View source for Adaptation

{{Short description|Process that fits organisms to their environment}}
{{About|the evolutionary process}}
{{Distinguish|Adoption|Acclimatization}}
{{Good article}}
{{use dmy dates|date=March 2022}}
{{Evolutionary biology}}
{{Complex systems}}

In [[biology]], '''adaptation''' has three related meanings. Firstly, it is the dynamic [[evolution]]ary process of [[natural selection]] that fits [[organism]]s to their environment, enhancing their [[Fitness (biology)|evolutionary fitness]]. Secondly, it is a state reached by the population during that process. Thirdly, it is a [[phenotypic trait]] or '''adaptive trait''', with a functional role in each individual [[organism]], that is maintained and has evolved through natural selection. 

Historically, adaptation has been described from the time of the ancient Greek philosophers such as [[Empedocles]] and [[Aristotle]]. In 18th and 19th-century [[natural theology]], adaptation was taken as evidence for the existence of a deity. [[Charles Darwin]] and [[Alfred Russel Wallace]] proposed instead that it was explained by natural selection.

Adaptation is related to [[biological fitness]], which governs the rate of evolution as measured by changes in [[allele frequencies]]. Often, two or more species co-adapt and [[co-evolve]] as they develop adaptations that interlock with those of the other species, such as with [[flowering plant]]s and [[pollinating insect]]s. In [[mimicry]], species evolve to resemble other species; in [[Müllerian mimicry|mimicry]] 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 [[Exaptation|co-opted]] for a different one, as when the insulating [[feather]]s of dinosaurs were co-opted for [[bird flight]].

Adaptation is a major topic in the [[philosophy of biology]], as it concerns function and purpose ([[Teleology in biology|teleology]]). Some biologists try to avoid terms which imply purpose in adaptation, not least because they suggest a deity's intentions, but others note that adaptation is necessarily purposeful.

==History ==
{{Main|History of evolutionary thought}}

Adaptation is an observable fact of life accepted by philosophers and natural historians from ancient times, independently of their views on [[evolution]], but their explanations differed. [[Empedocles]] did not believe that adaptation required a [[final cause]] (a purpose), but thought that it "came about naturally, since such things survived." [[Aristotle]] did believe in final causes, but assumed that [[Aristotle's biology#Scale of being|species were fixed]].<ref>{{cite book |author-link=Armand Marie Leroi |last=Leroi |first=Armand Marie |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |publisher=Bloomsbury |date=2015 |isbn=978-1-4088-3622-4 |pages=91–92, 273, 288}}</ref>

[[File:Lamarck's Two-Factor Theory.svg|thumb|upright=1.7|The second of [[Jean-Baptiste Lamarck]]'s two factors (the first being a complexifying force) was an adaptive force that causes animals with a given [[body plan]] to adapt to circumstances by [[inheritance of acquired characteristics]], creating a diversity of [[species]] and [[genus|genera]].]]

In [[natural theology]], adaptation was interpreted as the work of a deity and as evidence for the existence of God.<ref>{{harvnb|Desmond|1989|pp=31–32, fn 18}}</ref> [[William Paley]] believed that organisms were perfectly adapted to the lives they led, an argument that shadowed [[Gottfried Wilhelm Leibniz]], who had argued that God had brought about "[[best of all possible worlds|the best of all possible worlds]]." [[Voltaire]]'s satire [[Candide|Dr. Pangloss]]<ref>{{cite book |author=Voltaire |title=Candide |publisher=Cramer et al |date=1759}}</ref> is a parody of this optimistic idea, and [[David Hume]] also argued against design.<ref>{{harvnb|Sober|1993|loc=chpt. 2}}</ref> [[Charles Darwin]] broke with the tradition by emphasising the flaws and limitations which occurred in the animal and plant worlds.<ref>{{harvnb|Darwin|1872|p=[http://darwin-online.org.uk/content/frameset?pageseq=425&itemID=F391&viewtype=side 397: "Rudimentary, Atrophied, and Aborted Organs"]}}</ref>

[[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''.<ref>{{cite book |last=Bowler |first=Peter J. |author-link=Peter J. Bowler |title=Evolution: The History of an Idea |date=1989 |publisher=University of California Press |isbn=978-0-520-06386-0 |page=[https://archive.org/details/evolutionhistory0000bowl/page/86 86] |edition=Revised |orig-year=1983 |url=https://archive.org/details/evolutionhistory0000bowl/page/86 }}</ref> This second, subsidiary element of his theory is what is now called [[Lamarckism]], a proto-evolutionary hypothesis of the [[inheritance of acquired characteristics]], intended to explain adaptations by natural means.<ref>See, for example, the discussion in {{harvnb|Bowler|2003|pp=86–95}}: "Whatever the true nature of Lamarck's theory, it was his mechanism of adaptation that caught the attention of later naturalists." (p. 90)</ref>

Other natural historians, such as [[Georges-Louis Leclerc, Comte de 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]], and secondary figures such as [[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]] produced [[Evidence of common descent|evidence that natural selection]] was not only the 'engine' behind adaptation, but was a much stronger force than had previously been thought.<ref name="Provine 1986">{{harvnb|Provine|1986}}</ref><ref>{{harvnb|Ford|1975}}</ref><ref name="Orr_2005">{{cite journal |last=Orr |first=H. Allen |author-link=H. Allen Orr |date=February 2005 |title=The genetic theory of adaptation: a brief history |journal=[[Nature Reviews Genetics]] |volume=6 |issue=2 |pages=119–127 |doi=10.1038/nrg1523 |pmid=15716908|s2cid=17772950 }}</ref>

==General principles ==

{{Blockquote|''The significance of an adaptation can only be understood in relation to the total biology of the species.''|[[Julian Huxley]]|''[[Evolution: The Modern Synthesis]]''<ref>{{harvnb|Huxley|1942|p=449}}</ref>}}

