Fish are aquatic Aquatic means relating to water Water (chemical formula H2O) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the ...

fish
in African lakes, where the question of reproductive isolation is complex. Adaptation is not always a simple matter where the ideal phenotype evolves for a given environment. 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 main constraint, over which there has been much debate, is the requirement that each
genetic
genetic
and phenotypic change during evolution should be relatively small, because developmental systems are so complex and interlinked. However, it is not clear what "relatively small" should mean, for example
polyploid Polyploidy is a condition in which the biological cell, cells of an organism have more than two paired (Homologous chromosome, homologous) sets of chromosomes. Most species whose cells have Cell nucleus, nuclei (eukaryotes) are diploid, meaning t ...
y in plants is a reasonably common large genetic change. The origin of
eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

eukaryotic
endosymbiosis An endosymbiont or endobiont is any organism that lives within the body or cells of another organism most often, though not always, in a mutualism (biology), mutualistic relationship. (The term endosymbiosis is from the Greek language, Greek: ...

endosymbiosis
is a more dramatic example. All adaptations help organisms survive in their
ecological niche In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...

ecological niche
s. The adaptive traits may be structural, behavioural or
physiological Physiology (; ) is the scientific Science () is a systematic enterprise that builds and organizes knowledge Knowledge is a familiarity or awareness, of someone or something, such as facts A fact is an occurrence in the real world. ...
. Structural adaptations are physical features of an organism, such as shape, body covering, armament, and internal organization.
Behavioural Behavior (American English American English (AmE, AE, AmEng, USEng, en-US), sometimes called United States English or U.S. English, is the set of varieties of the English language native to the United States. Currently, American English ...
adaptations are inherited systems of behaviour, whether inherited in detail as
instinct Instinct is the inherent inclination of a living Living or The Living may refer to: Common meanings *Life, a condition that distinguishes organisms from inorganic objects and dead organisms ** extant taxon, Living species, one that is not extinct ...
s, or as a
neuropsychological Neuropsychology is a branch of psychology. It is concerned with how a person's cognition and behavior are related to the brain and the rest of the nervous system. Professionals in this branch of psychology often focus on how injuries or illnesse ...
capacity for
learning Learning is the process of acquiring new understanding Understanding is a psychological process related to an abstract or physical thing, such as a person, situation, or message whereby one is able to use concepts to model that thing. Under ...

learning
. Examples include searching for food,
mating In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...

mating
, and
vocalizations
vocalizations
. Physiological adaptations permit the organism to perform special functions such as making
venom Venom is a type of poison In biology, poisons are Chemical substance, substances that can cause death, injury or harm to organs, Tissue (biology), tissues, Cell (biology), cells, and DNA usually by chemical reactions or other activity (chemi ...

venom
, secreting
slime Slime may refer to: Biology * Slime mold Slime mold or slime mould is an informal name given to several kinds of unrelated eukaryotic organisms that can live freely as single cells, but can aggregate together to form multicellular reprodu ...
, and
phototropism File:Phototropism Diagram.svg, thumbnail, Auxin distribution controls phototropism. 1. Sunlight strikes the plant from directly above. Auxin (pink dots) encourages growth straight up. 2, 3, 4. Sunlight strikes the plant at an angle. Auxin is conc ...

phototropism
), but also involve more general functions such as growth and development,
temperature regulation Thermoregulation is the ability of an organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a sy ...
,
ionic Ionic or Ionian may refer to: Arts and entertainment * Ionic meter, a poetic metre in ancient Greek and Latin poetry * Ionian mode, a musical mode or a diatonic scale Places and peoples * Ionian, of or from Ionia, an ancient region in western An ...

ionic
balance and other aspects of
homeostasis In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanis ...
. Adaptation affects all aspects of the life of an organism. The following definitions are given by the evolutionary biologist
Theodosius Dobzhansky Theodosius Grygorovych Dobzhansky ( uk, Теодо́сій Григо́рович Добжа́нський; russian: Феодо́сий Григо́рьевич Добржа́нский; January 25, 1900 – December 18, 1975) was a prominent Ukra ...
: :1. ''Adaptation'' is the evolutionary process whereby an organism becomes better able to live in its
habitat In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers at the ...

habitat
or habitats. :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. :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.


