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All individuals have hereditary material in the form of [[gene]]s received from their parents, which they pass on to any offspring. Among offspring there are variations of genes due to the introduction of new genes via random changes called mutations or via reshuffling of existing genes during [[sexual reproduction]].<ref name="Gould">{{harvnb|Gould|2002|}}</ref><ref name="Gregory09">{{cite journal |last=Gregory |first=T. Ryan |author-link=T. Ryan Gregory |date=June 2009 |title=Understanding Natural Selection: Essential Concepts and Common Misconceptions |journal=Evolution: Education and Outreach |volume=2 |issue=2 |pages=156–175 |doi=10.1007/s12052-009-0128-1|doi-access=free }}</ref> The offspring differs from the parent in minor random ways. If those differences are helpful, the offspring is more likely to survive and reproduce. This means that more offspring in the next generation will have that helpful difference and individuals will not have equal chances of [[reproduction|reproductive]] success. In this way, traits that result in organisms being better [[adaptation|adapted]] to their living conditions become more common in descendant populations.<ref name="Gould" /><ref name="Gregory09" /> These differences accumulate resulting in changes within the population. This process is responsible for the many diverse life forms in the world.
The modern understanding of evolution began with the 1859 publication of [[Charles Darwin]]'s ''[[On the Origin of Species]]''. In addition, [[Gregor Mendel]]'s work with plants, between 1856 and 1863, helped to explain the hereditary patterns of [[genetics]].<ref name="gegme">{{cite web |url=http://www.accessexcellence.org/RC/AB/BC/Gregor_Mendel.php |title=Gregor Mendel (1822–1884) |last=Rhee |first=Seung Yon |website=Access Excellence |publisher=National Health Museum |___location=Atlanta, GA |access-date=2015-01-07 |url-status=dead |archive-url=https://web.archive.org/web/20141227004122/http://www.accessexcellence.org/RC/AB/BC/Gregor_Mendel.php |archive-date=2014-12-27}}</ref> Fossil discoveries in [[palaeontology]], advances in [[population genetics]] and a global network of scientific research have provided further details into the mechanisms of evolution. Scientists now have a good understanding of the origin of new species ([[speciation]]) and have observed the speciation process in the laboratory and in the wild. Evolution is the principal [[scientific theory]] that [[biologist]]s use to understand life and is used in many disciplines, including [[medicine]], [[psychology]], [[conservation biology]], [[anthropology]], [[Forensic science|forensics]], [[agriculture]] and other [[Sociocultural evolution|social-cultural]] applications.
== Simple overview ==
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{{main|Natural selection}}
In the 19th century, [[natural history]] collections and museums were popular. The European expansion and naval expeditions employed [[Natural history|naturalists]], while [[curator]]s of grand museums showcased preserved and live specimens of the varieties of life. Charles Darwin was an English graduate, educated and trained in the disciplines of natural history. Such natural historians would collect, catalogue, describe and study the vast collections of specimens stored and managed by curators at these museums. Darwin served as a ship's naturalist on board [[HMS Beagle|HMS ''Beagle'']], assigned to a five-year research expedition around the world. During his voyage, he observed and collected an abundance of organisms, being very interested in the diverse forms of life along the coasts of South America and the neighbouring [[Galápagos Islands]].<ref>{{harvnb|Farber|2000|p=136}}</ref><ref>{{harvnb|Darwin|2005}}</ref>
[[File:Haeckel Orchidae.jpg|thumb|left|upright|Darwin noted that [[Orchidaceae|orchids]] have complex [[adaptation]]s to ensure pollination, all derived from basic floral parts.]]