===What adaptation is===

Adaptation is primarily a process rather than a physical form or part of a body.<ref>{{harvnb|Mayr|1982|p=483}}: "Adaptation... could no longer be considered a static condition, a product of a creative past, and became instead a continuing dynamic process."</ref> An internal [[parasite]] (such as a [[liver fluke]]) 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 [[biological life cycle|life cycle]], which is often quite complex.<ref>{{harvnb |Price |1980}}</ref> However, as a practical term, "adaptation" often refers to a ''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 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.<ref>{{cite encyclopedia |editor1-last=Daintith |editor1-first=John |editor2-last=Martin |editor2-first=Elizabeth A. |encyclopedia=A Dictionary of Science |title=adaptation |orig-year=First published 1984 as ''Concise Science Dictionary'' |edition=6th |year=2010 |publisher=[[Oxford University Press]] |series=Oxford Paperback Reference |isbn=978-0-19-956146-9 |lccn=2010287468 |oclc=444383696 |page=13 |quote=Any change in the structure or functioning of successive generations of a population that makes it better suited to its environment.}}</ref><ref>{{harvnb |Bowler |2003 |p=10}}</ref><ref>{{harvnb |Patterson |1999 |p=1}}</ref><ref>{{harvnb |Williams |1966 |p=5}}: "Evolutionary adaptation is a phenomenon of pervasive importance in biology."</ref>

Adaptation is one of the two main processes that explain the observed diversity of species, such as the different species of [[Darwin's finches]]. The other process is [[speciation]], in which new species arise, typically through [[reproductive isolation]].<ref>{{harvnb |Mayr |1963}}</ref><ref>{{harvnb |Mayr |1982 |pp=562–566}}</ref> An example widely used today to study the interplay of adaptation and speciation is the evolution of [[cichlid]] fish in African lakes, where the question of reproductive isolation is complex.<ref name="Salzburger">{{cite journal |last1=Salzburger |first1=Walter |last2=Mack |first2=Tanja |last3=Verheyen |first3=Erik |last4=Meyer |first4=Axel |author-link4=Axel Meyer |date=21 February 2005 |title=Out of Tanganyika: Genesis, explosive speciation, key-innovations and phylogeography of the haplochromine cichlid fishes |journal=[[BMC Evolutionary Biology]] |volume=5 |pages=17 |number=17 |doi=10.1186/1471-2148-5-17 |pmc=554777 |pmid=15723698 |doi-access=free }}</ref><ref name="Kornfield">{{cite journal |last1=Kornfield |first1=Irv |last2=Smith |first2=Peter F. |date=November 2000 |title=African Cichlid Fishes: Model Systems for Evolutionary Biology |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=31 |issue=1 |pages=163–196 |doi=10.1146/annurev.ecolsys.31.1.163|bibcode=2000AnRES..31..163K }}</ref>

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 [[Genetics|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]]y in plants is a reasonably common large genetic change.<ref>{{harvnb|Stebbins|1950|loc=chs. 8 and 9}}</ref> The origin of [[eukaryote|eukaryotic]] [[endosymbiosis]] is a more dramatic example.<ref>{{harvnb|Margulis|Fester|1991}}</ref>

All adaptations help organisms survive in their [[ecological niche]]s. The adaptive traits may be structural, behavioural or [[Physiology|physiological]]. Structural adaptations are physical features of an organism, such as shape, body covering, armament, and [[comparative anatomy|internal organization]]. [[Ethology|Behavioural]] adaptations are inherited systems of behaviour, whether inherited in detail as [[instinct]]s, or as a [[neuropsychology|neuropsychological]] capacity for [[learning]]. Examples include [[Foraging|searching for food]], [[mating]], and [[Animal communication|vocalizations]]. Physiological adaptations permit the organism to perform special functions such as making [[venom]], secreting [[Snail slime|slime]], and [[phototropism]], but also involve more general functions such as [[developmental biology|growth and development]], [[Thermoregulation|temperature regulation]], [[ions|ionic]] balance and other aspects of [[homeostasis]]. Adaptation affects all aspects of the life of an organism.<ref>{{harvnb|Hutchinson|1965}}. The niche is the central concept in evolutionary ecology; see especially part II: "The niche: an abstractly inhabited hypervolume." (pp. 26–78)</ref>

The following definitions are given by the evolutionary biologist [[Theodosius Dobzhansky]]:
:1. ''Adaptation'' is the evolutionary process whereby an organism becomes better able to live in its [[habitat]] or habitats.<ref name="Dobzhansky T 1968">{{harvnb |Dobzhansky |1968 |pp=1–34}}</ref><ref>{{cite book |last1=Wang |first1=G |title=Analysis of Complex Diseases: A Mathematical Perspective |publisher=Taylor Francis |year=2014 |chapter=Chapter 5.6—Zero Order Adaptivity |pages=69 |isbn=978-1-4665-7223-2 |chapter-url=https://books.google.com/books?id=DmDSBQAAQBAJ&pg=PA69 |access-date=31 March 2018 |archive-date=30 June 2024 |archive-url=https://web.archive.org/web/20240630095058/https://books.google.com/books?id=DmDSBQAAQBAJ&pg=PA69#v=onepage&q&f=false |url-status=live }}</ref><ref>{{cite book |title=Climate Change Impact on Livestock: Adaptation and Mitigation |publisher=Springer |editor1-last=Sejian |editor1-first=V. |editor2-last=Gaughan |editor2-first=J. |editor3-last=Baumgard |editor3-first=L. |editor4-last=Prasad |editor4-first=C. |year=2015 |pages=515 |isbn=978-81-322-2265-1 |url=https://books.google.com/books?id=D-G9BwAAQBAJ&pg=PA515 |access-date=31 March 2018 |archive-date=24 July 2020 |archive-url=https://web.archive.org/web/20200724234323/https://books.google.com/books?id=D-G9BwAAQBAJ&pg=PA515 |url-status=live }}</ref>
: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.<ref>{{harvnb |Dobzhansky |1970 |pp=4–6; 79–82}}</ref>
: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.<ref>{{cite journal |last=Dobzhansky |first=Theodosius |author-link=Theodosius Dobzhansky |date=March 1956 |title=Genetics of Natural Populations. XXV. Genetic Changes in Populations of ''Drosophila pseudoobscura'' and ''Drosophila persimilis'' in Some Localities in California |journal=[[Evolution (journal)|Evolution]] |volume=10 |issue=1 |pages=82–92 |doi=10.2307/2406099 |jstor=2406099}}</ref>

===What adaptation is not===
[[File:Falke vor LKW.jpg|thumb|The common kestrel has adapted successfully to urban areas]]