What adaptation is not

Adaptation differs from flexibility,
acclimatization Acclimatization or acclimatisation ( also called acclimation or acclimatation) is the process in which an individual organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual co ...
, and learning, all of which are changes during life which are not inherited. Flexibility deals with the relative capacity of an organism to maintain itself in different habitats: its degree of specialization. Acclimatization describes automatic physiological adjustments during life; learning means improvement in behavioural performance during life. Flexibility stems from
phenotypic plasticity Phenotypic plasticity refers to some of the changes in an organism's behavior, morphology and physiology in response to a unique environment. Fundamental to the way in which organisms cope with environmental variation, phenotypic plasticity encompas ...
, the ability of an organism with a given
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
(genetic type) to change its
phenotype In genetics Genetics is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular inter ...

phenotype
(observable characteristics) in response to changes in its
habitat In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers at the ...

habitat
, or to move to a different habitat. The degree of flexibility is inherited, and varies between individuals. A highly specialized animal or plant lives only in a well-defined habitat, eats a specific type of food, and cannot survive if its needs are not met. Many
herbivore A herbivore is an animal Animals (also called Metazoa) are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of , the , during . Over 1.5 million animal have been —of ...
s are like this; extreme examples are
koala The koala or, inaccurately, koala bear (''Phascolarctos cinereus''), is an arboreal herbivorous marsupial native to Australia. It is the only Extant taxon, extant representative of the family Phascolarctidae and its closest living relative ...

koala
s which depend on ''
Eucalyptus ''Eucalyptus'' () is a of over seven hundred species of s, shrubs or in the , Myrtaceae. Along with several other genera in the , including ', they are commonly known as s. Plants in the genus ''Eucalyptus'' have bark that is either smooth, ...

Eucalyptus
'', and
giant panda The giant panda (''Ailuropoda melanoleuca''; ), also known as the panda bear (or simply the panda), is a bear Bears are carnivoran mammals of the family (biology), family Ursidae. They are classified as caniforms, or doglike carnivoran ...

giant panda
s which require
bamboo Bamboos are a diverse group of evergreen In botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in th ...

bamboo
. A generalist, on the other hand, eats a range of food, and can survive in many different conditions. Examples are humans,
rat Rats are various medium-sized, long-tailed rodent Rodents (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area a ...

rat
s,
crab Crabs are decapod crustacean Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such animals as crabs, lobsters, crayfish, Caridea, shrimp, krill, Dendrobranchiata, prawns, woodlice, barnacles, copepods, amphipoda ...

crab
s and many carnivores. The ''tendency'' to behave in a specialized or exploratory manner is inherited—it is an adaptation. Rather different is developmental flexibility: "An animal or plant is developmentally flexible if when it is raised in or transferred to new conditions, it changes in structure so that it is better fitted to survive in the new environment," writes
evolutionary biologist Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common descent, speciation) that produced the Biodiversity, diversity of life on Earth. In the 1930s, the discipline of evolutionary b ...
John Maynard Smith John Maynard Smith (6 January 1920 – 19 April 2004) was a British theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War World War II or the Secon ...

John Maynard Smith
. If humans move to a higher altitude, respiration and physical exertion become a problem, but after spending time in high altitude conditions they acclimatize to the reduced partial pressure of oxygen, such as by producing more
red blood cell Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek language, Greek ''erythros'' for "red" and ''k ...

red blood cell
s. The ability to acclimatize is an adaptation, but the acclimatization itself is not. The reproductive rate declines, but deaths from some tropical diseases also go down. Over a longer period of time, some people are better able to reproduce at high altitudes than others. They contribute more heavily to later generations, and gradually by natural selection the whole population becomes adapted to the new conditions. This has demonstrably occurred, as the observed performance of long-term communities at higher altitude is significantly better than the performance of new arrivals, even when the new arrivals have had time to acclimatize.