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Darwin was still researching and experimenting with his ideas on natural selection when he received a letter from [[Alfred Russel Wallace]] describing a theory very similar to his own. This led to an immediate joint publication of both theories. Both Wallace and Darwin saw the history of life like a [[family tree]], with each fork in the tree's limbs being a common ancestor. The tips of the limbs represented modern species and the branches represented the common ancestors that are shared amongst many different species. To explain these relationships, Darwin said that all living things were related, and this meant that all life must be descended from a few forms, or even from a single common ancestor. He called this process ''descent with modification''.<ref name="wyhe">{{cite web |url=http://darwin-online.org.uk/darwin.html |title=Charles Darwin: gentleman naturalist |last=van Wyhe |first=John |author-link=John van Wyhe |year=2002 |work=[[The Complete Works of Charles Darwin Online]] |oclc=74272908 |access-date=2008-01-16}}</ref>
Darwin published his theory of evolution by natural selection in ''On the Origin of Species'' in 1859.{{sfn|Darwin|1859}} His theory means that all life, including [[human]]ity, is a product of continuing natural processes. The implication that all life on Earth has a common ancestor has been met with [[Objections to evolution|objections]] from some [[Creation–evolution controversy|religious groups]]. Their objections are in contrast to the level of support for the theory by [[level of support for evolution|more than 99 percent]] of those within the [[scientific community]] today.<ref name="delgado">{{cite journal |last=Delgado |first=Cynthia |date=July 28, 2006 |title=Finding the Evolution in Medicine |url=http://nihrecord.nih.gov/newsletters/2006/07_28_2006/story03.htm |journal=NIH Record |volume=58 |issue=15 |pages=1, 8–9 |access-date=January 9, 2015 |archive-date=September 24, 2015 |archive-url=https://web.archive.org/web/20150924003836/https://nihrecord.nih.gov/newsletters/2006/07_28_2006/story03.htm |url-status=dead }}</ref>
Natural selection is commonly equated with ''survival of the fittest'', but this expression originated in [[Herbert Spencer]]'s ''Principles of Biology'' in 1864, five years after Charles Darwin published his original works. ''Survival of the fittest'' describes the process of natural selection incorrectly, because natural selection is not only about survival and it is not always the fittest that survives.<ref>{{harvnb|Futuyma|2005b|pp=93–98}}</ref>{{Clear}}
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== Source of variation ==
Darwin's theory of natural selection laid the groundwork for modern evolutionary theory, and his experiments and observations showed that the organisms in populations varied from each other, that some of these variations were inherited, and that these differences could be acted on by natural selection. However, he could not explain the source of these variations. Like many of his predecessors, Darwin mistakenly thought that heritable traits were a product of use and disuse, and that features acquired during an organism's lifetime could be passed on to its offspring. He looked for examples, such as large ground feeding [[bird]]s getting stronger legs through exercise, and weaker wings from not flying until, like the [[ostrich]], they could not fly at all.<ref name=toos>{{harvnb|Darwin|1872|p=[http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=136 108]}}, "Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection"</ref> This misunderstanding was called the [[Inheritance of acquired characteristics|inheritance of acquired characters]] and was part of the theory of [[transmutation of species]] put forward in 1809 by [[Jean-Baptiste Lamarck]]. In the late 19th century this theory became known as [[Lamarckism]]. Darwin produced an unsuccessful theory he called [[pangenesis]] to try to explain how acquired characteristics could be inherited. In the 1880s [[August Weismann]]'s experiments indicated that changes from use and disuse could not be inherited, and Lamarckism gradually fell from favour.<ref name="ImaginaryLamarck">{{cite journal |last=Ghiselin |first=Michael T. |author-link=Michael Ghiselin |date=September–October 1994 |title=The Imaginary Lamarck: A Look at Bogus 'History' in Schoolbooks |url=http://www.textbookleague.org/54marck.htm |journal=The Textbook Letter |___location=Sausalito, CA |publisher=The Textbook League |oclc=23228649 |access-date=2008-01-23 |archive-url=https://web.archive.org/web/20080212174536/http://www.textbookleague.org/54marck.htm |archive-date=2008-02-12 |url-status=
The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of [[Gregor Mendel]]. Mendel's experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilisation. This is like mixing different hands of [[playing cards]], with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other. Mendel called the information ''factors''; however, they later became known as genes. Genes are the basic units of heredity in living organisms. They contain the information that directs the physical development and behaviour of organisms.