Adaptation differs from flexibility, [[acclimatization]], 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 [[Generalist and specialist species|specialization]]. Acclimatization describes automatic physiological adjustments during life;<ref name="Rymer2013">{{cite journal |last1=Rymer |first1=Tasmin |last2=Pillay |first2=Neville |last3=Schradin |first3=Carsten |title=Extinction or Survival? Behavioral Flexibility in Response to Environmental Change in the African Striped Mouse Rhabdomys |journal=Sustainability |volume=5 |issue=1 |year=2013 |doi=10.3390/su5010163 |pages=163–186|doi-access=free |bibcode=2013Sust....5..163R }}</ref> learning means alteration in behavioural performance during life.<ref>{{cite book |last=Gross |first=Richard |title=Psychology: The Science of Mind and Behaviour |edition=6th |url=https://books.google.com/books?id=Cle1Fcr_6_QC&pg=PT335 |year=2012 |publisher=Hodder |isbn=978-1-4441-6436-7 |page=335 |access-date=31 March 2018 |archive-date=30 June 2024 |archive-url=https://web.archive.org/web/20240630095059/https://books.google.com/books?id=Cle1Fcr_6_QC&pg=PT335 |url-status=live }}</ref>

Flexibility stems from [[phenotypic plasticity]], the ability of an organism with a given [[genotype]] (genetic type) to change its [[phenotype]] (observable characteristics) in response to changes in its [[habitat]], or to move to a different habitat.<ref>{{cite journal |last1=Price |first1=Trevor D. |last2=Qvarnström |first2=Anna |last3=Irwin |first3=Darren E. |date=July 2003 |title=The role of phenotypic plasticity in driving genetic evolution |journal=[[Proceedings of the Royal Society#Proceedings of the Royal Society B|Proceedings of the Royal Society B]] |volume=270 |issue=1523 |pages=1433–1440 |doi=10.1098/rspb.2003.2372 |pmc=1691402 |pmid=12965006}}</ref><ref>{{cite journal |last=Price |first=Trevor D. |date=June 2006 |title=Phenotypic plasticity, sexual selection and the evolution of colour patterns |journal=[[The Journal of Experimental Biology]]  |volume=209 |issue=12 |pages=2368–2376 |doi=10.1242/jeb.02183 |pmid=16731813|doi-access= |bibcode=2006JExpB.209.2368P }}</ref> 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]]s are like this; extreme examples are [[koala]]s which depend on ''[[Eucalyptus]]'', and [[giant panda]]s which require [[bamboo]]. A generalist, on the other hand, eats a range of food, and can survive in many different conditions. Examples are humans, rats, crabs 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 the [[evolutionary biology|evolutionary biologist]] [[John Maynard Smith]].<ref>{{harvnb|Maynard Smith|1993|p=33}}</ref>

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]]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.<ref>{{cite journal |last1=Moore |first1=Lorna G. |last2=Regensteiner |first2=Judith G. |date=October 1983 |title=Adaptation to High Altitude |journal=[[Annual Review of Anthropology]] |volume=12 |pages=285–304 |doi=10.1146/annurev.an.12.100183.001441}}</ref>

===Adaptedness and fitness ===
{{main|Fitness (biology)|Fitness landscape}}

There is a relationship between adaptedness and the concept of fitness used in [[population genetics]]. 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 [[Heritability|heritable]].<ref name="Endler 33">{{harvnb|Endler|1986|pp=[https://books.google.com/books?id=MYk1XbelDssC&pg=PA33 33–51]}}</ref> However, a phenotype with high adaptedness may not have high fitness. Dobzhansky mentioned the example of the [[Sequoia sempervirens|Californian redwood]], which is highly adapted, but a [[Relict (biology)|relict]] species in danger of [[extinction]].<ref name="Dobzhansky T 1968" /> [[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.<ref name="Sober2">{{harvnb|Sober|1984|p=210}}</ref>

: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.<ref name="Futuyma_Evolution">{{harvnb|Futuyma|1986|p=552}}</ref> This is also known as ''Darwinian fitness'', ''[[selection coefficient]]'', 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 growth model|Malthusian parameter]] when applied to the population as a whole.<ref name="Endler 33" /><ref>{{harvnb|Fisher|1930|p=25}}</ref>
:3. Adaptedness. The extent to which a phenotype fits its local ecological niche. Researchers can sometimes test this through a [[Transplant experiment|reciprocal transplant]].<ref>{{cite journal |last1=de Villemereuil |first1=P. |last2=Gaggiotti |first2=O. E. |last3=Mouterde |first3=M. |last4=Till-Bottraud |first4=I |title=Common garden experiments in the genomic era: new perspectives and opportunities |journal=Heredity |volume=116 |issue=3 |date=21 October 2015 |doi=10.1038/hdy.2015.93 |pmid=26486610 |pages=249–254 |pmc=4806574 }}</ref>

[[File:fitness-landscape-cartoon.png|thumb|In this sketch of a [[fitness landscape]], a population can evolve by following the arrows to the adaptive peak at point B, and the points A and C are local optima where a population could become trapped.]]

[[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.<ref>{{harvnb|Wright|1932|pp=[http://www.esp.org/books/6th-congress/facsimile/contents/6th-cong-p356-wright.pdf 356–366]}}</ref>

==Types==

{{Blockquote|Adaptation is the heart and soul of evolution.|[[Niles Eldredge]]|''Reinventing Darwin: The Great Debate at the High Table of Evolutionary Theory''<ref>{{harvnb|Eldredge|1995|p=33}}</ref>}}

===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 (ecology)|biota]]. Also, habitats are subject to changes in their biota: for example, [[Invasive species|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.
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 insects or oceanic organisms, which have wide (though not unlimited) opportunity for movement.<ref>{{harvnb|Eldredge|1985|p=136: "Of glaciers and beetles"}}</ref> This common response is called ''habitat tracking''. It is one explanation put forward for the periods of apparent stasis in the [[Fossil#Dating|fossil record]] (the [[punctuated equilibrium]] theory).<ref>{{harvnb|Eldredge|1995|p=64}}</ref>