Adaptedness and fitness

There is a relationship between adaptedness and the concept of fitness used in
population genetics Population genetics is a subfield of that deals with genetic differences within and between s, and is a part of . Studies in this branch of examine such phenomena as , , and . Population genetics was a vital ingredient in the of the . Its pri ...
. Differences in fitness between genotypes predict the rate of evolution by natural selection. Natural selection changes the relative frequencies of alternative phenotypes, insofar as they are
heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, the offspring cell (biology), cells or orga ...

heritable
. However, a phenotype with high adaptedness may not have high fitness. Dobzhansky mentioned the example of the
Californian redwood
Californian redwood
, which is highly adapted, but a
relict A relict is a surviving remnant of a natural phenomenon. Biology A relict (or relic) is an organism that at an earlier time was abundant in a large area but now occurs at only one or a few small areas. Geology and geomorphology In geology ...
species in danger of
extinction Extinction is the termination of a kind of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...

extinction
.
Elliott Sober Elliott R. Sober (born 6 June 1948, Baltimore Baltimore ( , locally: ) is the most populous city in the U.S. state of Maryland, as well as the 30th most populous city in the United States, with an estimated population of 593,490 in 2019. Ba ...
commented that adaptation was a retrospective concept since it implied something about the history of a trait, whereas fitness predicts a trait's future. :1. Relative fitness. The average contribution to the next generation by a genotype or a class of genotypes, relative to the contributions of other genotypes in the population. This is also known as ''Darwinian fitness'', ''
selection coefficient In population genetics, a selection coefficient, usually denoted by the letter ''s'', is a measure of differences in relative Fitness (biology), fitness. Selection coefficients are central to the quantitative description of evolution, since fitness ...
'', and other terms. :2. Absolute fitness. The absolute contribution to the next generation by a genotype or a class of genotypes. Also known as the Malthusian parameter when applied to the population as a whole. :3. Adaptedness. The extent to which a phenotype fits its local ecological niche. Researchers can sometimes test this through a reciprocal transplant.
Sewall Wright Sewall Green Wright FRS(For) H FRSE (December 21, 1889March 3, 1988) was an American geneticist known for his influential work on evolutionary theory Evolutionary thought, the recognition that species change over time and the perceived unde ...

Sewall Wright
proposed that populations occupy ''adaptive peaks'' on a fitness landscape. To evolve to another, higher peak, a population would first have to pass through a valley of maladaptive intermediate stages, and might be "trapped" on a peak that is not optimally adapted.


Types


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 Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

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 An invasion is a Offensive (military), military offensive in which large numbers of combatants of one geopolitics, geopolitical Legal entity, entity aggressively enter territory (country subdivision), territory owned by another such entity, gene ...
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. When the habitat changes, three main things may happen to a resident population: habitat tracking, genetic change or extinction. In fact, all three things may occur in sequence. Of these three effects only genetic change brings about adaptation. When a habitat changes, the resident population typically moves to more suitable places; this is the typical response of flying
insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to be in ...

insect
s or oceanic organisms, which have wide (though not unlimited) opportunity for movement. This common response is called ''habitat tracking''. It is one explanation put forward for the periods of apparent stasis in the
fossil record A fossil (from Classical Latin: , literally 'obtained by digging') is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, Seashell, shells, exoskeletons, stone imprints of a ...
(the
punctuated equilibrium Image:PunctuatedEquilibrium.png, The punctuated equilibrium model (top) consists of Morphology (biology), morphological stability followed by rare bursts of evolutionary change via rapid cladogenesis. It is contrasted (below) to phyletic gradualism ...

punctuated equilibrium
theory).


