View source for Evolution

[[File:Tree of life.svg|thumb|250px|The [[Tree of life (biology)|tree of life]] showing the three domains of life on [[Earth]].]]

'''Evolution''' is a [[scientific theory]] used by [[biologist]]s. It explains how [[living thing]]s change over a long time, and how they have come to be the way they are.<ref name=Mayr>Mayr, Ernst. 2001. ''What evolution is''. Weidenfeld & Nicolson, London. ISBN 0-465-04426-3</ref>

The [[Earth]] has been around for a very long time.<ref>about 4.5 [[billion]] years: see [[Age of the Earth]].</ref><ref name=Age>Dalrymple G. Brent 2004. ''Ancient Earth and ancient skies: the age of the Earth and its cosmic surroundings''. Stanford University Press.</ref> By doing [[research]] on the layers of [[Rock (geology)|rock]], we can find out about its past. That kind of research is called [[historical geology]].

We know that living things have changed over time, because we can see their [[wikt:remains|remains]] in the rocks. These remains are called '[[fossil]]s'. So we know that the animals and plants of today are different from those of long ago. And the further we go back, the more different the fossils are.<ref name=Levin>Levin, Harold L. 2005. ''The Earth through time''. 8th ed, Wiley, N.Y. Chapter 6: Life on Earth: what do fossils reveal?</ref> How has this come about? Evolution has taken place. That evolution has taken place is a [[fact]], because it is overwhelmingly supported by many lines of [[evidence]].<ref name=Fut3>Futuyma, Douglas J. 1997. ''Evolutionary biology'', 3rd ed. Sinauer Associates. ISBN 0-87893-189-9.</ref><ref>{{cite web |url=http://www.guardian.co.uk/science/2005/sep/01/schools.research |title=One side can be wrong |last1=Dawkins |first1=Richard |last2=Coyne |first2=Jerry |date=1 September 2005 |work=guardian.co.uk |accessdate=29 January 2011}}</ref><ref name=Muller>{{cite journal | url= http://www.skepticfiles.org/evolut/100pcnts.htm | first = H. J.  | last = Muller | title = One hundred years without Darwin are enough| journal = School Science and Mathematics | volume = 59 | pages = 304–305 | year = 1959}}
Reprinted in:
{{cite book | title = Evolution versus creationism: the public education controversy | first = Peter (ed.) | last = Zetterberg | publisher = ORYX Press | location = Phoenix AZ | date = 1983 | isbn = 0897740610 }}</ref> At the same time, evolutionary questions are still being actively researched by biologists.

Comparison of [[DNA sequence]]s allows organisms to be grouped by how similar their sequences are. In 2010 an analysis compared sequences to phylogenetic trees, and supported the idea of [[common descent]]. There is now "strong quantitative support, by a formal test",<ref name="theobald">{{Cite journal| author=Theobald, Douglas L. | name=A formal test of the theory of universal common ancestry | date= 2010 | journal=Nature | volume=465 | pages=219–222 | doi=10.1038/nature09014 | issue=7295 | pmid=20463738 | title=A formal test of the theory of universal common ancestry}}</ref> for the unity of life.<ref name = "steel">{{cite journal | last1 = Steel | first1 = Mike | last2 = Penny | first2 = David | title = Origins of life: common ancestry put to the test | journal = Nature | volume = 465 | issue = 7295 | pages = 168–9 | year = 2010 | pmid = 20463725 | doi = 10.1038/465168a }}</ref>

The theory of evolution is the basis of modern [[biology]]. "Nothing in biology makes sense except in the light of evolution".<ref>[[Theodosius Dobzhansky|Dobzhansky, Theodosius]] 1973. Nothing in biology makes sense except in the light of evolution. ''American Biology Teacher'' '''35''', 125-129. [http://www.2think.org/dobzhansky.shtml http://www.2think.org/dobzhansky.shtml]</ref>

== Evidence for evolution ==
The evidence for evolution is given in a number of books.<ref name=Costa/><ref name=Dawkins>Dawkins, Richard 2009. ''The greatest show on Earth: the evidence for evolution''. Transworld, [[Ealing]]. ISBN 978-0-593-06173-2</ref><ref name=Coyne/><ref name=Trial>Futuyma D.J. 1983. ''Science on trial: the case for evolution''. Pantheon Books, New York. ISBN 0-394-52371-7; 2nd ed 1995 Sinauer Associates, Sunderland, Massachusetts. ISBN 0-87893-184-8</ref> Some of this evidence is discussed here.

=== Fossils show that change has occurred ===
The realization that some rocks contain fossils was a landmark in [[natural history]]. There are three parts to this story:
#Realizing that things in rocks which looked [[organic matter|organic]] actually were the altered remains of living things. This was settled in the 16th and 17th centuries by [[Conrad Gessner]], [[Nicolaus Steno]], [[Robert Hooke]] and others.<ref name=mjs>Rudwick M.J.S. 1972. ''The meaning of fossils: episodes in the history of palaeontology''. Chicago University Press.</ref><ref>Whewell, William 1837. ''History of the inductive sciences, from the earliest to the present time''. vol III, Parker, London. Book XVII The palaeotiological sciences. Chapter 1 Descriptive geology, section 2. Early collections and descriptions of fossils, p405.</ref>
#Realizing that many fossils represented [[species]] which do not exist today. It was [[Georges Cuvier]], the [[comparative anatomy|comparative anatomist]], who proved that [[extinction]] occurred, and that different [[strata]] contained different fossils.<ref name=Bowler>Bowler, Peter H. 2003. ''Evolution: the history of an idea''. 3rd ed, University of California Press.</ref><sup>p108</sup>
#Realizing that early fossils were simpler [[organism]]s than later fossils. Also, the later the rocks, the more like the present day are the fossils.<ref>Prothero, Donald R. 2007. ''Evolution: what the fossils say and why it matters''. Columbia University Press, New York. ISBN 978-0-231-13962-5</ref>
{{quotation|The most convincing evidence for the occurrence of evolution is the discovery of extinct organisms in older geological strata... The older the [[strata]] are...the more different the fossil will be from living representatives... that is to be expected if the [[fauna]] and [[flora]] of the earlier strata had gradually evolved into their descendants.|''[[Ernst Mayr]]'' <ref name=Mayr/><sup>p13</sup>}}

==== Evolution of horses ====
[[Image:Hyracotherium Eohippus hharder.jpg|thumb|200px|right|The [[ancestor]]s of our horses lived in forests.]]

The evolution of the [[horse]] family ([[Equidae]]) is a good example of the way that evolution works. The oldest [[fossil]] of a [[horse]] is about 52 [[million]] years old. It was a small animal with five [[toes]] on the front feet and four on the hind feet. At that time, there were more [[forest]]s in the world than today. This horse lived in [[woodland]], eating [[leaves]], [[nut (fruit)|nuts]] and [[fruit]] with its simple teeth. It was only about as big as a [[fox]].<ref name=GG/>

About 30 [[million]] years ago the world started to become cooler and drier. Forests shrank; [[grassland]] expanded, and horses changed. They ate grass, they grew larger, and they ran faster because they had to escape faster [[predators]]. Because grass wears teeth out, horses with longer-lasting teeth had an advantage.

For most of this long period of time, there were a number of horse types ([[genera]]). Now, however, only one genus exists: the modern horse, ''Equus''. It has teeth which grow all its life, [[hoof|hooves]] on single toes, great long legs for running, and the animal is big and strong enough to survive in the open [[plain]].<ref name=GG>Simpson G.G. 1951. ''Horses: the story of the horse family in the modern world and through sixty million years of history''. Oxford, New York. (despite the title, the earliest record of the horse is 52 mya)</ref> Horses lived in western Canada until 12,000 years ago,<ref>Singer, Ben 2005. A brief history of the horse in America. ''Canadian Geographic Magazine''. [http://www.canadiangeographic.ca/Magazine/ma05/indepth/#cnd]</ref> but all horses in North America became extinct about 11,000 years ago. The causes of this extinction are not yet clear. [[Climate change]] and over-[[hunting]] by humans are suggested.

So, scientists can see that changes have happened. They have happened slowly over a long time. How these changes have come about is explained by the theory of evolution.

=== Geographical distribution ===
[[File:Protea cynaroides 3.jpg|thumb|right|200px|''Protea''. The [[Proteaceae]] are a family of [[flowering plant]]s entirely limited to the southern continents.]]

This is a topic which fascinated both [[Charles Darwin]] and [[Alfred Russel Wallace]].<ref name=6th/><ref>Wallace, Alfred Russel 1876. ''The geographical distribution of animals''. 2 vols, Macmillan, London.</ref><ref>Wallace, Alfred Russel 1880. ''Island life''. Macmillan, London.</ref> When new species occur, usually by the splitting of older species, this takes place in one place in the world. Once it is established, a new species may spread to some places and not others.

==== Australasia ====
[[Australasia]] has been separated from other continents for many millions of years. In the main part of the continent, [[Australia]], 83% of mammals, 89% of reptiles, 90% of fish and insects and 93% of [[amphibian]]s are [[endemism|endemic]].<ref name = "Williams">Williams, J. et al. 2001. [http://www.environment.gov.au/soe/2001/publications/theme-reports/biodiversity/pubs/biodiversity.pdf ''Biodiversity, Australia State of the Environment Report 2001'' (Theme Report)], CSIRO Publishing on behalf of the Department of the Environment and Heritage, Canberra. ISBN 0-643-06749-3</ref> Its native mammals are mostly [[marsupial]]s like [[kangaroo]]s, [[bandicoot]]s, and [[quoll]]s.<ref>{{cite book|last1=Menkhorst|first1=Peter|last2=Knight|first2=Frank|title=A field guide to the mammals of Australia|publisher=Oxford Uniersity Press|year=2001|isbn=0-19-550870-X|page=14}}</ref> By contrast, marsupials are today totally absent from Africa and form a small portion of the mammalian fauna of [[South America]], where [[opossum]]s, [[shrew opossum]]s, and the [[Microbiotheria|monito del monte]] occur (see the [[Great American Interchange]]).