=== Genetic change ===

Without [[mutation]], the ultimate source of all [[genetic variation]], there would be no genetic changes and no subsequent adaptation through evolution by natural selection. Genetic change occurs in a population when mutation increases or decreases in its initial frequency followed by random genetic drift, migration, recombination or natural selection act on this genetic variation.<ref>{{cite encyclopedia |last=Hogan |first=C. Michael |editor-last=Monosson |editor-first=Emily |encyclopedia=[[Encyclopedia of Earth]] |title=Mutation |url=https://www.eoearth.org/view/article/159530/ |access-date=18 August 2015 |date=12 October 2010 |publisher=Environmental Information Coalition, [[National Council for Science and the Environment]] |oclc=72808636 |archive-date=24 September 2015 |archive-url=https://web.archive.org/web/20150924044631/http://www.eoearth.org/view/article/159530/ |url-status=live }}</ref> One example is that the first pathways of enzyme-based metabolism at the very origin of life on Earth may have been co-opted components of the already-existing [[Purine metabolism|purine nucleotide metabolism]], a metabolic pathway that evolved in an ancient [[RNA world]]. The co-option requires new mutations and through natural selection, the population then adapts genetically to its present circumstances.<ref name="Orr_2005" /> Genetic changes may result in entirely new or gradual change to visible structures, or they 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 adaptive mutations in the ALX1 gene.<ref>{{cite journal |last1=Lamichhaney |first1=Sangeed |last2=Berglund |first2=Jonas |date=19 February 2015 |title=Evolution of Darwin's finches and their beaks revealed by genome sequencing |journal=Nature |volume=518 |issue=7539 |pages=371–375 |doi=10.1038/nature14181 |pmid=25686609 |bibcode=2015Natur.518..371L |s2cid=4462253 }}</ref> The coat color of different wild mouse species matches their environments, whether black lava or light sand, owing to adaptive mutations in the [[melanocortin 1 receptor]] and other [[melanin]] pathway genes.<ref>{{Cite journal |last1=Nachman |first1=Michael W. |last2=Hoekstra |first2=Hopi E. |last3=D'Agostino |first3=Susan L. |date=2003-04-29 |title=The genetic basis of adaptive melanism in pocket mice |journal=Proceedings of the National Academy of Sciences |language=en |volume=100 |issue=9 |pages=5268–5273 |doi=10.1073/pnas.0431157100 |issn=0027-8424 |pmid=12704245 |pmc=154334 |bibcode=2003PNAS..100.5268N |doi-access=free}}</ref><ref>{{Cite journal |last1=Hoekstra |first1=Hopi E. |last2=Hirschmann |first2=Rachel J. |last3=Bundey |first3=Richard A. |last4=Insel |first4=Paul A. |last5=Crossland |first5=Janet P. |date=7 July 2006 |title=A Single Amino Acid Mutation Contributes to Adaptive Beach Mouse Color Pattern |url=https://www.science.org/doi/10.1126/science.1126121 |journal=Science |volume=313 |issue=5783 |pages=101–104 |doi=10.1126/science.1126121 |pmid=16825572 |bibcode=2006Sci...313..101H |s2cid=33376626 |access-date=18 December 2021 |archive-date=30 June 2024 |archive-url=https://web.archive.org/web/20240630095102/https://www.science.org/doi/10.1126/science.1126121 |url-status=live |url-access=subscription }}</ref> Physiological resistance to the heart poisons ([[cardiac glycoside]]s) that [[Monarch butterfly|monarch butterflies]] store in their bodies to protect themselves from predators<ref>{{Cite journal |last1=Brower |first1=Lincoln P. |last2=Glazier |first2=Susan C. |date=4 April 1975 |title=Localization of Heart Poisons in the Monarch Butterfly |url=|journal=Science |volume=188 |issue=4183 |pages=19–25 |doi=10.1126/science.188.4183.19 |pmid=17760150 |bibcode=1975Sci...188...19B |s2cid=44509809 |issn=0036-8075}}</ref><ref>{{Cite book |last=Agrawal |first=Anurag |url=|title=Monarchs and Milkweed |date=7 March 2017 |publisher=[[Princeton University Press]] |doi=10.1515/9781400884766 |isbn=978-1-4008-8476-6}}</ref> are driven by adaptive mutations in the target of the poison, the [[Na+/K+-ATPase|sodium pump]], resulting in target site insensitivity.<ref>{{Cite journal |last1=Reichstein |first1=T. |last2=von Euw |first2=J. |last3=Parsons |first3=J. A. |last4=Rothschild |first4=Miriam |date=30 August 1968 |title=Heart Poisons in the Monarch Butterfly |url=|journal=Science |volume=161 |issue=3844 |pages=861–866 |doi=10.1126/science.161.3844.861 |pmid=4875496 |bibcode=1968Sci...161..861R |issn=0036-8075}}</ref><ref>{{Cite journal |last1=Holzinger |first1=F. |last2=Frick |first2=C. |last3=Wink |first3=M. |date=21 December 1992 |title=Molecular basis for the insensitivity of the Monarch (Danaus plexippus) to cardiac glycosides |journal=FEBS Letters |volume=314 |issue=3 |pages=477–480 |doi=10.1016/0014-5793(92)81530-y |issn=0014-5793 |pmid=1334851 |s2cid=7427771|doi-access=free |bibcode=1992FEBSL.314..477H }}</ref><ref>{{Cite journal |last1=Karageorgi |first1=Marianthi |last2=Groen |first2=Simon C. |last3=Sumbul |first3=Fidan |last4=Pelaez |first4=Julianne N. |last5=Verster |first5=Kirsten I. |last6=Aguilar |first6=Jessica M. |last7=Hastings |first7=Amy P. |last8=Bernstein |first8=Susan L. |last9=Matsunaga |first9=Teruyuki |last10=Astourian |first10=Michael |last11=Guerra |first11=Geno |display-authors=3 |date=2 October 2019 |title=Genome editing retraces the evolution of toxin resistance in the monarch butterfly |url=|journal=Nature |volume=574 |issue=7778 |pages=409–412 |doi=10.1038/s41586-019-1610-8 |pmid=31578524 |pmc=7039281 |issn=0028-0836}}</ref> These same adaptive mutations and similar changes at the same amino acid sites were found to evolve in a parallel manner in distantly related insects that feed on the same plants, and even in a bird that feeds on monarchs through [[convergent evolution]], a hallmark of adaptation.<ref>{{Cite journal |last1=Dobler |first1=S. |last2=Dalla |first2=S. |last3=Wagschal |first3=V. |last4=Agrawal |first4=A. A. |date=23 July 2012 |title=Community-wide convergent evolution in insect adaptation to toxic cardenolides by substitutions in the Na,K-ATPase |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=32 |pages=13040–13045 |doi=10.1073/pnas.1202111109 |pmid=22826239 |pmc=3420205 |issn=0027-8424 |doi-access=free}}</ref><ref>{{Cite journal |last1=Groen |first1=Simon C. |last2=Whiteman |first2=Noah K. |date=November 2021 |title=Convergent evolution of cardiac-glycoside resistance in predators and parasites of milkweed herbivores |url=|journal=Current Biology |volume=31 |issue=22 |pages=R1465–R1466 |doi=10.1016/j.cub.2021.10.025 |pmid=34813747 |pmc=8892682 |bibcode=2021CBio...31R1465G |s2cid=244485686 |issn=0960-9822}}</ref> Convergence at the gene-level across distantly related species can arise because of evolutionary constraint.<ref>{{cite book |last=Losos |first=Jonathan B. |url=http://worldcat.org/oclc/1024108339 |title=Improbable destinies: fate, chance, and the future of evolution |date=7 August 2018 |publisher=Penguin |isbn=978-0-525-53413-6 |oclc=1024108339 |access-date=18 December 2021 |archive-date=30 June 2024 |archive-url=https://web.archive.org/web/20240630095104/https://search.worldcat.org/title/1024108339 |url-status=live }}</ref>