Genetic change

Genetic change occurs in a population when natural selection and
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s act on its
genetic variabilityGenetic variability is either the presence of, or the generation of, genetic differences. It is defined as "the formation of individuals differing in genotype Here the relation between genotype and phenotype is illustrated, using a Punnett square ...
. The first pathways of enzyme-based metabolism may have been parts of purine nucleotide metabolism, with previous metabolic pathways being part of the ancient
RNA world The RNA world is a hypothetical stage in the evolutionary history of life The history of life on Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surfa ...
. By this means, the population adapts genetically to its circumstances. Genetic changes may result in visible structures, or may adjust physiological activity in a way that suits the habitat. The varying shapes of the beaks of Darwin's finches, for example, are driven by differences in the ALX1 gene. Habitats and biota 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 genetic ''tracking process'', which goes on all the time to some extent, but especially when the population cannot or does not move to another, less hostile area. Given enough genetic change, as well as specific demographic conditions, an adaptation may be enough to bring a population back from the brink of extinction in a process called evolutionary rescue. Adaptation does affect, to some extent, every species in a particular
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

ecosystem
.
Leigh Van Valen Leigh Van Valen (August 12, 1935 – October 16, 2010) was a U.S. The United States of America (USA), commonly known as the United States (U.S. or US), or America, is a country primarily located in North America North America i ...
thought that even in a stable environment, competing species constantly had to adapt to maintain their relative standing. This became known as the
Red Queen hypothesis The Red Queen hypothesis, also referred to as Red Queen's, the Red Queen effect, the Red Queen model, Red Queen's race, and Red Queen dynamics, is a hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. Fo ...
, as seen in host-
parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is adaptation (biology), adapted structurally to this w ...
interaction. Existing genetic variation and mutation were the traditional sources of material on which natural selection could act. In addition,
horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between unicellular A unicellular organism, also known as a single-celled organism, is an organism In biology, an organism (from Ancient G ...
is possible between organisms in different species, using mechanisms as varied as
gene cassette In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ''birth'' or wiktionary:γένος, ...
s,
plasmid A plasmid is a small, extrachromosomal DNA Extrachromosomal DNA (abbreviated ecDNA) is any DNA that is found off the chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. Most eukaryo ...
s,
transposon A transposable element (TE, transposon, or jumping gene) is a DNA sequence DNA sequencing is the process of determining the nucleic acid sequence A nucleic acid sequence is a succession of bases signified by a series of a set of five diff ...
s and
virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecu ...

virus
es such as
bacteriophage A bacteriophage (), also known informally as a ''phage'' (), is a virus A virus is a submicroscopic infectious agent In biology Biology is the natural science that studies life and living organisms, including their anatomy, ...

bacteriophage
s.


Co-adaptation

In
coevolution In biology, coevolution occurs when two or more species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined ...
, 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. These
co-adaptationIn biology, co-adaptation is the process by which two or more species, genes or phenotypic traits undergo adaptation as a pair or group. This occurs when two or more interacting characteristics undergo natural selection together in response to the sa ...
al relationships are intrinsically dynamic, and may continue on a trajectory for millions of years, as has occurred in the relationship between flowering plants and pollination, pollinating insects.


Mimicry

Bates' work on Amazonian Butterfly, butterflies led him to develop the first scientific account of
mimicry In evolutionary biology, mimicry is an evolved resemblance between an organism and another object, often an organism of another species. Mimicry may evolve between different species, or between individuals of the same species. Often, mimicry f ...

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 (the model), gaining a selective advantage as predators avoid the model and therefore also the mimic. Mimicry is thus an anti-predator adaptation. A common example seen in temperate gardens is the hoverfly, many of which—though bearing no sting—mimic the warning coloration of hymenoptera (wasps and bees). Such mimicry does not need to be perfect to improve the survival of the palatable species. Bates, Wallace and Fritz Müller believed that Batesian and
Müllerian mimicry Müllerian mimicry is a natural phenomenon in which two or more well-defended species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversit ...
provided coloration evidence for natural selection, evidence for the action of natural selection, a view which is now standard amongst biologists.