The only living representatives of primitive egg-laying mammals ([[monotreme]]s) are the [[echidna]]s and the [[platypus]]. They are only found in Australasia, which includes [[Tasmania]], [[New Guinea]], and [[Kangaroo Island]]. These monotremes are totally absent in the rest of the world.<ref>{{cite book | title=Echidna: Extraordinary egg-laying mammal| edition=| author=Michael Augee, Brett Gooden, and Anne Musser| year=2006| pages=| publisher=CSIRO Publishing| isbn=}}</ref> On the other hand, Australia is missing many groups of [[placental]] mammals that are common on other continents ([[carnivora]], [[artiodactyl]]s, [[shrew]]s, [[squirrel]]s, [[lagomorph]]s), although it does have indigenous [[bat]]s and [[rodent]]s, which arrived later.<ref>Bats arrived 15 million years ago (mya), and rodents 5–10 mya. Egerton L. (ed) 2005. ''Encyclopedia of Australian wildlife''. Reader's Digest ISBN 1-876689-34-X</ref>

The evolutionary story is that placental mammals evolved in [[Eurasia]], and wiped out the marsupials and monotremes wherever they spread. They did not reach Australasia until more recently. That is the simple reason why Australia has most of the world's marsupials and all the world's monotremes.

==== Hawaiian ''Drosophila'' ====
In about {{convert|6500|sqmi|km2|abbr=on}}, the [[Hawaiian Islands]] have the most diverse collection of ''[[Drosophila]]'' flies in the world, living from [[rainforest]]s to [[mountain]] [[meadow]]s. About 800 Hawaiian drosophilid species are known.

[[Genetics|Genetic]] evidence shows that all the native drosophilid species in Hawai{{okina}}i have descended from ''a single ancestral species'' that colonized the islands, about 20 million years ago. The subsequent [[adaptive radiation]] was spurred by a lack of [[competition]] and a wide variety of vacant [[niche]]s. Although it would be possible for a single [[pregnant]] female to colonise an island, it is more likely to have been a group from the same species.<ref>Carson H.L. 1970. Chromosomal tracers of evolution. ''Science'' '''168''', 1414–1418.</ref><ref>Carson H.L. 1983. Chromosomal sequences and interisland colonizations in Hawaiian ''Drosophila''. ''Genetics'' '''103''', 465-482.</ref><ref>Carson H.L. 1992. Inversions in Hawaiian ''Drosophila''. In: Krimbas C.B. & Powell J.R. (eds) ''Drosophila inversion polymorphism''. CRC Press, Boca Raton, FL. 407-439.</ref><ref>Kaneshiro K.Y. Gillespie R.G. and Carson H.L. 1995. In Wagner W.L. & Funk E. (eds) ''Hawaiian biogeography: evolution on a hot spot archipelago''. Smithsonian, Washington D.C. 57-71</ref>

==== Distribution of ''Glossopteris'' ====
[[File:Pangaea Glossopteris.jpg|thumb|right|230px| Current distribution of ''Glossopteris'' on a Permian map showing the connection of the continents. (1. South America 2. Africa 3. Madagascar 4. India 5. Antarctica and 6. Australia)]]

The combination of [[continental drift]] and evolution can explain what is found in the fossil record. ''[[Glossopteris]]'' is an extinct species of [[seed fern]] plants from the [[Permian]] period on the ancient [[supercontinent]] of [[Gondwana]].<ref>Davis, Paul and Kenrick, Paul. 2004. ''Fossil plants''. Smithsonian, Washington D.C. ISBN 1-58834-156-9</ref>

''Glossopteris'' fossils are found in [[Permian]] [[strata]] in southeast South America, southeast Africa, all of [[Madagascar]], northern [[India]], all of Australia, all of New Zealand, and scattered on the southern and northern edges of [[Antarctica]].

During the Permian, these continents were connected as Gondwana. This is known from [[magnetic]] striping in the rocks, other fossil distributions, and glacial scratches pointing away from the temperate climate of the South Pole during the Permian.<ref name=Coyne/><sup>p103</sup><ref>{{Cite episode |title= |episodelink=http://www.history.com/shows/how-the-earth-was-made/episodes/#slide-11 |series=How the Earth was made|serieslink= |credits= Pioneer Productions |network=History channel |station= |airdate= |season=2 |seriesno=21 |number=8 |quote= |language=English}}</ref>

=== Common descent ===
When [[biologist]]s look at living things, they see that animals and plants belong to groups which have something in common. Charles Darwin explained that this followed naturally if "we admit the common parentage of allied forms, together with their modification through variation and natural selection".<ref name=6th>Darwin, Charles 1884. ''The origin of species''. 6th ed, Murray, London.</ref><sup>p402</sup><ref name=Costa>Darwin, Charles; Costa, James T. 2009. ''The Annotated Origin: a facsimile of the first edition of On the Origin of Species annotated by James T. Costa''. Cambridge, Massachusetts, and London: Belknap Press of Harvard University. ISBN 978-0-674-03281-1</ref><sup>p456</sup>

For example, all [[insect]]s are related. They share a basic body plan, whose development is controlled by master regulatory genes.<ref name=Gehring>Gehring, Walter. 1998. ''Master control genes in development and evolution: the [[homeobox]] story''. Yale University Press, New Haven, Connecticut. ISBN 0-300-07409-3</ref> They have six legs; they have hard parts on the outside of the body (an [[exoskeleton]]); they have eyes formed of many separate chambers, and so on. Biologists explain this with evolution. All insects are the [[wikt:descendant|descendant]]s of a group of animals who lived a long time ago. They still keep the basic plan (six legs and so on) but the details change. They look different now because they changed in different ways: this is evolution.<ref>Grimaldi D. and Engel M.S. 2005. ''Evolution of the insects''. Cambridge University Press. ISBN 0-521-82149-5</ref>

It was Darwin who first suggested that all life on Earth had a single origin, and from that beginning "endless forms most beautiful and most wonderful have been, and are being, evolved".<ref name=Costa>Darwin, Charles; Costa, James T. 2009. ''The Annotated Origin: a facsimile of the first edition of On the Origin of Species'' annotated by James T. Costa. Cambridge, Massachusetts, and London, England: Belknap Press of Harvard University. ISBN 978-0-674-03281-1</ref><sup>p490</sup><ref name=6th/> Evidence from [[molecular biology]] in recent years has supported the idea that all life is related by [[common descent]].<ref name=Tree>[http://tolweb.org/Life_on_Earth/1 Tree of Life Web Project] - explore complete phylogenetic tree interactively.</ref>

=== Vestigial structures ===
Strong evidence for common descent comes from [[vestigial]] structures.<ref name=6th/><sup>p397</sup> The useless wings of flightless [[beetle]]s are sealed under fused wing covers. This can be simply explained by their descent from ancestral beetles which had wings that worked.<ref name=Trial/><sup>p49</sup>

Rudimentary body parts, those that are smaller and simpler in structure than corresponding parts in ancestral species, are called [[vestigial organ]]s. Those organs are functional in the ancestral species but are now either nonfunctional or re-adapted to a new function. Examples are the [[Pelvis|pelvic girdles]] of whales, [[halteres]] (hind [[wing]]s) of [[fly|flies]], wings of [[flightless bird]]s, and the [[leaf|leaves]] of some [[xerophyte]]s (''e.g.'' [[cactus]]) and [[parasitic]] [[plant]]s (''e.g.'' [[Cuscuta|dodder]]).

However, vestigial structures may have their original function replaced with another. The [[ear ossicles]] in mammals are former bones of the lower jaw.

:"Rudimentary organs plainly declare their origin and meaning..." (p262). "Rudimentary organs... are the record of a former state of things, and have been retained solely though the powers of inheritance...  far from being a difficulty, as they assuredly do on the old doctrine of creation, might even have been anticipated in accordance with the views here explained" (p402). Charles Darwin.<ref name=6th/>

In 1893, Robert Wiedersheim published a book on human anatomy and its relevance to man's evolutionary history. This book contained a list of 86 human organs that he considered vestigial.<ref>Wiedersheim, Robert 1893. ''The structure of Man: an index to his past history.'' Macmillan, London.</ref> This list included examples such as the [[Vermiform appendix|appendix]] and the 3rd [[molar]] [[teeth]] (wisdom teeth).