Habitats and biota do frequently change over time and space. Therefore, it follows that the process of adaptation is never fully complete.<ref>{{harvnb|Mayr|1982|pp=481–483}}: This sequence tells how Darwin's ideas on adaptation developed as he came to appreciate it as "a continuing dynamic process."</ref> Over time, it may happen that the environment changes little, and the species comes to fit its surroundings better and better, resulting in stabilizing selection. On the other hand, it may happen that changes in the environment occur suddenly, and then the species becomes less and less well adapted. The only way for it to climb back up that fitness peak is via the introduction of new genetic variation for natural selection to act upon. 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]].<ref>{{harvnb|Sterelny|Griffiths|1999|p=217}}</ref><ref>{{harvnb|Freeman|Herron|2007|p=364}}</ref>

[[Leigh Van Valen]] thought that even in a stable environment, because of antagonistic species interactions and limited resources, a species must constantly had to adapt to maintain its relative standing. This became known as the [[Red Queen hypothesis]], as seen in host-[[parasite]] interactions.<ref>{{cite journal |doi=10.1126/sciadv.1501548 |pmid=26973878 |pmc=4783124 |last=Rabajante |first=J. |title=Host-parasite Red Queen dynamics with phase-locked rare genotypes |journal=[[Science Advances]] |year=2016 |volume=2 |issue=3 |pages=e1501548 |display-authors=etal |bibcode=2016SciA....2E1548R}}</ref>

Existing genetic variation and mutation were the traditional sources of material on which natural selection could act. In addition, [[horizontal gene transfer]] is possible between organisms in different species, using mechanisms as varied as [[gene cassette]]s, [[plasmid]]s, [[transposon]]s and viruses such as [[bacteriophage]]s.<ref>{{cite journal |last1=Gogarten |first1=J. Peter |last2=Doolittle |first2=W. Ford |date=1 December 2002 |title=Prokaryotic Evolution in Light of Gene Transfer |journal=Molecular Biology and Evolution |volume=19 |issue=12 |pages=2226–2238 |doi=10.1093/oxfordjournals.molbev.a004046 |pmid=12446813 |doi-access=free }}</ref><ref>{{Cite journal |last1=de la Cruz |first1=Fernando |last2=Davies |first2=Julian |date=1 March 2000 |title=Horizontal gene transfer and the origin of species: lessons from bacteria |journal=Trends in Microbiology |volume=8 |issue=3 |pages=128–133 |doi=10.1016/s0966-842x(00)01703-0 |pmid=10707066}}</ref><ref>{{Cite book |title=Genetics: From Genes to Genomes |last1=Hartwell |first1=Leland |last2=Goldberg |first2=Michael |last3=Fischer |first3=Janice |last4=Hood |first4=Lee |last5=Aquardo |first5=Charles |last6=Bejcek |first6=Bruce |publisher=McGraw- Hill Education |year=2015 |isbn=978-0-07-352531-0 |edition=5th |location=New York City |pages=475–479}}</ref>

=== Co-adaptation ===
{{main|Co-adaptation}}

[[File:Plumpollen0060.jpg|thumb|Pollinating insects are [[Co-adaptation|co-adapted]] with flowering plants.]]

In [[coevolution]], 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. In other words, each species triggers reciprocal natural selection in the other. These [[co-adaptation]]al relationships are intrinsically dynamic, and may continue on a trajectory for millions of years, as has occurred in the relationship between [[flowering plant]]s and [[pollination|pollinating]] insects.<ref>{{cite book |title=Coevolution |editor1=Futuyma, D. J. |editor-link1=Douglas J. Futuyma |editor2=M. Slatkin |year=1983 |publisher=[[Sinauer Associates]] |isbn=978-0-87893-228-3 |pages=whole book}}</ref><ref>{{cite book |last=Thompson |first= J. N. |title=The Coevolutionary Process |year=1994 |publisher=University of Chicago Press |isbn=978-0-226-79759-5 |pages=whole book}}</ref>

=== Mimicry ===
{{main|Mimicry}}
[[File:Wasp mimicry.jpg|thumb|Images A and B show real [[wasps]]; the others show [[Batesian mimicry|Batesian mimics]]: three [[Hoverfly|hoverflies]] and one [[beetle]].]]

Bates' work on Amazonian [[Butterfly|butterflies]] led him to develop the first scientific account of [[mimicry]], especially the kind of mimicry which bears his name: [[Batesian mimicry]].<ref>{{harvnb|Carpenter|Ford|1933}}</ref> This is the mimicry by a palatable species of an unpalatable or noxious species (the model), gaining a selective advantage as [[predator]]s 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]] (Syrphidae), many of which—though bearing no sting—mimic the [[warning coloration]] of aculeate [[Hymenoptera]] ([[wasp]]s and [[bee]]s). Such mimicry does not need to be perfect to improve the survival of the palatable species.<ref>{{harvnb|Wickler|1968}}</ref>

Bates, Wallace and [[Fritz Müller]] believed that Batesian and [[Müllerian mimicry]] provided [[coloration evidence for natural selection|evidence for the action of natural selection]], a view which is now standard amongst biologists.<ref>{{harvnb|Moon|1976}}</ref><ref>{{harvnb|Ruxton|Sherratt|Speed|2004}}</ref><ref>{{cite journal |last=Mallet |first=James |author-link=James Mallet |date=November 2001 |title=The speciation revolution |url=http://www.ucl.ac.uk/taxome/jim/pap/malletjeb01.pdf |journal=[[Journal of Evolutionary Biology]] |volume=14 |issue=6 |pages=887–888 |doi=10.1046/j.1420-9101.2001.00342.x |s2cid=36627140 |doi-access=free |access-date=28 December 2010 |archive-date=3 March 2016 |archive-url=https://web.archive.org/web/20160303203454/http://www.ucl.ac.uk/taxome/jim/pap/malletjeb01.pdf |url-status=live }}</ref>