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Genes are made of [[DNA]]. DNA is a long [[molecule]] made up of individual molecules called [[nucleotide]]s. Genetic information is encoded in the sequence of nucleotides, that make up the DNA, just as the sequence of the letters in words carries information on a page. The genes are like short instructions built up of the "letters" of the DNA alphabet. Put together, the entire set of these genes gives enough information to serve as an "instruction manual" of how to build and run an organism. The instructions spelled out by this DNA alphabet can be changed, however, by mutations, and this may alter the instructions carried within the genes. Within the [[Cell (biology)|cell]], the genes are carried in [[chromosome]]s, which are packages for carrying the DNA. It is the reshuffling of the chromosomes that results in unique combinations of genes in offspring. Since genes interact with one another during the development of an organism, novel combinations of genes produced by sexual reproduction can increase the genetic variability of the population even without new mutations.<ref>{{cite web |url=http://evolution.berkeley.edu/evolibrary/article/evo_22 |title=Sex and genetic shuffling |website=Understanding Evolution |publisher=University of California, Berkeley |access-date=2015-01-08}}</ref> The genetic variability of a population can also increase when members of that population [[hybrid (biology)|interbreed]] with individuals from a different population causing [[gene flow]] between the populations. This can introduce genes into a population that were not present before.<ref>{{cite web |url=http://evolution.berkeley.edu/evolibrary/article/evo_21 |title=Gene flow |website=Understanding Evolution |publisher=University of California, Berkeley |access-date=2015-01-08}}</ref>
Evolution is not a random process. Although mutations in DNA are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. Evolution is an inevitable result of imperfectly copying, self-replicating organisms reproducing over billions of years under the selective pressure of the environment. The outcome of evolution is not a perfectly designed organism. The end products of natural selection are organisms that are adapted to their present environments. Natural selection does not involve [[orthogenesis|progress towards an ultimate goal]]. Evolution does not strive for [[Evolution of biological complexity|more advanced]], more intelligent, or more sophisticated life forms.<ref>{{harvnb|Gould|1980|p=24}}</ref> For example, [[flea]]s (wingless parasites) are descended from a winged, ancestral [[Mecoptera|scorpionfly]], and [[snake]]s are [[lizard]]s that no longer require limbs—although [[Python (genus)|pythons]] still grow tiny structures that are the remains of their ancestor's hind legs.<ref>{{cite journal |last1=Bejder |first1=Lars |last2=Hall |first2=Brian K. |author-link2=Brian K. Hall |date=November 2002 |title=Limbs in whales and limblessness in other vertebrates: mechanisms of evolutionary and developmental transformation and loss |journal=[[Evolution & Development]] |volume=4 |issue=6 |pages=445–458 |doi=10.1046/j.1525-142X.2002.02033.x |pmid=12492145 |s2cid=8448387 }}</ref><ref>{{cite journal |last1=Boughner |first1=Julia C. |last2=Buchtová |first2=Marcela |last3=Fu |first3=Katherine |last4=Diewert |first4=Virginia |last5=Hallgrímsson |first5=Benedikt |last6=Richman |first6=Joy M. |date=June 25, 2007 |title=Embryonic development of ''Python sebae'' – I: Staging criteria and macroscopic skeletal morphogenesis of the head and limbs |journal=Zoology |volume=110 |issue=3 |pages=212–230 |doi=10.1016/j.zool.2007.01.005 |pmid=17499493 |bibcode=2007Zool..110..212B |display-authors=3 }}</ref> Organisms are merely the outcome of variations that succeed or fail, dependent upon the environmental conditions at the time.