The strong grip of a baby is another example.<ref>{{cite book |title=Psychology |edition=5th|author=Peter Gray |year=2007 |page=66 |publisher=Worth |isbn=0-7167-0617-2}}</ref> It is a vestigial reflex, a remnant of the past when pre-human babies clung to their mothers' hair as the mothers swung through the trees. This is borne out by the babies' feet, which curl up when it is sitting down (primate babies grip with the feet as well). All [[primate]]s except modern man have thick body hair to which an infant can cling, unlike modern humans. The grasp reflex allows the mother to escape danger by climbing a tree using both hands and feet.<ref name=Coyne>{{cite book |title=Why evolution is true |author=Jerry Coyne |year=2009 |pages=85–86 |publisher=Penguin Group |isbn=978-0-670-02053}}</ref><ref>{{cite book |title=Archetype: a natural history of the self |author=Anthony Stevens |year=1982 |pages=87 |publisher=Routledge & Kegan Paul |isbn=0-7100-0980-1}}</ref>

Vestigial organs often have some selection against them. The original organs took resources, sometimes huge resources. If they no longer have a function, reducing their size improves fitness. And there is direct evidence of selection. Some cave [[crustacea]] reproduce more successfully with smaller eyes than do those with larger eyes. This may be because the nervous tissue dealing with sight now becomes available to handle other sensory input.<ref name=FutEvol>Futuyma D.J. 2005. ''Evolution''. Sinauer, Sunderland, Massachusetts. Sinauer. ISBN 0-87893-187-2</ref><sup>p310</sup>

=== Embryology ===
From the eighteenth century it was known that embryos of different species were much more similar than the adults. In particular, some parts of embryos reflect their evolutionary past. For example, the embryos of land [[vertebrate]]s develop [[gill]] slits like [[fish]] embryos. Of course, this is only a temporary stage, which gives rise to many structures in the neck of reptiles, birds and mammals. The proto-gill slits are part of a complicated system of development: that is why they persisted.<ref name=Gehring/>

Another example are the embryonic teeth of [[baleen whale]]s.<ref>Baleen whales, such as the [[blue whale]], use a [[comb]]-like structure made of [[keratin]] to filter the [[plankton]] from the water.</ref> They are later lost. The baleen [[filter]] is developed from different [[tissue (biological)|tissue]], called [[keratin]]. Early fossil baleen whales did actually have teeth as well as the baleen.<ref>Fudge D.S., Szewciw L.J. and A.N. Schwalb. 2009. Morphology and development of blue whale baleen: an annotated translation of Tycho Tullberg's classic 1883 paper. ''Aquatic Mammals'' '''35'''(2):226-252.</ref>

A good example is the [[barnacle]]. It took many centuries before natural historians discovered that barnacles were crustacea. Their adults look so unlike other crustacea, but their larvae are very similar to those of other crustacea.<ref>Charles Darwin 1851 and 1854. ''A Monograph of the Sub-class Cirripedia, with figures of all the species''.</ref>

=== Artificial selection ===
[[Image:Big and little dog 1.jpg|thumb|right|220px|This mixed-breed [[Chihuahua (dog)|Chihuahua]] and [[Great Dane]] show the range of dog breed sizes produced by artificial selection.]]
[[Image:Cornselection.jpg|thumb|right|220px|Selective breeding transformed [[teosinte]]'s few fruitcases (left) into modern corn's rows of exposed kernels (right).]]

[[Charles Darwin]] lived in a world where [[animal husbandry]] and [[Domestication|domesticated]] [[crop]]s were vitally important. In both cases farmers selected for breeding individuals with special properties, and prevented the breeding of individuals with less desirable characteristics. The eighteenth and early nineteenth century saw a growth in scientific agriculture, and artificial breeding was part of this.

Darwin discussed artificial selection as a model for [[natural selection]] in the 1859 first edition of his work ''[[On the Origin of Species]]'', in Chapter IV: Natural selection: 
:"Slow though the process of selection may be, if feeble man can do much by his powers of artificial selection, I can see no limit to the amount of change... which may be effected in the long course of time by nature's power of selection".<ref name=Costa/><sup>p109</sup><ref>van Wyhe, John (ed) 2002. ''The complete work of Charles Darwin online''. [http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=124]</ref>

[[File:Secale cereale.jpg|right|thumb|[[Rye]] is a now a crop. Originally it was a mimetic weed of [[wheat]]]]
[[Nikolai Vavilov]] showed that [[rye]], originally a [[weed]], came to be a crop plant by [[wikt:unintentional|unintentional]] selection. Rye is a tougher plant than wheat: it survives in harsher conditions. Having become a crop like the wheat, rye was able to become a crop plant in harsh areas, such as [[hill]]s and [[mountains]].<ref>Vavilov N. 1951. The origin, variation, immunity and breeding of cultivated plants (translated by K. Starr Chester). ''Chronica Botanica'' '''13''':1–366.</ref><ref>Vavilov N. 1992. ''Origin and geography of cultivated plants'' (translated by Doris Love). Cambridge University Press, Cambridge. ISBN 0-521-40427-4</ref>
  
There is no real difference in the genetic processes underlying artificial and natural selection, and the concept of artificial selection was used by Charles Darwin as an illustration of the wider process of natural selection. There are practical differences. Experimental studies of artificial selection show that "the rate of evolution in selection experiments is at least two orders of magnitude (that is 100 times) greater than any rate seen in nature or the fossil record".<ref name=popgen>Gillespie J.H. 2004. ''Population genetics: a concise guide'' 2nd ed, Johns Hopkins University Press, Baltimore. ISBN 0-8018-8009-2</ref><sup>p157</sup>

==== Artificial new species ====
Some have thought that artificial selection could not produce new species. It now seems that it can.

New species have been created by domesticated [[animal husbandry]], but the details are not known or not clear. For example, [[domestic sheep]] were created by hybridisation, and no longer produce viable offspring with ''[[Ovis]] orientalis'', one species from which they are descended.<ref>{{cite journal | doi = 10.1098/rspb.2002.1975 | last1 = Hiendleder | first1 = S | year = 2002 | last2 = Kaupe | first2 = B | last3 = Wassmuth | first3 = R | pmc = 1690972 | last4 = Janke | first4 = A.| title = Molecular analysis of wild and domestic sheep questions current nomenclature and provides evidence for domestication from two different subspecies | url = | journal = Proceedings of the Royal Society B: Biological Sciences | volume = 269 | issue = 1494| pages = 893–904 | pmid=12028771}}</ref> Domestic [[cattle]], on the other hand, can be considered the same species as several varieties of wild [[ox]], [[gaur]], [[yak]], etc., as they readily produce fertile offspring with them.<ref>Nowak R. 1999 ''Walker's Mammals of the World'' 6th ed. Baltimore: Johns Hopkins University Press.</ref>

The best-documented new species came from laboratory experiments in the late 1980s. William Rice and G.W. Salt bred fruit flies, ''[[Drosophila melanogaster]],'' using a [[maze]] with three different choices of [[habitat]] such as light/dark and wet/dry. Each generation was put into the maze, and the groups of flies that came out of two of the eight exits were set apart to breed with each other in their respective groups.

After thirty-five generations, the two groups and their offspring were isolated reproductively because of their strong habitat preferences: they mated only within the areas they preferred, and so did not mate with flies that preferred the other areas.<ref>{{cite journal|author=Rice W.R. and G.W. Salt|title=Speciation via disruptive selection on habitat preference: experimental evidence|journal=The American Naturalist|volume=131|year=1988|pages=911–917 | doi = 10.1086/284831}}</ref><ref>{{cite journal|title=Laboratory experiments on speciation: what have we learned in forty years?|url=http://jstor.org/stable/2410209|author=W.R. Rice and E.E. Hostert|journal=Evolution|volume=47|issue=6|year=1993|pages=1637–1653 | doi = 10.2307/2410209}}</ref>

Diane Dodd was also able to show how [[reproductive isolation]] can develop from  preferences in ''[[Drosophila pseudoobscura]]'' fruit flies after only eight generations using different food types, starch and maltose.<ref>{{cite journal | doi = 10.2307/2409365 | last1 = Dodd | first1 = D.M.B. | year = 1989 | title = Reproductive isolation as a consequence of adaptive divergence in ''Drosophila pseudoobscura'' | url = http://jstor.org/stable/2409365| journal = Evolution | volume = 43 | issue = 6| pages = 1308–1311 }}</ref>

[[File:Drosophila speciation experiment.svg|Drosophila speciation experiment]]

Dodd's experiment has been easy for others to repeat. It has also been done with other fruit flies and foods.<ref>Kirkpatrick M. and V. Ravigné 2002. Speciation by natural and sexual selection: models and experiments ''The American Naturalist'' '''159''':S22–S35 [http://www.journals.uchicago.edu/doi/abs/10.1086/338370 DOI]</ref>

=== Observable changes ===
Some biologists say that evolution has happened when a [[Trait (biology)|trait]] that is caused by [[genetics]] becomes more or less common in a group of organisms.<ref>Stephen Jay Gould, ''[http://www.stephenjaygould.org/library/gould_fact-and-theory.html Evolution as fact and theory]'' from ''Hen's teeth and horse's toes'', New York: Norton 1994, pp253-262.</ref> Others call it evolution when new [[species]] appear.

Changes can happen quickly in the smaller, simpler organisms. For example, many [[bacteria]] that cause disease can no longer be killed with some of the [[antibiotic]] [[medicine]]s. These medicines have only been in use about eighty years, and at first worked extremely well. The bacteria have evolved so that they are no longer affected by antibiotics anymore.<ref>Quammen, David, [http://magma.nationalgeographic.com/ngm/0411/feature1/fulltext.html ''Was Darwin wrong?''], National Geographic, November 2004</ref> The drugs killed off all the bacteria except a few which had some resistance. These few resistant bacteria produced the next generation.

The [[Colorado beetle]] is famous for its ability to resist [[pesticide]]s. Over the last 50 years it has become [[Resistance|resistant]] to 52 [[chemical compound]]s used in [[insecticide]]s, including [[cyanide]].<ref>Plant pests: The biggest threats to food security? BBC News Science & Environment [http://www.bbc.co.uk/news/science-environment-15623490]</ref> This is natural selection speeded up by the artificial conditions. However, not every population is resistant to every chemical.<ref>Alyokhin A. ''et al.'' 2008. Colorado potato beetle resistance to insecticides. ''American Journal of Potato Research'' '''85''': 395–413.</ref> The populations only become resistant to chemicals used in their area.

== History ==
Although there were a number of natural historians in the 18th century who had some idea of evolution, the first well-formed ideas came in the 19th century. Three biologists are most important.

=== Lamarck ===
[[Image:Jean-Baptiste Lamarck.jpg|thumb|left|120px|Lamarck]]
[[Jean-Baptiste de Lamarck]] (1744–1829), a [[France|French]] [[biologist]], claimed that animals changed according to natural laws. He said that animals could pass on [[Trait (biology)|traits]] they had acquired during their lifetime to their offspring, using [[inheritance]]. Today, his theory is known as [[Lamarckism]]. Its main purpose is to explain [[adaptation]]s by natural means.<ref>see, for example, the discussion in Bowler, Peter H. 2003. ''Evolution: the history of an idea''. 3rd ed, California. p86–95, especially "Whatever the true nature of Lamarck's theory, it was his mechanism of adaptation that caught the attention of later naturalists". p90</ref> He proposed a tendency for organisms to become more complex, moving up a ladder of progress, plus ''use and disuse''.