=== Trade-offs ===

All adaptations have a downside: horse legs are great for running on grass, but they cannot scratch their backs; [[mammal]]s' hair helps temperature, but offers a niche for [[Parasitism#Basic concepts|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.<ref>{{cite journal |last=Jacob |first=François |s2cid=29756896 |author-link=François Jacob |date=10 June 1977 |title=Evolution and Tinkering |journal=[[Science (journal)|Science]] |volume=196 |issue=4295 |pages=1161–1166 |doi=10.1126/science.860134 |pmid=860134 |bibcode=1977Sci...196.1161J }}</ref>{{Blockquote|It is a profound truth that Nature does not know best; that genetical evolution... is a story of waste, makeshift, compromise and blunder.|[[Peter Medawar]]|''The Future of Man''<ref>{{harvnb|Medawar|1960}}</ref>}}

{{Blockquote|Since the phenotype as a whole is the target of selection, it is impossible to improve simultaneously all aspects of the phenotype to the same degree.|[[Ernst Mayr]]|''[[The Growth of Biological Thought|The Growth of Biological Thought: Diversity, Evolution, and Inheritance]]''<ref>{{harvnb|Mayr|1982|p=589}}</ref>}}

==== Examples ====
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). Antlers serve positively for defence against [[Predation|predator]]s, and to score victories in the annual [[rut (mammalian reproduction)|rut]]. But they are costly in terms of resources. 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.<ref>{{cite journal |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |date=June 1974 |title =The Origin and Function of 'Bizarre' Structures: Antler Size and Skull Size in the 'Irish Elk,' ''Megaloceros giganteus'' |journal=Evolution |volume=28 |issue=2 |pages=191–220 |doi=10.2307/2407322 |pmid=28563271 |jstor=2407322}}</ref> 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.<ref name="Garcia2013">{{cite journal |last1=Garcia |first1=J. E. |last2=Rohr |first2=D. |last3=Dyer |first3=A. G. |title=Trade-off between camouflage and sexual dimorphism revealed by UV digital imaging: the case of Australian Mallee dragons (Ctenophorus fordi) |journal=Journal of Experimental Biology |volume=216 |issue=22 |year=2013 |doi=10.1242/jeb.094045 |pmid=23997198 |pages=4290–4298|doi-access=free }}</ref>

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|brook]]s. Elongated body protects their [[larva]]e 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.<ref>{{cite journal |last=Tarkhnishvili |first=David N. |year=1994 |title=Interdependences between Populational, Developmental and Morphological Features of the Caucasian salamander, ''Mertensiella caucasica'' |url=http://eprints.iliauni.edu.ge/usr/share/eprints3/data/814/1/Caucasian%20Salamander%20Ecological%20Constraints.pdf |journal=Mertensiella |volume=4 |pages=315–325 |access-date=18 August 2015 |archive-url=https://web.archive.org/web/20160304084347/http://eprints.iliauni.edu.ge/usr/share/eprints3/data/814/1/Caucasian%20Salamander%20Ecological%20Constraints.pdf |archive-date=4 March 2016 |url-status=dead }}</ref>

[[File:Pfau imponierend.jpg|thumb|left|An [[Indian peafowl|Indian peacock]]'s train<br />in full display]]

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."<ref>{{harvnb|Darwin|1871|p=[http://darwin-online.org.uk/content/frameset?pageseq=269&itemID=F937.1&viewtype=side 256]}}</ref> 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.<ref>The case was treated by {{harvnb|Fisher|1930|pp=134–139}}</ref> The recognition of sexual selection was for a long time in abeyance, but has been rehabilitated.<ref>{{harvnb|Cronin|1991}}</ref>

The conflict between the size of the human [[Fetus|foetal]] brain at birth, (which cannot be larger than about 400&nbsp;cm<sup>3</sup>, else it will not get through the mother's [[pelvis]]) and the size needed for an adult brain (about 1400 cm<sup>3</sup>), 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. [[Neanderthal]]s had a similar problem.<ref>{{cite journal |last=Rosenberg |first=Karen R. |year=1992 |title=The evolution of modern human childbirth |journal=American Journal of Physical Anthropology |volume=35 |issue=Supplement S15 |pages=89–124 |doi=10.1002/ajpa.1330350605|doi-access= }}</ref><ref>{{cite journal |last1=Friedlander |first1=Nancy J. |last2=Jordan |first2=David K. |author-link2=David K. Jordan |date=October–December 1994 |title=Obstetric implications of Neanderthal robusticity and bone density |journal=Human Evolution  |volume=9 |issue=4 |pages=331–342 |doi=10.1007/BF02435519|s2cid=86590348 }}</ref><ref>{{harvnb|Miller|2007}}</ref>

As another example, the long neck of a giraffe brings benefits but at a cost. The neck of a giraffe can be up to {{convert|2|m|abbr=on}} in length.<ref>{{harvnb|Williams|2010|p=29}}</ref> 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.<ref>{{cite journal |last1=Altwegg |first1=Robert E. |last2=Simmons |first2=Res |title=Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe |date=September 2010 |journal=[[Journal of Zoology]] |volume=282 |issue=1 |pages=6–12 |doi=10.1111/j.1469-7998.2010.00711.x}}</ref>

==Shifts in function ==
{{Blockquote|Adaptation and function are two aspects of one problem.|Julian Huxley|''Evolution: The Modern Synthesis''<ref>{{harvnb|Huxley|1942|p=417}}</ref>}}