Trade-offs

All adaptations have a downside: horse legs are great for running on grass, but they can't scratch their backs; mammals' hair helps temperature, but offers a niche for Parasitism#Types, ectoparasites; the only flying penguins do is under water. Adaptations serving different functions may be mutually destructive. Compromise and makeshift 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 Allometry, allometric relationship to body size). Obviously, antlers serve positively for defence against Predation, predators, and to score victories in the annual rut (mammalian reproduction), 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. As another example, camouflage to avoid detection is destroyed when vivid animal coloration, coloration is displayed at mating time. Here the risk to life is counterbalanced by the necessity for reproduction. Stream-dwelling salamanders, such as Caucasian salamander or Gold-striped salamander have very slender, long bodies, perfectly adapted to life at the banks of fast small rivers and mountain Stream, brooks. Elongated body protects their larvae from being washed out by current. However, elongated body increases risk of desiccation and decreases dispersal ability of the salamanders; it also negatively affects their fecundity. As a result, fire salamander, less perfectly adapted to the mountain brook habitats, is in general more successful, have a higher fecundity and broader geographic range. The Peafowl, 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: "This 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 recognition of sexual selection was for a long time in abeyance, but has been rehabilitated. The conflict between the size of the human Fetus, foetal brain at birth, (which cannot be larger than about 400 cm3, else it will not get through the mother's pelvis) and the size needed for an adult brain (about 1400 cm3), 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 Bipedalism, bipedal stance, without which our pelvis could be shaped more suitably for birth. Neanderthals had a similar problem. As another example, the long neck of a giraffe brings benefits but at a cost. The neck of a giraffe can be up to in length. The benefits are that it can be used for inter-species competition or for foraging on tall trees where shorter herbivores cannot reach. The cost is that a long neck is heavy and adds to the animal's body mass, requiring additional energy to build the neck and to carry its weight around.


Shifts in function


Pre-adaptation

Pre-adaptation occurs when a population has characteristics which by chance are suited for a set of conditions not previously experienced. For example, the polyploid Spartina, cordgrass ''Spartina townsendii'' is better adapted than either of its parent species to their own habitat of saline marsh and mud-flats. Among domestic animals, the Leghorn chicken, White Leghorn chicken is markedly more resistant to Thiamine, vitamin B1 deficiency than other breeds; on a plentiful diet this makes no difference, but on a restricted diet this preadaptation could be decisive. Pre-adaptation may arise because a natural population carries a huge quantity of genetic variability. In Ploidy#Diploid, diploid eukaryotes, this is a consequence of the system of sexual reproduction, where mutant alleles get partially shielded, for example, by dominance (genetics), genetic dominance. Microorganisms, 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 microorganisms was provided by Salvador Luria and Max Delbrück who developed the Luria–Delbrück experiment, Fluctuation Test, a method to show the random fluctuation of pre-existing genetic changes that conferred resistance to bacteriophages in ''Escherichia coli''.


Co-option of existing traits: exaptation

Features that now appear as adaptations sometimes arose by co-option of existing traits, evolved for some other purpose. The classic example is the Evolution of mammalian auditory ossicles, ear ossicles of mammals, which we know from Paleontology, paleontological and Embryology, embryological evidence originated in the upper and lower jaws and the hyoid bone of their synapsid ancestors, and further back still were part of the Branchial arch, gill arches of early fish. The word ''exaptation'' was coined to cover these common evolutionary shifts in function. The flight
feather Feathers are epidermal growths that form a distinctive outer covering, or plumage Plumage ( "feather") is a layer of feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on dinosaurs, both avi ...

feather
s of birds evolved from the much earlier Feathered dinosaur#List of non-avian dinosaur species preserved with evidence of feathers, feathers of dinosaurs, which might have been used for insulation or for display.


Niche construction

Animals including earthworms, beavers and humans use some of their adaptations to modify their surroundings, so as to maximize their chances of surviving and reproducing. Beavers create dams and lodges, changing the ecosystems of the valleys around them. Earthworms, as Darwin noted, improve the topsoil in which they live by incorporating organic matter. Humans have constructed extensive civilizations with cities in environments as varied as the Arctic and hot deserts. In all three cases, the construction and maintenance of ecological niches helps drive the continued selection of the genes of these animals, in an environment that the animals have modified.