Rapid environmental changes typically cause extinctions.<ref name="PBS_Evolution_library">{{cite web |url=https://www.pbs.org/wgbh/evolution/library/faq/cat03.html |title=Frequently Asked Questions About Evolution |year=2001 |website=Evolution Library |series=[[Evolution (TV series)|Evolution]] |publisher=[[WGBH Educational Foundation]]; Clear Blue Sky Productions, Inc. |___location=Boston, MA |type=Web resource |oclc=48165595 |access-date=2008-01-23}}</ref> Of all species that have existed on Earth, 99.9 percent are now extinct.<ref name="PBS_Evolution_extintion">{{cite web |url=https://www.pbs.org/wgbh/evolution/extinction/massext/index.html |title=A Modern Mass Extinction? |year=2001 |website=Evolution Library |series=Evolution |publisher=WGBH Educational Foundation; Clear Blue Sky Productions, Inc. |___location=Boston, MA |type=Web resource |oclc=48165595 |access-date=2008-01-23}}</ref> Since life began on Earth, five major mass extinctions have led to large and sudden drops in the variety of species. The most recent, the [[Cretaceous–Paleogene extinction event]], occurred 66 million years ago.<ref>{{cite journal |last1=Bambach |first1=Richard K. |last2=Knoll |first2=Andrew H. |author-link2=Andrew H. Knoll |last3=Wang |first3=Steve C. |date=December 2004 |title=Origination, extinction, and mass depletions of marine diversity |url=http://biology.kenyon.edu/courses/biol241/extinction%20and%20marine%20diversity%20banbach%202004.pdf |journal=[[Paleobiology (journal)|Paleobiology]] |volume=30 |issue=4 |pages=522–542 |doi=10.1666/0094-8373(2004)030<0522:OEAMDO>2.0.CO;2|bibcode=2004Pbio...30..522B |s2cid=17279135 }}</ref>
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== Genetic drift ==
{{Further|Genetic drift}}
Genetic drift is a cause of allelic frequency change within populations of a species. [[Allele]]s are different variations of specific genes. They determine things like [[Human hair color|hair colour]], [[Human skin color|skin tone]], [[eye colour]] and [[blood type]]; in other words, all the genetic traits that vary between individuals. Genetic drift does not introduce new alleles to a population, but it can reduce variation within a population by removing an allele from the gene pool. Genetic drift is caused by random sampling of alleles. A truly random sample is a sample in which no outside forces affect what is selected. It is like pulling marbles of the same size and weight but of different colours from a brown paper bag. In any offspring, the alleles present are samples of the previous
Genetic drift affects smaller populations more than it affects larger populations.<ref name="Ellstrand1993">{{cite journal |last1=Ellstrand |first1=Norman C. |last2=Elam |first2=Diane R. |date=November 1993 |title=Population Genetic Consequences of Small Population Size: Implications for Plant Conservation |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=24 |issue=1 |pages=217–242 |doi=10.1146/annurev.es.24.110193.001245|bibcode=1993AnRES..24..217E }}</ref>
=== Hardy–Weinberg principle ===
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=== Population bottleneck ===
[[File:Population bottleneck.jpg|thumb|left|250px|Model of population bottleneck illustrates how [[allele]]s can be lost.]]