Lamarck's idea was that a giraffe's neck grew longer because it tried to reach higher up. This idea failed because it cannot be reconciled with [[Mendelian inheritance|heredity]] ([[Gregor Mendel|Mendel]]'s work). Mendel made his discoveries about half a century after Lamarck's work.

=== Darwin ===
[[Image:Principe_variation.svg|thumb|150px|Variation]]

[[Charles Darwin]] (1809–1882) wrote his ''[[On the Origin of Species]]'' in 1859. In this book, he put forward much evidence that evolution had occurred. He also proposed natural selection as the way evolution had taken place. But Darwin did not understand about genetics and how traits were actually passed on. He could not accurately explain what made children look like their parents.

Nevertheless, Darwin's explanation of evolution was fundamentally correct. In contrast to Lamarck, Darwin's idea was that the giraffe's neck became longer because ''those with longer necks survived better''.<ref name=6th/><sup>p177/8</sup> These survivors passed their [[genes]] on, and in time the whole race got longer necks.

=== Mendel ===
An [[Austrian]] [[monk]] called [[Gregor Mendel]] (1822–1884) bred plants. In the mid-19th century, he discovered how traits were passed on from one generation to the next.

He used [[pea]]s for his experiments: some peas have white flowers and others have red ones. Some peas have green seeds and others have yellow seeds. Mendel used artificial [[pollination]] to breed the peas. His results are discussed further in [[Mendelian inheritance]]. Darwin thought that the inheritance from both parents blended together. Mendel proved that the genes from the two parents stay separate, and may be passed on unchanged to later generations.

Mendel published his results in a journal that was not well-known, and his discoveries were overlooked. Around 1900, his work was rediscovered.<ref>Stern, Curt and Underwood, Eva R. (eds) 1966. ''The origin of genetics: a Mendel source book''. Freeman, San Francisco.</ref><ref>Carlson, E.A. 1966. ''The gene: a critical history''. Saunders, N.Y.</ref> [[Gene]]s are bits of information made of [[DNA]] which work like a set of instructions. A set of genes are in every living [[cell]]. Together, genes organise the way an egg develops into an adult. With [[mammals]], and many other living things, a copy of each gene comes from the father and another copy from the mother. Some living organisms, including some plants, only have one parent, so get all their genes from them. These genes produce the genetic differences which evolution acts on.

== Darwin's theory ==
Darwin's ''On the Origin of Species'' has two themes: the evidence for evolution, and his ideas on how evolution took place. This section deals with the second issue.

=== Variation ===
[[Image:Famiglia.jpeg|thumb|200px|The members of this family are similar in some ways, different in others.]]
[[Image:Pinguicula moranensis variation.jpg|thumb|200px|''Variation''. The flower on the right has a different colour.]]

The first two chapters of the ''Origin'' deal with variation in domesticated plants and animals, and variation in nature.

All living things show [[Variation (biology)|variation]]. Every [[population]] which has been studied shows that animal and plants vary as much as humans do.<ref name=JMS>Maynard Smith, John 1993. ''The theory of evolution''. Cambridge University Press. p133</ref><ref name=Fut2/><sup>p90</sup> This is a great fact of nature, and without it evolution would not occur. Darwin said that, just as man selects what he wants in his [[farm]] animals, so in nature the variations allow natural selection to work.<ref>A paraphrase of Darwin C.D. 1859. ''On the origin of species''. Chapters 1 and 2, especially p45.</ref>

The features of an individual are influenced by two things, [[heredity]] and [[environment]]. First, development is controlled by genes [[Heredity|inherited]] from the parents. Second, living brings its own influences. Some things are entirely inherited, others partly, and some not inherited at all.

The colour of eyes is entirely inherited; they are a genetic [[Trait (biology)|trait]]. Height or weight is only partly inherited, and the language is not at all inherited. Just to be clear: the fact that humans can speak is inherited, but what language is spoken depends on where a person lives and what they are taught. Another example: a person inherits a brain of somewhat variable capacity. What happens after birth depends on many things such as home environment, education and other experiences. When a person is adult, their brain is what their inheritance and life experience have made it.

''Evolution only concerns the traits which can be inherited'', wholly or partly. The hereditary traits are passed on from one generation to the next through the genes. A person's genes contain all the traits which they inherit from their parents. The accidents of life are not passed on. Also, of course, each person lives a somewhat different life: that increases the differences.

Organisms in any population vary in reproductive success.<ref name=Ridley>Ridley, Mark 1996. ''Evolution''. 2nd ed, Wiley-Blackwell ISBN 0-632-04292-3</ref><sup>p81</sup> From the point of view of evolution, 'reproductive success' means the total number of offspring which live to breed and leave offspring themselves.

==== Inherited variation ====
Variation can only affect future generations if it is inherited. Because of the work of Gregor Mendel, we know that much variation is inherited. Mendel's 'factors' are now called genes. Research has shown that ''almost every individual in a sexually reproducing species is genetically unique''.<ref name=FutEv>Futuyma D.J. 2005. ''Evolution''. Sinauer, Sunderland, Massachusetts. ISBN 0-87893-187-2</ref><sup>p204</sup>

Genetic variation is increased by gene [[mutation]]s. DNA does not always reproduce exactly. Rare changes occur, and these changes can be inherited. Many changes in DNA cause faults; some are neutral or even advantageous. This gives rise to genetic variation, which is the seed-corn of evolution. [[Sexual reproduction]], by the [[crossing over]] of [[chromosome]]s during [[meiosis]], spreads variation through the population. Other events, like natural selection and drift, reduce variation. So a population in the wild always has variation, but the details are always changing.<ref name=Fut2>Futuyma D.J. 1986. ''Evolutionary biology''. Sinauer Associates, Sunderland, Massachusetts. 2nd ed, Sinauer. ISBN 0-87893-188-0</ref><sup>p90</sup>

=== Natural selection ===
{{main|natural selection}}
Evolution mainly works by natural selection. What does this mean? Animals and plants which are best suited to their environment will, on average, survive better. There is a [[struggle for existence]]. Those who survive will produce the next generation. Their genes will be passed on, and the genes of those who did not reproduce will not. This is the basic mechanism which changes a population and causes evolution.

Natural selection explains why living organisms change over time to have the anatomy, the functions and behaviour that they have. It works like this:
# All living things have such fertility that their population size could increase rapidly for ever.
# We see that the size of populations does not increase to this extent. Mostly, numbers remain about the same.
# The food and other resources are limited. Therefore, there is competition for food and resources.
# No two individuals are alike. Therefore, they will not have the same chances to live and reproduce.
# Much of this variation can be inherited. Parents pass such traits to the children through their genes.
# ''The next generation can only come from those that survive and reproduce''. After many generations of this, the population will have more helpful genetic differences, and fewer harmful ones.<ref>[http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_25 Evolution 101: Natural selection] from the ''Understanding evolution'' webpages made by the University of California at Berkeley</ref> ''Natural selection is really a process of elimination''.<ref name=Mayr/><sup>p117</sup> The elimination is being caused by the relative fit between individuals and the environment they live in.

==== Selection in natural populations ====
There are now many cases where natural selection has been proved to occur in wild populations.<ref name=Fut3/><ref name=Endler>Endler J.A. 1986. ''Natural selection in the wild''. Princeton, New Jersey: Princeton University Press. ISBN 0-691-00057-3.</ref><ref name=Ecgen/> Almost every case investigated of [[camouflage]], [[mimicry]] and [[polymorphism]] has shown strong effects of selection.<ref name=Attack>Ruxton G.D; Sherratt T.N. and Speed M.P. 2004. ''Avoiding attack: the evolutionary ecology of crypsis, warning signals and mimicry''. Oxford University Press. ISBN 0-19-852860-4</ref>

The force of selection can be much stronger than was thought by the early population geneticists. The resistance to pesticides has grown quickly. Resistance to ''warfarin'' in Norway [[rat]]s (''Rattus norvegicus'') grew rapidly because those that survived made up more and more of the population. Research showed that, in the absence of ''warfarin'', the resistant homozygote was at a 54% disadvantage to the normal [[wild type]] homozygote.<ref name=Fut2/><sup>p182</sup><ref>Wood R.J. 1981. Insecticide resistance: genes and mechanisms. In Bishop J.A. & Cook L.M. (eds) ''Genetic consequences of man-made change''. Academic Press, London. 97–127</ref> This great disadvantage was quickly overcome by the selection for ''warfarin'' resistance.