===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.<ref>{{cite journal |last=Huskins |first=C. Leonard |author-link=Charles Leonard Huskins |year=1930 |title=The origin of Spartina Townsendii |journal=[[Genetica]] |volume=12 |issue=6 |pages=531–538 |doi=10.1007/BF01487665|s2cid=30321360 }}</ref> Among domestic animals, the [[Leghorn chicken|White Leghorn]] chicken is markedly more resistant to [[Thiamine|vitamin B<sub>1</sub>]] deficiency than other breeds; on a plentiful diet this makes no difference, but on a restricted diet this preadaptation could be decisive.<ref>{{cite journal |last1=Lamoreux |first1=Wilfred F. |last2=Hutt |first2=Frederick B. |date=15 February 1939 |title=Breed differences in resistance to a deficiency in vitamin B<sub>1</sub> in the fowl |url=http://naldc.nal.usda.gov/download/IND43969284/PDF |journal=Journal of Agricultural Research |volume=58 |issue=4 |pages=307–316 |access-date=20 August 2015 |archive-date=23 February 2016 |archive-url=https://web.archive.org/web/20160223003741/http://naldc.nal.usda.gov/download/IND43969284/PDF |url-status=dead }}</ref>

Pre-adaptation may arise because a natural population carries a huge quantity of genetic variability.<ref name="Dobzhansky T 1981">{{harvnb|Dobzhansky|1981}}</ref> 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]].<ref>{{cite book |first=R. C. |last=King |date=2006 |title=A Dictionary of Genetics |edition=7th |page=129 |publisher=Oxford University Press |isbn=978-0-19-530761-0 |url=https://books.google.com/books?id=ykp-7oJ5pREC&pg=PA129 |quote=Dominance [refers] to alleles that fully manifest their phenotype when present in the [[zygosity|heterozygous]] ... state. |display-authors=etal}}</ref> [[Microorganism]]s, 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]]''.<ref name="Luria 1943 491–511">{{cite journal |last1=Luria |first1=S. E. |last2=Delbrück |first2=M. |year=1943 |title=Mutations of Bacteria from Virus Sensitivity to Virus Resistance |journal=[[Genetics (journal)|Genetics]] |volume=28 |issue=6 |pages=491–511 |doi=10.1093/genetics/28.6.491 |pmid=17247100 |pmc=1209226 |url=https://www.genetics.org/cgi/reprint/28/6/491 |access-date=19 December 2019 |archive-date=30 June 2024 |archive-url=https://web.archive.org/web/20240630095104/https://academic.oup.com/genetics/article/28/6/491/6033179 |url-status=live }}</ref> The word is controversial because it is [[Teleology|teleological]] and the entire concept of natural selection depends on the presence of genetic variation, regardless of the population size of a species in question.

===Co-option of existing traits: exaptation ===
[[File:Sinosauropteryxfossil.jpg|thumb|upright=1.2|The feathers of dinosaur ''[[Sinosauropteryx]]'', were used for insulation or display, making them an [[exaptation]] for flight.]]
{{main|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 [[jaw]]s and the [[hyoid bone]] of their [[synapsid]] ancestors, and further back still were part of the [[Branchial arch|gill arch]]es of early fish.<ref>{{harvnb|Allin|Hopson|1992|pp=587–614}}</ref><ref>{{harvnb|Panchen|1992|loc=chpt. 4, "Homology and the evidence for evolution"}}</ref> The word ''exaptation'' was coined to cover these common evolutionary shifts in function.<ref>{{cite journal |last1=Gould |first1=Stephen Jay |author1-link=Stephen Jay Gould |last2=Vrba |first2=Elizabeth S. |author-link2=Elisabeth Vrba |date=Winter 1982 |title=Exaptation–A Missing Term in the Science of Form |journal=[[Paleobiology (journal)|Paleobiology]] |volume=8 |issue=1 |pages=4–15 |jstor=2400563|doi=10.1017/S0094837300004310 |bibcode=1982Pbio....8....4G |s2cid=86436132 }}</ref> The flight [[feather]]s of birds evolved from the much earlier [[Feathered dinosaur#Non-avian dinosaur species preserved with evidence of feathers|feathers of dinosaur]]s,<ref name="Ornithoscelida">{{cite journal |last1=Baron |first1=M.G. |last2=Norman |first2=D.B. |last3=Barrett |first3=P.M. |year=2017 |title=A new hypothesis of dinosaur relationships and early dinosaur evolution |journal=Nature |volume=543 |issue=7646 |pages=501–506 |doi=10.1038/nature21700 |pmid=28332513 |bibcode=2017Natur.543..501B |s2cid=205254710 |url=http://eprints.esc.cam.ac.uk/3857/2/Supplementary%20Information%20_%20MGB%20_%20DBN%20_%20PMB.pdf |access-date=11 February 2019 |archive-date=26 April 2021 |archive-url=https://web.archive.org/web/20210426223325/http://eprints.esc.cam.ac.uk/3857/2/Supplementary%20Information%20_%20MGB%20_%20DBN%20_%20PMB.pdf |url-status=live }}</ref> which might have been used for insulation or for display.<ref name="Dimond et al">{{Cite journal|author1=Dimond, C. C. |author2=Cabin, R. J. |author3=Brooks, J. S. |journal=BIOS |title=Feathers, Dinosaurs, and Behavioral Cues: Defining the Visual Display Hypothesis for the Adaptive Function of Feathers in Non-Avian Theropods |volume=82|year=2011 |pages=58–63 |doi=10.1893/011.082.0302 |issue=3|s2cid=98221211 }}</ref><ref>{{Cite journal |author1=Sumida, S. S. |author2=C. A. Brochu |journal=American Zoologist|title=Phylogenetic Context for the Origin of Feathers |volume=40 |issue=4 |year=2000 |pages=485–503 |doi=10.1093/icb/40.4.486 |doi-access=free }}</ref>

== Niche construction ==

Animals including [[earthworm]]s, [[beaver]]s 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.<ref>{{Cite web |last1=Odling-Smee |first1=John |last2=Laland |first2=Kevin |title=Niche Construction and Evolution |url=https://synergy.st-andrews.ac.uk/niche/niche-construction-and-evolution/ |publisher=University of St Andrews |access-date=17 October 2019 |archive-date=8 August 2019 |archive-url=https://web.archive.org/web/20190808041055/https://synergy.st-andrews.ac.uk/niche/niche-construction-and-evolution/ |url-status=live }}</ref>

==Non-adaptive traits ==
{{main|Spandrel (biology)|Vestigiality}}

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.<ref name=Wagner2014>[[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</ref>

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]].<ref>{{harvnb|Barrett|Gautrey|Herbert|Kohn|1987}}. Charles Darwin was the first to put forward such ideas.</ref>