A [[population bottleneck]] occurs when the population of a species is reduced drastically over a short period of time due to external forces.<ref>{{cite encyclopedia |editor-last=Robinson |editor-first=Richard |encyclopedia=Genetics |title=Population Bottleneck |url=https://archive.org/details/genetics0000unse |access-date=2011-04-07 |year=2003 |publisher=[[Gale (publisher)|Macmillan Reference US]] |___location=New York |isbn=978-0-02-865606-9 |lccn=2002003560 |oclc=49278650 |url-access=registration }}</ref> In a true population bottleneck, the reduction does not favour any combination of alleles; it is totally random chance which individuals survive. A bottleneck can reduce or eliminate genetic variation from a population. Further drift events after the bottleneck event can also reduce the population's [[genetic diversity]]. The lack of diversity created can make the population at risk to other selective pressures.<ref>{{harvnb|Futuyma|1998|pp=303–304}}</ref>
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* {{cite book |last1=Carroll |first1=Sean B. |author-link1=Sean B. Carroll |last2=Grenier |first2=Jennifer K. |last3=Weatherbee |first3=Scott D. |year=2005 |title=From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design |edition=2nd |___location=Malden, MA |publisher=[[Wiley-Blackwell|Blackwell Publishing]] |isbn=978-1-4051-1950-4 |lccn=2003027991 |oclc=53972564 }}
* {{cite book |last=Coyne |first=Jerry A. |author-link=Jerry A. Coyne |year=2009 |title=Why Evolution is True |___location=New York |publisher=[[Viking Press|Viking]] |isbn=978-0-670-02053-9 |lccn=2008033973 |oclc=233549529 |url=https://archive.org/details/whyevolutionistr00coyn }}
* {{cite book |last=Dalrymple |first=G. Brent |author-link=Brent Dalrymple |year=2001 |chapter=The age of the Earth in the twentieth century: a problem (mostly) solved |editor1-last=Lewis |editor1-first=C. L. E. |editor2-last=Knell |editor2-first=S. J. |title=The Age of the Earth: from 4004 BC to AD 2002
* {{cite book |last=Darwin |first=Charles |author-link=Charles Darwin |year=1859 |title=On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1 |edition=1st |___location=London |publisher=[[John Murray (publishing house)|John Murray]] |lccn=06017473 |oclc=741260650 }}
* {{cite book |last=Darwin |first=Charles |year=1872 |title=The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |url=http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=1 |edition=6th |___location=London |publisher=John Murray |lccn=13003393 |oclc=228738610 }}
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* {{cite book |last=Mayr |first=Ernst |author-link=Ernst Mayr |year=1970 |title=Populations, Species, and Evolution: An Abridgment of Animal Species and Evolution |___location=Cambridge, MA |publisher=Belknap Press of Harvard University Press |isbn=978-0-674-69010-3 |lccn=79111486 |oclc=114063 |url=https://archive.org/details/populationsspeci00mayr }}
* {{cite book |last=Mayr |first=Ernst |year=2001 |title=What Evolution Is |url=https://archive.org/details/whatevolutionis0000mayr |url-access=registration |___location=New York |publisher=Basic Books |isbn=978-0-465-04426-9 |lccn=2001036562 |oclc=47443814 }}
* {{cite book |author=National Academy of Sciences |author-link=National Academy of Sciences |year=1998 |title=Teaching About Evolution and the Nature of Science |url=http://books.nap.edu/openbook.php?isbn=0309063647 |___location=Washington, D.C. |publisher=[[National Academies Press]] |isbn=978-0-309-06364-7 |lccn=98016100 |oclc=245727856 |access-date=2015-01-10 |ref=NAS 1998 }}
* {{cite book |author1=National Academy of Sciences |author2=Institute of Medicine |author-link2=Institute of Medicine |year=2008 |title=Science, Evolution, and Creationism |url=https://archive.org/details/isbn_9780309105866 |___location=Washington, D.C. |publisher=National Academies Press |isbn=978-0-309-10586-6 |lccn=2007015904 |oclc=123539346 |access-date=2014-07-31 |ref=NAS 2008 }}
* {{cite book |last1=Raven |first1=Peter H. |author-link1=Peter H. Raven |last2=Johnson |first2=George B. |author-link2=George B. Johnson |year=2002 |title=Biology |url=https://archive.org/details/biologyrave00rave |url-access=registration |edition=6th |___location=Boston, MA |publisher=[[McGraw-Hill Education|McGraw-Hill]] |isbn=978-0-07-112261-0 |lccn=2001030052 |oclc=45806501 }}
* {{cite book |last=Tattersall |first=Ian |author-link=Ian Tattersall |year=1995 |title=The Fossil Trail: How We Know What We Think We Know About Human Evolution |url=https://archive.org/details/fossiltrailhowwe00tatt |url-access=registration |publisher=Oxford University Press |___location=New York |isbn=978-0-19-506101-7 |lccn=94031633 |oclc=30972979 }}
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