Mammals normally cannot drink [[milk]] as adults, but humans are an exception. Milk is digested by the [[enzyme]] [[lactase]], which switches off as mammals stop taking milk from their mothers. The human ability to drink milk during adult life is supported by a lactase mutation which prevents this switch-off. Human populations have a high proportion of this mutation wherever milk is important in the [[diet]]. The spread of this 'milk tolerance' is promoted by natural selection, because it helps people survive where milk is available. Genetic studies suggest that the oldest mutations causing [[lactase persistence]] only reached high levels in human populations in the last ten thousand years.<ref name="CoehloEtal2002">Coelho M. ''et al.'' 2002. Microsatellite variation and evolution of human lactase persistence. ''Human Genetics'' '''117'''(4): 329–339.</ref><ref name="BersaglieriEtal2004">Bersaglieri T. ''et al.'' 2004. Genetic signatures of strong recent positive selection at the lactase gene. ''American Journal of Human Genetics'' '''74'''(6): 1111–20.</ref> Therefore, lactase persistence is often cited as an example of [[Human evolution#Recent and current human evolution|recent human evolution]].<ref name="Wade2006">Wade N. 2006. [http://www.nytimes.com/2006/12/10/science/10cnd-evolve.html Study detects recent instance of human evolution]. ''The New York Times''. December 10, 2006.</ref><ref name="Swaminathan2006">Swaminathan, N. 2006. [http://www.scientificamerican.com/article.cfm?id=african-adaptation-to-dig African adaptation to digesting milk is "strongest signal of selection ever"]. ''Scientific American''.</ref> As lactase persistence is genetic, but animal husbandry a cultural trait, this is gene–[[culture]] [[coevolution]].<ref name="Aoki2001">Aoki K. 2001. Theoretical and empirical aspects of gene–culture coevolution. ''Theoretical Population Biology'' '''59'''(4): 253–261.</ref>

=== Adaptation ===
{{main|adaptation}}
[[Adaptation]] is one of the basic [[phenomenon|phenomena]] of [[biology]].<ref>Williams, George C. 1966. ''Adaptation and natural selection: a critique of some current evolutionary thought''. Princeton. "Evolutionary adaptation is a phenomenon of pervasive importance in biology." p5</ref> Through the process of adaptation, an organism becomes better suited to its [[habitat]].<ref>The ''Oxford Dictionary of Science'' defines ''adaptation'' as "Any change in the structure or functioning of an organism that makes it better suited to its environment".</ref>

Adaptation is one of the two main processes that explain the diverse species we see in biology.  The other is [[speciation]] (species-splitting or [[cladogenesis]]).<ref>{{cite book |last1=Mayr |first= Ernst |title=Animal species and evolution |publisher=Belknap Press of Harvard University Press |location=Cambridge |year=1963 |pages= |isbn=0-674-03750-2 |edition=1st}}</ref><ref>{{cite book |last1=Mayr |first1= Ernst |title=The growth of biological thought: diversity, evolution, and inheritance |publisher=Belknap Press |location=Cambridge, Mass |year=1982 | edition=1st |pages= 562–566|isbn=0-674-36445-7}}</ref> A favourite example used today to study the interplay of adaptation and speciation is the evolution of [[cichlid]] fish in African rivers and lakes.<ref name="Salzburger">{{cite journal | author = Salzburger W., Mack T., Verheyen E., Meyer A.|year = 2005 | title = Out of Tanganyika: genesis, explosive speciation, key-innovations and phylogeography of the haplochromine cichlid fishes | journal = BMC Evolutionary Biology | volume = 5 | number = 17 | url = http://www.biomedcentral.com/content/pdf/1471-2148-5-17.pdf | format = PDF | doi = 10.1186/1471-2148-5-17  | pages = 17 | pmid = 15723698 | issue = 1 | pmc = 554777}}</ref><ref name = "Kornfield">{{cite journal | author = Kornfield, Irv | coauthors = Smith, Peter| url = http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.ecolsys.31.1.163 | title = African Cichlid fishes: model systems for evolutionary biology| journal = Annual Review of Ecology and Systematics | volume = 31 | pages = 163 | month = November | year = 2000 | doi= 10.1146/annurev.ecolsys.31.1.163}}</ref>

When people speak about adaptation they often mean something which helps an animal or plant survive. One of the most widespread adaptations in animals is the [[evolution of the eye]]. Another example is the adaptation of [[horse]]s' [[tooth|teeth]] to grinding [[grass]]. [[Camouflage]] is another adaptation; so is [[mimicry]]. The better adapted animals are the most likely to survive, and to reproduce successfully (natural selection).

An internal [[parasite]] (such as a [[Trematoda|fluke]]) is a good example: it has a very simple bodily structure, but still the organism is highly adapted to its particular environment. From this we see that adaptation is not just a matter of visible [[Trait (biology)|traits]]: in such parasites critical adaptations take place in the [[life cycle]], which is often quite complex.<ref>Price P.W. 1980. ''The evolutionary biology of parasites''. Princeton.</ref>

==== Limitations ====
Not all features of an organism are adaptations.<ref name=Fut2/><sup>p251</sup> Adaptations tend to reflect the past life of a species. If a species has recently changed its life style, a once valuable adaptation may become useless, and eventually become a dwindling [[vestige]].

Adaptations are never perfect. There are always tradeoffs between the various functions and structures in a body. It is the organism as a whole which lives and reproduces, therefore it is the complete set of adaptations which gets passed on to future generations.

=== Genetic drift and its effect ===
{{main|genetic drift}}
[[File:Random sampling genetic drift.gif|thumb|right|250px|'''Click for action''' <br />In this [[simulation]], there is fixation of the blue "allele" in five generations.]]

In populations, there are forces which add variation to the population (such as [[mutation]]), and forces which remove it. [[Genetic drift]] is the name given to random changes which remove variation from a population. Genetic drift gets rid of variation at the rate of 1/(2N) where N = population size.<ref name=popgen/><sup>p29</sup> It is therefore "a very weak evolutionary force in large populations".<ref name=popgen/><sup>p55</sup>
  
Genetic drift explains how random chance can affect evolution in surprisingly big ways, but only when populations are quite small.  Overall, its action is to make the individuals more similar to each other, and hence more vulnerable to disease or to chance events in their environment.

# Drift reduces genetic variation in populations, potentially reducing a population’s ability to survive new selective pressures.
# Genetic drift acts faster and has more drastic results in smaller populations. Small populations usually become extinct.
# Genetic drift may contribute to speciation, if the small group does survive.
# Bottleneck events: when a large population is suddenly and drastically reduced in size by some event, the genetic variety will be very much reduced. [[Infection]]s and extreme [[climate]] events are frequent causes. Occasionally, [[invasions]] by more competitive species can be devastating.<ref>The [[extinction]] of many [[Australian]] [[marsupial]]s by foreign species is a famous example.</ref> <br />♦ In the 1880/90s, hunting reduced the Northern elephant seal to only about 20 individuals. Although the population has rebounded, its genetic variability is much less than that of the Southern elephant seal.<br />♦ [[Cheetah]]s have very little variation. We think the species was reduced to a small number at some recent time. Because it lacks genetic variation, it is in danger from infectious diseases.<ref>O'Brien S. Wildt D. & Bush M. 1986. The Cheetah in genetic peril. ''Scientific American'' '''254''': 68–76. Skin grafts between non-related cheetahs illustrate this point: there is no rejection of the donor skin.</ref>
#Founder events: these occur when a small group buds off from a larger population. The small group then lives separately from the main population. The human species is often quoted as having been through such stages. For example, when groups left Africa to set up elsewhere (see [[human evolution]]). Apparently, we have less variation than would be expected from our worldwide distribution. <br />Groups that arrive on islands far from the mainland are also good examples. These groups, by virtue of their small size, cannot carry the full range of [[allele]]s to be found in the parent population.<ref>[http://evolution.berkeley.edu/evosite/evo101/IIID1Samplingerror.shtml Evolution 101:Sampling error and evolution] and [http://evolution.berkeley.edu/evosite/evo101/IIID2Genesdrift.shtml Effects of genetic drift] from the ''Understanding evolution'' webpages of the University of California at Berkeley</ref><ref>[http://evolution.berkeley.edu/evosite/evo101/VC1cPeripatric.shtml Evolution 101: Peripatric speciation] from the ''Understanding evolution'' webpages of the University of California at Berkeley</ref>

=== Species ===
{{main|speciation}}
How [[species]] form is a major part of [[evolutionary biology]]. Darwin interpreted 'evolution' (a word he did not use at first) as being about speciation. That is why he called his famous book ''[[On the Origin of Species]]''.

[[Charles Darwin|Darwin]] thought most species arose directly from pre-existing species. This is called ''anagenesis'': new species by older species changing. Now we think most species arise by previous species splitting: ''cladogenesis''.<ref>{{cite journal | doi = 10.1126/science.23.587.506 | author = Cook O.F. | year = 1906 | title = Factors of species-formation | url = | journal = Science | volume = 23 | issue = 587| pages = 506–507 | pmid = 17789700 }}</ref><ref>{{cite journal | doi = 10.1086/279001 | author = Cook O.F. | year = 1908 | title = Evolution without isolation | url = | journal = American Naturalist | volume = 42 | issue = | pages = 727–731 }}</ref>

==== Species splitting ====
Two groups that start the same can also become very different if they live in different places. When a [[species]] gets split into two geographical regions, a process starts. Each adapts to its own situation. After a while, individuals from one group can no longer reproduce with the other group. Two good species have evolved from one.

A German explorer, [[Moritz Wagner]], during his three years in [[Algeria]] in the 1830s, studied flightless [[beetles]]. Each species is confined to a stretch of the north coast between rivers which descend from the Atlas mountains to the [[Mediterranean]]. As soon as one crosses a river, a different but closely related species appears.<ref>Wagner M. ''Reisen in der Regentschaft Algier in den Jahren 1836, 1837 & 1838''. Voss, Leipzig. p199-200</ref> He wrote later:
:"... a [new] species will only [arise] when a few individuals [cross] the limiting borders of their range... the formation of a new race will never succeed... without a long continued separation of the colonists from the other members of their species".<ref>Wagner M. 1873. ''The Darwinian theory and the law of the migration of organisms''. Translated by I.L. Laird, London.</ref>
This was an early account of the importance of geographical separation. Another biologist who thought geographical separation was critical was [[Ernst Mayr]].<ref name=Provine>Provine, William B. 2004. [http://www.genetics.org/cgi/content/full/167/3/1041 Genetics and speciation], Genetics '''167''', 1041-1046.</ref>

[[Image:Gasterosteus aculeatus.jpg|thumb|The three-spined stickleback (''Gasterosteus aculeatus'')]]
One example of natural [[speciation]] is the three-spined [[stickleback]], a [[sea]] fish that, after the last ice age, invaded [[freshwater]], and set up colonies in isolated lakes and streams. Over about 10,000 generations, the sticklebacks show great differences, including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.<ref name=sciamstickleback>Kingsley D.M. January 2009. From atoms to traits. ''Scientific American'' p57</ref>

The [[wombat]]s of Australia fall into two main groups, Common wombats and Hairy-nosed wombats. The two types look very similar, apart from the hairiness of their noses. However, they are adapted to different environments. Common wombats live in forested areas and eat mostly green food with lots of moisture. They often feed in the daytime. Hairy-nosed wombats live on hot dry plains where they eat dry grass with very little water or goodness in it. Their metabolic system is slow and they sleep most of the day underground.