==Extinction and coextinction ==
{{Main|Extinction|Coextinction}}

If a population cannot move or change sufficiently to preserve its long-term viability, then 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.<ref>{{cite journal |last=Van Valen |first=Leigh |author-link=Leigh Van Valen |date=July 1973 |title=A New Evolutionary Law |url=https://dl.dropboxusercontent.com/u/18310184/evolutionary-theory/vol-01/Vol.1%2CNo.1%2C1-30%2CL.%20Van%20Valen%2C%20A%20new%20evolutionary%20law..pdf |journal=Evolutionary Theory |volume=1 |pages=1–30 |access-date=22 August 2015 |archive-url=https://web.archive.org/web/20141222094258/https://dl.dropboxusercontent.com/u/18310184/evolutionary-theory/vol-01/Vol.1%2CNo.1%2C1-30%2CL.%20Van%20Valen%2C%20A%20new%20evolutionary%20law..pdf |archive-date=22 December 2014 |url-status=dead }}</ref>

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.<ref name="Koh">{{cite journal |last1=Koh |first1=Lian Pin |last2=Dunn |first2=Robert R. |author-link2=Robert Dunn (biologist) |last3=Sodhi |first3=Navjot S. |last4=Colwell |first4=Robert K. |last5=Proctor |first5=Heather C. |last6=Smith |first6=Vincent S. |s2cid=30713492 |display-authors=3 |title=Species Coextinctions and the Biodiversity Crisis |date=September 2004 |journal=Science |volume=305 |issue=5690 |pages=1632–1634  |doi=10.1126/science.1101101 |pmid=15361627|bibcode=2004Sci...305.1632K }}</ref><ref>{{harvnb|Darwin|1872|pp=[http://darwin-online.org.uk/content/frameset?pageseq=84&itemID=F391&viewtype=side 57–58]}}. Darwin in tells the story of "a web of complex relations" involving heartsease (''[[Viola tricolor]]''), red clover (''[[Trifolium pratense]]''), [[bumblebee]]s, mice and cats.</ref>

==Origin of adaptive capacities==

The first stage in the evolution of life on earth is often hypothesized to be the [[RNA world]] in which short self-replicating [[RNA]] molecules proliferated before the evolution of [[DNA]] and [[protein]]s. By this hypothesis, [[biogenesis|life started]] when RNA chains began to self-replicate, initiating the three mechanisms of Darwinian selection: heritability, variation of type, and competition for resources. The fitness of an RNA replicator (its per capita rate of increase) would likely have been a function of its intrinsic adaptive capacities, determined by its [[nucleic acid sequence|nucleotide sequence]], and the availability of resources.<ref name = Bernstein1983>Bernstein, H., Byerly, H.C., Hopf, F.A., Michod, R.E., and Vemulapalli, G.K. (1983) The Darwinian dynamic. Q. Rev. Biol. 58(2), 185–207. {{doi|10.1086/413216}}. {{JSTOR|2828805}}. {{S2CID|83956410}}</ref><ref name = Michod1999>Michod, R. E. 1999. Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality. Princeton University Press, Princeton, NJ. {{ISBN|978-0-691-02699-2}}. {{LCCN|98004166}}. {{OCLC|38948118}}</ref> The three primary adaptive capacities may have been: (1) replication with moderate fidelity, giving rise to heritability while allowing variation of type, (2) resistance to decay, and (3) acquisition of resources.<ref name = Bernstein1983/><ref name = Michod1999/> These adaptive capacities would have been determined by the folded configurations of the RNA replicators resulting from their nucleotide sequences.

==Philosophical issues==
[[File:Springbok pronk.jpg|thumb|"Behaviour with a purpose": a young [[springbok]] [[stotting]]. A biologist might argue that this has the [[function (biology)|function]] of [[signalling theory|signalling to predators]], helping the springbok to survive and allowing it to reproduce.<ref name=Caro>{{cite journal |title=The functions of stotting in Thomson's gazelles: Some tests of the predictions |author=Caro, TM |author-link=Tim Caro |journal=Animal Behaviour |year=1986 |volume=34 |issue=3 |pages=663–684 |doi=10.1016/S0003-3472(86)80052-5|s2cid=53155678 }}</ref><ref name=Stanford>{{cite web |title=Teleological Notions in Biology |url=http://plato.stanford.edu/entries/teleology-biology/ |website=Stanford Encyclopedia of Philosophy |access-date=28 July 2016 |date=18 May 2003 |archive-date=13 March 2020 |archive-url=https://web.archive.org/web/20200313041857/https://plato.stanford.edu/entries/teleology-biology/ |url-status=live }}</ref>]]

{{main|Adaptationism|Teleology in biology}}

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.<ref name="Sober1">{{harvnb|Sober|1993|pp=85–86}}</ref><ref>{{harvnb|Williams|1966|pp=8–10}}</ref> [[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.<ref>{{cite journal |last=Nagel |first=Ernest |author-link=Ernest Nagel |date=May 1977 |title=Goal-Directed Processes in Biology |journal=[[The Journal of Philosophy]] |volume=74 |issue=5 |pages=261–279 |doi=10.2307/2025745 |jstor=2025745}} Teleology Revisisted: The Dewy Lectures 1977 (first lecture)</ref><ref>{{cite journal |last=Nagel |first=Ernest |author-link=Ernest Nagel |date=May 1977 |title=Functional Explanations in Biology |journal=The Journal of Philosophy |volume=74 |issue=5 |pages=280–301 |doi=10.2307/2025746 |jstor=2025746}} Teleology Revisisted: The Dewy Lectures 1977 (second lecture)</ref> Modern biologists continue to face the same difficulty.<ref>{{harvnb|Pittendrigh|1958}}</ref><ref>{{harvnb|Mayr|1965|pp=33–50}}</ref><ref>{{harvnb|Mayr|1988|loc=ch. 3, "The Multiple Meanings of Teleological"}}</ref><ref>{{harvnb|Williams|1966|loc="The Scientific Study of Adaptation"}}</ref><ref>{{harvnb|Monod|1971}}</ref> On the one hand, adaptation is 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 (philosopher)|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."<ref>{{harvnb|Hull|1982}}</ref> [[Ernst Mayr]] stated that "adaptedness... is an [[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.<ref>[[Ernst Mayr|Mayr, Ernst W.]] (1992). "The idea of teleology" ''Journal of the History of Ideas'', 53, 117–135.</ref>

==See also==
{{Div col|colwidth=30em}}
* [[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]]
{{div col end}}

==References==
{{Reflist|30em}}

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{{Refend}}

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