Non-adaptive traits

Some traits do not appear to be adaptive as they have a neutral or deleterious effect on fitness in the current environment. Because genes often have Pleiotropy, pleiotropic effects, not all traits may be functional: they may be what Stephen Jay Gould and Richard Lewontin called Spandrel (biology), spandrels, features brought about by neighbouring adaptations, on the analogy with the often highly decorated triangular areas between pairs of arches in architecture, which began as functionless features.Günter P. Wagner, Wagner, Günter P., ''Homology, Genes, and Evolutionary Innovation''. Princeton University Press. 2014. Chapter 1: The Intellectual Challenge of Morphological Evolution: A Case for Variational Structuralism. Page 7 Another possibility is that a trait may have been adaptive at some point in an organism's evolutionary history, but a change in habitats caused what used to be an adaptation to become unnecessary or even maladaptation, maladapted. Such adaptations are termed Vestigiality, vestigial. Many organisms have vestigial organs, which are the remnants of fully functional structures in their ancestors. As a result of changes in lifestyle the organs became redundant, and are either not functional or reduced in functionality. Since any structure represents some kind of cost to the general economy of the body, an advantage may accrue from their elimination once they are not functional. Examples: Wisdom tooth, wisdom teeth in humans; the loss of pigment and functional eyes in cave fauna; the loss of structure in Intestinal parasite, endoparasites.


Extinction and coextinction

If a population cannot move or change sufficiently to preserve its long-term viability, then obviously, it will become extinct, at least in that locale. The species may or may not survive in other locales. Species extinction occurs when the death rate over the entire species exceeds the birth rate for a long enough period for the species to disappear. It was an observation of Van Valen that groups of species tend to have a characteristic and fairly regular rate of extinction. Just as there is co-adaptation, there is also coextinction, the loss of a species due to the extinction of another with which it is coadapted, as with the extinction of a parasitism, parasitic insect following the loss of its host, or when a flowering plant loses its pollinator, or when a food chain is disrupted.


Philosophical issues

Adaptation raises Philosophy of biology, philosophical issues concerning how biologists speak of function and purpose, as this carries implications of evolutionary history – that a feature evolved by natural selection for a specific reason – and potentially of supernatural intervention – that features and organisms exist because of a deity's conscious intentions. Aristotle's biology, In his biology, Aristotle introduced teleology to describe the adaptedness of organisms, but without accepting the supernatural intention built into Plato's thinking, which Aristotle rejected. Modern biologists continue to face the same difficulty. On the one hand, adaptation is obviously purposeful: natural selection chooses what works and eliminates what does not. On the other hand, biologists by and large reject conscious purpose in evolution. The dilemma gave rise to a famous joke by the evolutionary biologist J. B. S. Haldane, Haldane: "Teleology is like a mistress to a biologist: he cannot live without her but he's unwilling to be seen with her in public.'" David Hull commented that Haldane's mistress "has become a lawfully wedded wife. Biologists no longer feel obligated to apologize for their use of teleological language; they flaunt it." Ernst Mayr stated that "adaptedness... is ''Empirical evidence, a posteriori'' result rather than an a priori goal-seeking", meaning that the question of whether something is an adaptation can only be determined after the event.Ernst Mayr, Mayr, Ernst W. (1992). "The idea of teleology" ''Journal of the History of Ideas'', 53, 117–135.


See also

* Adaptive evolution in the human genome * Adaptive memory * Adaptive mutation * Adaptive system * Anti-predator adaptation * Body reactivity * Ecological trap * Evolutionary pressure * Evolvability * Intragenomic conflict * Neutral theory of molecular evolution


References


Sources

* "Based on a conference held at the Mote Marine Laboratory in Sarasota, Fla., May 20–24, 1990." * * * * * * * * * * "Papers by Dobzhansky and his collaborators, originally published 1937-1975 in various journals." * * * * * * * * * * * "Based on a conference held in Bellagio, Italy, June 25–30, 1989" * * * * * * * * * * * * * * * * * * * * * * * * {{Authority control Evolutionary biology