When two groups that started the same become different enough, then they become two different [[species]]. Part of the theory of evolution is that all living things started off the same, but then split off into different groups over billions of years.<ref>[http://evolution.berkeley.edu/evosite/evo101/VIADefinition.shtml Evolution 101: Definition: What is Macroevolution?] from the ''Understanding Evolution'' webpages made by the University of California at Berkeley</ref>

== Modern evolutionary synthesis ==
{{main|modern evolutionary synthesis}}

This was an important movement in evolutionary biology, which started in the 1930s and finished in the 1950s.<ref name=JSH>Huxley, Julian S. 1942. ''Evolution: the modern synthesis''. Reprint 2010, The MIT Press. ISBN 0-262-51366-8.</ref><ref name=M&P>Mayr E. and W.B. Provine eds. 1998. ''The evolutionary synthesis: perspectives on the unification of biology''. Harvard University Press. ISBN 0-674-27225-0</ref> It has been updated regularly ever since.
The synthesis explains how the ideas of Charles Darwin fit with the discoveries of Gregor Mendel, who found out how we inherit our genes. The modern synthesis brought Darwin's idea up to date. It bridged the gap between different types of biologists: geneticists, naturalists, and palaeontologists.

When the theory of evolution was developed, it was not clear that natural selection and genetics worked together. But [[Ronald Fisher]] showed that natural selection would work to change species.<ref name=Larson>Larson, Edward J. 2004. ''Evolution: the remarkable history of a scientific theory'' 221-243</ref> [[Sewall Wright]] explained genetic drift in 1931.<ref>Provine, William B. 1986. ''Sewell Wright and evolutionary biology''. University of Chicago Press. ISBN 0-226-68474-1</ref>

*Evolution and genetics: evolution can be explained by what we know about genetics, and what we see of animals and plants living in the wild.<ref name=JSH/><ref name=M&P/>

*Thinking in terms of [[Population genetics|populations]], rather than individuals, is important. The genetic variety existing in natural populations is a key factor in evolution.<ref>Dobzhansky T. 1951. ''Genetics and the Origin of Species''. 3rd ed, Columbia University Press New York.</ref>

*Evolution and fossils: ''the same factors which act today also acted in the past''.<ref>George lord Simpson 1953. ''The major features of evolution''. Columbia University Press, New York.</ref>

*Gradualism: evolution is gradual, and usually takes place by small steps. There are some exceptions to this, notably [[polyploidy]], especially in plants.<ref>Stebbins G. Ledyard 1940. The significance of polyploidy in plant evolution. ''The American Naturalist'' '''74''':54–66</ref><ref name=Steb>Stebbins, G. Ledyard. 1950. ''Variation and evolution in plants''. Columbia University Press, New York.</ref>

*Natural selection: the struggle for existence of animals and plant in the wild causes natural selection. ''The strength of natural selection in the wild was greater than even Darwin expected''.<ref name=Ecgen>Ford E.B. 1964, 4th edn 1975. ''Ecological genetics''. Chapman and Hall, London.</ref>

*Genetic drift can be important in small populations.<ref name=popgen/>

*The rate of evolution can vary. There is very good evidence from fossils that different groups can evolve at different rates, and that different parts of an animal can evolve at different rates.<ref name=Fut2/><sup>p292, 397</sup>

== Some areas of research ==
[[Image:TwoBees.jpg|thumb|right|280px|'''Pollinator constancy''': these two honeybees, active at the same time and place, selectively visit flowers from only one species, as can be seen by the colour of the pollen in their baskets]]

=== Co-evolution ===
{{main|co-evolution}}
Co-evolution is where the existence of one species is tightly bound up with the life of one or more other species.

New or 'improved' adaptations which occur in one species are often followed by the appearance and spread of related features in the other species.

These relationships may continue for millions of years, as it has in the [[pollination]] of flowering plants by insects. The gut contents, wing structures, and mouthparts of fossilized [[Coleoptera|beetles]] and [[Diptera|flies]] suggest that they acted as early pollinators.<ref>Stebbins, G. Ledyard, Jr. 1974. ''Flowering plants: evolution above the species level''. Harvard.</ref>

=== Tree of life ===
{{main|tree of life (biology)}} 
Charles Darwin was the first to use this [[metaphor]] in biology. The ''evolutionary tree'' shows the relationships among various biological groups. It includes data from DNA, RNA and protein analysis. Tree of life work is a product of traditional [[comparative anatomy]], and modern [[molecular evolution]] and [[molecular clock]] research.

The major figure in this work is [[Carl Woese]], who defined the [[Archaea]], the third domain (or kingdom) of life.<ref name="Woese_1990">{{cite journal | author = Woese C, Kandler O, Wheelis M | title = Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya. | url=http://www.pnas.org/cgi/reprint/87/12/4576 | journal = Proc Natl Acad Sci USA | volume = 87 | issue = 12 | pages = 4576–9 | year = 1990 | pmid = 2112744 | doi = 10.1073/pnas.87.12.4576 | pmc = 54159}}</ref> Below is a simplified version of present-day understanding.<ref>Woese C.R. 2000. Interpreting the universal phylogenetic tree. Proc Natl Acad Sci USA, 97(15):8392–6.</ref> 
[[File:Simplified tree.png|center|400px|Simplified universal phylogenetic tree]]

=== Macroevolution ===
{{main|macroevolution}}

[[Macroevolution]]: the study of changes above the species level, and how they take place. The basic data for such a study are fossils ([[palaeontology]]) and the reconstruction of ancient [[environment]]s. Some subjects whose study falls within the realm of macroevolution:
* Adaptive radiation, such as the [[Cambrian]] Explosion.
* Changes in [[Biodiversity#Biodiversity and evolution|biodiversity]] through time.
* Mass extinctions.
* [[Speciation]] and extinction rates.
* The debate between punctuated equilibrium and gradualism.
* The role of development in shaping evolution: [[heterochrony]]; [[hox gene]]s.
* Origin of major categories: [[cleidoic egg]]; [[origin of birds]].
It is a term of convenience: for most biologists it does not suggest any change in the [[process]] of evolution.<ref name=Fut3/><ref>Rensch B. 1959. ''Evolution above the species level''. Columbia University Press.</ref><ref>Hoffman, Antoni 1989. ''Arguments on evolution: a paleontologist's perspective''. Oxford University Press. ISBN 0-19-504443-6</ref><sup>p87</sup> For some palaeontologists, what they see in the fossil record cannot be explained just by the gradualist evolutionary synthesis.<ref>Stanley S.M. 1979. ''Evolution: patterns and processes''. Freeman, San Francisco. p3, table 7.1, p183. ISBN 0-7167-1092-7</ref> They are in the minority.

=== Altruism and group selection ===
{{main|kin selection}}
Altruism – the willingness of some to sacrifice themselves for others – is widespread in social animals. As explained above, the next generation can only come from those who survive and reproduce. Some biologists have thought that this meant altruism could not evolve by the normal process of selection. Instead a process called "group selection" was proposed.<ref>Wynne-Edwards V. 1962. ''Animal dispersion in relation to social behavior''. Oliver & Boyd, London.</ref><ref>Wynne-Edwards V. 1986. ''Evolution through group selection''. Blackwell, Oxford. ISBN 0-632-01541-1.</ref> Group selection refers to the idea that [[allele]]s can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the [[fitness]] of individuals within that group.

For several decades, critiques cast serious doubt on group selection as a major mechanism of evolution.<ref>Williams, George C. 1972. ''Adaptation and natural selection: a critique of some current evolutionary thought''. Princeton University Press.ISBN 0-691-02357-3</ref><ref>Williams G.C. 1986. ''Evolution through group selection''. Blackwell. ISBN 0-632-01541-1</ref><ref>Maynard Smith, John 1964. Group selection and kin selection ''Nature'' 201:1145–1147</ref><ref>Maynard Smith, John 1998. ''Evolutionary genetics'', 2nd ed. Oxford.</ref>

In simple cases it can be seen at once that traditional selection suffices. For example, if one sibling sacrifices itself for three siblings, the genetic disposition for the act will be increased. This is because siblings share on average 50% of their genetic inheritance, and the sacrificial act has led to greater representation of the genes in the next generation.

Altruism is now generally seen as emerging from standard selection.<ref name="Koeslag97">Koeslag J.H. 1997. Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation. ''J. Theor. Biol.'' '''189''', 53–61</ref><ref name="Koeslag03">Koeslag J.H. 2003. Evolution of cooperation: cooperation defeats defection in the cornfield model. ''J. Theor. Biol.''  '''224''', 399–410</ref><ref>Hamilton W. 1963. "The evolution of altruistic behavior." ''American Naturalist'' '''97''':354-356</ref><ref>Dawkins R. 1976. ''The selfish gene''. Oxford.</ref><ref>Dawkins R. 1982. ''The extended phenotype''. Freeman, Oxford.</ref> The warning note from Ernst Mayr, and the work of William Hamilton are both important to this discussion.<ref>Mayr, Ernst 1997. The objects of selection. ''PNAS'' '''94''' 2091-2094 [http://www.pnas.org/cgi/content/full/94/6/2091 The objects of selection].</ref><ref>Hamilton W.D. 1996. ''Narrow roads of geneland: the collected papers of W.D. Hamilton'', vol 1. Freeman, Oxford.</ref>

==== Hamilton's equation ====
Hamilton's equation describes whether or not a gene for altruistic behaviour will spread in a population. The gene will spread if '''r'''x'''b''' is greater than '''c''':
:<math>rb > c \ </math>
where:
* <math>c \ </math> is the reproductive cost to the altruist,
* <math>b \ </math> is the reproductive benefit to the recipient of the altruistic behavior, and
* <math>r \ </math> is the probability, above the population average, of the individuals sharing an altruistic gene – the "degree of relatedness".

=== Sexual reproduction ===
{{main|sex}}
At first, [[sexual reproduction]] might seem to be at a disadvantage compared with [[asexual reproduction]]. In order to be advantageous, sexual reproduction (cross-fertilisation) has to overcome a two-fold disadvantage (takes two to reproduce) plus the difficulty of finding a mate. Why, then, is sex so nearly universal among [[eukaryote]]s? This is one of the oldest questions in biology.<ref>Maynard Smith J. 1978. ''The evolution of sex''. Cambridge.</ref>

The answer has been given since Darwin's time: because the sexual populations adapt better to changing circumstances. A recent laboratory experiment suggests this is indeed the correct explanation.<ref name="Agrawal">Agrawal A.F. 2009. Why reproduction often takes two. ''Nature'' '''462''' p294</ref><ref>Morran L.T. Parmenter M.D. & Phillips P.C. 2009. Mutation load and rapid adaptation favour outcrossing over self-fertilisation. ''Nature'' '''462''' p350</ref>

:"When populations are [[Outcrossing|outcrossed]]<ref>Normal sexual reproduction with unrelated members of the population.</ref> genetic [[recombination]] occurs between different parental genomes. This allows beneficial [[mutation]]s to escape deleterious [[allele]]s on its original background, and to combine with other beneficial alleles that arise elsewhere in the population. In [[selfing]]<ref>Self-fertilisation (which is possible in some animals and plants) is an extreme form of [[inbreeding]] which leads to loss of genetic variability.</ref> populations, individuals are largely [[homozygous]] and recombination has no effect".<ref name="Agrawal"/>

In the main experiment, [[nematode]] worms were divided into two groups. One group was entirely [[outcrossing]], the other was entirely selfing. The groups were subjected to a rugged terrain and repeatedly subjected to a mutagen.<ref>Mutagen = chemical which causes mutations.</ref> After 50 generations, the selfing population showed a substantial decline in fitness (= survival), whereas the outcrossing population showed no decline.<ref>"Non-sexual", because obligatory selfing is effectively asexual in its genetic effect.</ref>

== What evolution is used for today ==
An important activity is [[artificial selection]] for [[domestication]]. This is when people choose which animals to breed from, based on their traits. Humans have used this for thousands of years to domesticate plants and animals.<ref>{{cite journal |author=Doebley JF, Gaut BS, Smith BD |title=The molecular genetics of crop domestication |journal=Cell |volume=127 |issue=7 |pages=1309–21 |year=2006 |pmid=17190597 |doi=10.1016/j.cell.2006.12.006 }}</ref>

More recently, it has become possible to use [[genetic engineering]]. New techniques such as 'gene targeting' are now available. The purpose of this is to insert new genes or knock out old genes from the genome of a plant or animal. A number of [[Nobel Prize]]s have already been awarded for this work.

However, the real purpose of studying evolution is to explain and help our understanding of biology. After all, it is the first good explanation of how living things came to be the way they are. That is a big achievement. The practical things come mostly from [[genetics]], the science started by Gregor Mendel, and from [[Molecular biology|molecular]] and [[cell biology]].

== Evolution gems ==
In 2010 the journal ''Nature'' selected 15 [[wikt:topic|topics]] as 'Evolution gems'. These were:

=== Gems from the fossil record ===
#Land-living ancestors of [[whale]]s
#From water to land (see [[tetrapod]])
#The origin of [[feather]]s (see [[origin of birds]])
#The evolutionary history of [[teeth]]
#The origin of [[vertebrate]] [[skeleton]]

=== Gems from habitats ===
#[[Natural selection]] in [[speciation]]
#Natural selection in [[lizard]]s
#A case of co-[[adaptation]]
#Differential dispersal in wild birds
#Selective survival in wild [[Guppy|guppies]]
#Evolutionary history matters

=== Gems from molecular processes ===
#Darwin's [[Galapagos]] finches
#[[Microevolution]] meets [[macroevolution]]
#[[Toxin]] resistance in [[snake]]s and [[clam]]s
#[[Variation (biology)|Variation]] versus [[Stable|stability]]

*''Nature'' is the oldest scientific weekly journal. The link downloads as a free text file, complete with references. The idea is to make the information available to teachers.<ref name=gems>[http://www.nature.com/nature/newspdf/evolutiongems.pdf Gee H; Howlett R. & Campbell P. 2009. 15 evolutionary gems. ''Nature'']</ref>

== Responses to the idea of evolution ==
{{main|social responses to the idea of evolution}}

=== Debates about the fact of evolution ===
[[Image:Editorial cartoon depicting Charles Darwin as an ape (1871).jpg|right|150px|thumb|As [[Darwinism]] became accepted in the 1870s, [[caricature]]s of [[Charles Darwin]] with an [[ape]] body symbolized evolution.<ref name=Browne2003e>{{cite book |author=Browne, Janet |title=Charles Darwin: The power of place |publisher=Pimlico |location=London |year=2003 |pages=376-379 |isbn=0-7126-6837-3 }}</ref>]]

The idea that all life evolved had been proposed before Charles Darwin published ''On the Origin of species''. Even today, some people still discuss the concept of evolution and what it means to them, their [[philosophy]] and their [[religion]]. Evolution does seem to explain some things about about our [[human nature]].<ref name=Human>Stevenson, Leslie and Haberman, David L. 2009. ''Ten theories of human nature''. 5th ed, Oxford University Press. Chapter 10: Darwinian theories of human nature. ISBN 978-0-19-536825-3</ref> People also talk about the social implications of evolution.

In order to fit in the idea of evolution with their religious views, people have come up with ideas like ''guided evolution'' or ''[[theistic evolution]]''. They say that evolution is real, but is being guided in some way.<ref name=Bowler/><ref name=Dennett>For an overview of the controversies see: {{cite book|last=Dennett|first=D|title=Darwin's dangerous idea: Evolution and the meanings of life|publisher=Simon & Schuster|date=1995|isbn=978-0684824710}}</ref><ref>For the reception of evolution in the 19th and early 20th centuries, see: {{cite web | last = Johnston | first = Ian C. | title = History of science: origins of evolutionary theory | work = And still we evolve | publisher = Liberal Studies Department, Malaspina University College | url =http://www.mala.bc.ca/~johnstoi/darwin/sect3.htm| accessdate =2007-05-24}}</ref><ref>{{cite journal 
|author=Zuckerkandl E 
|title=Intelligent design and biological complexity 
|journal=Gene 
|volume=385 
|issue= 
|pages=2–18 
|year=2006 
|pmid=17011142 
|doi=10.1016/j.gene.2006.03.025 }}</ref>

There are many different concepts of ''theistic evolution''. Many [[creationism|creationists]] believe that the [[creation myth]] found in their religion goes against the idea of evolution.<ref name=Ross2005>{{cite journal | author = Ross M.R. | year = 2005 | title = Who believes what? Clearing up confusion over intelligent design and young-Earth creationism | journal = Journal Of Geoscience Education | volume = 53 | issue = 3 | pages = 319 | url = http://www.nagt.org/files/nagt/jge/abstracts/Ross_v53n3p319.pdf  | accessdate = 2008-04-28}}</ref> As Darwin realised, the most controversial part of the evolutionary thought is what it means for [[human evolution|human origins]].

In some countries, especially in the [[United States]], there is tension between people who accept the idea of evolution and those who do not accept it. The debate is mostly about whether evolution should be taught in [[school]]s, and in what way this should be done.<ref>{{cite journal | author = Spergel D.N. |title=Science communication. Public acceptance of evolution |journal=Science |volume=313 |issue=5788 |pages=765–66 |year=2006 |pmid=16902112 |doi=10.1126/science.1126746 }}</ref>

Other fields, like [[physical cosmology|cosmology]]<ref name="wmap">{{cite journal | doi=10.1086/377226 | title = First-year Wilkinson microwave anisotropy probe (WMAP) observations: determination of cosmological parameters | first = D N. | last = Spergel | coauthors = et al. | journal = The Astrophysical Journal Supplement Series | volume = 148 | year = 2003 | pages = 175–94}}</ref> and [[earth science]]<ref name="zircon">{{cite journal |author=Wilde SA, Valley JW, Peck WH, Graham CM |title=Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago |journal=Nature |volume=409 |issue=6817 |pages=175&section=1275–78 |year=2001 |pmid=11196637 |doi=10.1038/35051550 }}</ref> also do not match with the original writings of many religious texts. These ideas were once also fiercely opposed. Death for [[heresy]] was threatened to those who wrote against the idea that the Earth was the centre of the universe. [[Giordano Bruno]] was burnt at the stake by the Holy [[Inquisition]] for teaching that the Earth moved around the Sun (and other ideas).

Evolutionary biology is a more recent idea.  Certain religious groups oppose the idea of evolution more than other religious groups do. For instance, the [[Roman Catholic Church]] now has the following position on evolution: [[Pope Pius XII]] said in his [[encyclical]] ''Humani Generis'' published in the 1950s:
:"The Church does not forbid that (...) research and discussions (..) take place with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent and living matter." Pope Pius XII ''Humani Generis''<ref name="Humani_Generis">{{cite web|url=http://www.vatican.va/holy_father/pius_xii/encyclicals/documents/hf_p-xii_enc_12081950_humani-generis_en.html|title=English version of ''Humani Generis''|publisher=The Holy See}}</ref>

[[Pope John Paul II]] updated this position in 1996. He said that Evolution was "more than a [[hypothesis]]":
:"In his encyclical Humani Generis, my predecessor Pius XII has already [said] that there is no conflict between evolution and the doctrine of the faith regarding man and his vocation. (...) Today, more than a half-century after (..) that encyclical, some new findings lead us toward the recognition of evolution as more than an hypothesis. In fact it is remarkable that this theory has had progressively greater influence on the spirit of researchers, following a series of discoveries in different scholarly disciplines." Pope John Paul II speaking to the [[Pontifical Academy of Science]]<ref name="JP2_Evolution">{{cite web|url=http://www.ewtn.com/library/PAPALDOC/JP961022.HTM|title=Message to the Pontifical Academy of Science: On Evolution|publisher=EWTN}}</ref>

The [[Anglican Communion]] also does not oppose the scientific account of evolution.

=== Using evolution for other purposes ===
Many of those who accepted evolution were not much interested in biology. They were interested in using the theory to support their own ideas on society.

==== Racism ====
Some people have tried to use evolution to support [[racism]]. People wanting to justify racism claimed that certain groups, such as black people, were inferior. In nature, some animals do ''survive'' better than others, and it does lead to animals better [[adaptation|adapted]] to their circumstances. With humans groups from different parts of the world, all evolution can say is that each group is probably well suited to its original situation. Evolution makes no judgements about better or worse. It does ''not'' say that any human group is superior to any other.<ref>Higgins A.C. [http://www.math.buffalo.edu/mad/special/scientific-racism.html Scientific racism: a review of the science and politics of racial research by William H. Tucker (Chicago: University of Illinois Press, 1994)].</ref>

==== Eugenics ====
{{main|eugenics}}
This amazing idea of [[eugenics]] was rather different. Two things had been noticed as far back as the 18th century. One was the great success of farmers in [[breeding]] [[cattle]] and [[crops|crop]] plants. They did this by selecting which animals or plants would produce the next generation ([[artificial selection]]). The other observation was that lower [[Social class|class]] people had more children than upper class people. If (and it's a big if) the higher classes were there on ''merit'', then their lack of children was the exact reverse of what should be happening. Faster breeding in the lower classes would lead to the society getting worse.

The idea to improve the human species by [[selective breeding]] is called [[eugenics]]. The name was proposed by [[Francis Galton]], a bright [[scientist]] who meant to do good.<ref>Today, Galton is remembered for many things he did in statistics and psychology.
*Forrest D.W 1974. Francis Galton: ''The life and work of a Victorian genius''. Taplinger. ISBN 0-8008-2682-5
*Bulmer, Michael 2003. ''Francis Galton: pioneer of heredity and biometry''. Johns Hopkins University Press. ISBN 0-8018-7403-3</ref>  He said that the human stock (gene pool) should be improved by selective breeding policies. This would mean that those who were considered "good stock" would receive a reward if they reproduced. The German [[Nazi]] government (1933–1945) used eugenics as a cover for their extreme racial policies, with dreadful results.<ref>{{cite journal |author=Kevles DJ |title=Eugenics and human rights |url=http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=10445929 |journal=[[British Medical Journal|BMJ]] |volume=319 |issue=7207 |pages=435–8 |year=1999 |pmid=10445929}}</ref>

The problem with Galton's idea is how to decide which features to select. There are so many different skills people could have, you could not agree who was "good stock" and who was "bad stock". There was rather more agreement on who should ''not'' be breeding. Several countries passed laws for the compulsory sterilisation of unwelcome groups.<ref>Though the details vary from country to country, and in the U.S. from state to state, here is some idea of the groups who might be subject to sterilization: violent criminals; the insane; mentally disabled persons; unmarried mothers taken into public care; orphans in public care. Eugenics ideas also influenced legislation on immigration in some countries (see Kevles 1998 Chapter 7 Eugenic enactments).</ref> Most of these laws were passed between 1900 and 1940. After [[World War II]], disgust at what the Nazis had done squashed any more attempts at eugenics.

==== Algorithm design ====
Some [[equation]]s can be [[wikt:solve|solved]] using [[algorithm]]s that [[simulation|simulate]] evolution. [[Evolutionary algorithm]]s work like that.

==== Social Darwinism ====
Another example of using ideas about evolution to support social action is [[Social Darwinism]]. Social Darwinism is a term given to the ideas of the 19th century [[social]] [[philosopher]] [[Herbert Spencer]]. Spencer believed the ''[[survival of the fittest]]'' could and should be applied to [[commerce]] and human societies as a whole.

Again, some people used these ideas to claim that racism, and ruthless economic policies were justified.<ref>On the history of eugenics and evolution, see {{cite book|first=D |last=Kevles |date=1998 |title=In the name of Eugenics: genetics and the uses of human heredity |publisher=Harvard University Press|isbn=978-0674445574}}</ref> Today, most [[biologist]]s and philosophers say that the theory of evolution should not be applied to social policy.<ref>Darwin strongly disagreed with attempts by Herbert Spencer and others to apply evolutionary ideas to all possible subjects; see {{cite book|first=M|last=Midgley|date=2004|title=The myths we live by|publisher=Routledge|page=62|isbn=978-0415340779}}</ref><ref>{{cite journal |author=Allhoff F |title=Evolutionary ethics from Darwin to Moore |journal=History and philosophy of the life sciences |volume=25 |issue=1 |pages=51–79 |year=2003 |pmid=15293515 |doi=10.1080/03919710312331272945}}</ref>

=== Controversy ===
Some people disagree with the idea of evolution. They disagree with it for a number of reasons. Most often these reasons are influenced by or based on their religious beliefs. People who do not agree with evolution usually believe in [[creationism]] or [[intelligent design]].

Despite this, evolution is one of the most successful theories in science. People have discovered it to be useful for different kinds of research. None of the other suggestions explain things, such as [[fossil]] records, as well. So, for almost all scientists, evolution is not in doubt.<ref name="Dob">Theodosius Dobzhansky, [http://people.delphiforums.com/lordorman/light.htm ''Nothing in biology makes sense except in the light of evolution''], from ''The American Biology Teacher'', March 1973 (35:125-129)</ref><ref>Moran, Laurence 1993. TalkOrigins: [http://www.talkorigins.org/faqs/evolution-fact.html Evolution is a fact and a theory]</ref><ref>{{cite news | url = http://www.nytimes.com/2009/10/25/books/review/Letters-t-THEFACTOFEVO_LETTERS.html?_r=1&partner=rss&emc=rss |title = Letters - The fact of evolution - NY Times.com | date = 2009-10-23 |accessdate = 2009-11-03 | publisher = [[New York Times]]}}</ref><ref>{{cite news|title=Letters: Scientists respond to our review of Richard Dawkins’s ''Greatest Show on Earth''|url=http://artsbeat.blogs.nytimes.com/2009/10/23/letters-scientists-respond-to-our-review-of-richard-dawkinss-greatest-show-on-earth/|accessdate=24 April 2011|newspaper=[[New York Times]]|date=23 October 2009}}</ref>

== Further reading ==
=== Evidence for evolution ===
These books are mostly about the evidence for evolution.
*Coyne, Jerry A. 2009 ''Why evolution is true''. Oxford University Press, Oxford. ISBN 0670-02053-2 (pbk)
*Dawkins, Richard 2009. ''The greatest show on Earth''. Bantam, London. ISBN 978-0-593-06173-2 (hbk)
*Futuyma D.J. 1983. ''Science on trial: the case for evolution''. Pantheon Books, New York. ISBN 0-394-52371-7; 2nd ed 1995 Sinauer Associates, Sunderland, Massachusetts. ISBN 0-87893-184-8.
*Prothero, Donald R. 2007. ''Evolution: what the fossils say and why it matters''. Columbia University Press, New York. ISBN 978-0-231-13962-5 (hbk)

===  The process of evolution===
These books cover most evolutionary topics.
*Futuyma D.J. 1979. ''Evolutionary Biology''. 1st ed. Sinauer Associates, Sunderland, Massachusetts. ISBN 0-87893-199-6; 2nd ed 1986 Sinauer. ISBN 0-87893-188-0; 3rd ed 1998 Sinauer. ISBN 0-87893-189-9. Widely used textbook, available second-hand. For students and teachers.
* Futuyma D.J. 2005. ''Evolution''. Sinauer Associates, Sunderland, Massachusetts. ISBN 0-87893-187-2; 2nd ed 2009 Sinauer. ISBN 978-0-87893-223-8. Successor to above; but basically a different book. For students and teachers.
*Freeman, Scott & Herron, Jon; 1997. ''Evolutionary analysis''. Prentice Hall ISBN 0-13-568023-9; 2nd ed 2000 ISBN 0-13-017291-X; 3rd ed 2004 Cummings ISBN 978-0-13-101859-4; 4th ed 2007 Cummings ISBN 0-13-227584-8. Modern topics such as phylogenetic trees based on genomics, genetics, molecular biology. Has website: [http://www.aw-bc.com/freeman/] For students and teachers.
*Ridley, Mark 1993. ''Evolution''. Blackwell ISBN 0-86542-226-5; 2nd ed 1996 Wiley-Blackwell ISBN 0-86542-495-0; 3rd ed 2003 Wiley ISBN 978-1-4051-0345-9. Comprehensive: case studies, commentary, dedicated website and CD. For students and teachers.
* Mayr, Ernst. 2001. ''What evolution is''. Weidenfeld & Nicolson, London. ISBN 0-297-60741-3. Clearly written, for a general audience.

== References ==
{{reflist|2}}

== Other websites ==
{{commonscat|Evolution}}
{{wiktionary}}
* [http://evolution.berkeley.edu/ Understanding Evolution] - a guide prepared by the University of California at Berkeley
* [http://darwin-online.org.uk/ Darwin Online] Darwin's publications; papers and bibliography; biographies, obituaries and reviews.
*[http://www.talkorigins.org/ Talk Origins] in depth website on information about evoluton and the evidence for it
*[http://ncse.com/ National Center for Science Education] Information on how evolution works
*[http://www.pbs.org/wgbh/evolution/ PBS] on evolution site

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[[Category:Evolution| ]]
[[Category:History of science]]