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}}</ref> This helped to demonstrate how DNA serves as the hereditary code and how genetic variations in a population arise by chance mutations in DNA. Although such mutations are random, natural selection is not a process of chance: the environment determines the probability of reproductive success. 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 necessarily strive for [[evolution of complexity|more advanced]], more intelligent, or more sophisticated life forms.<ref>{{Harv |Gould (a)|1981| p=}}</ref><ref name= wgbh>{{citation| year =2001 | publication-date = | contribution =Frequently Asked Questions About Evolution | contribution-url =http://www.pbs.org/wgbh/evolution/library/faq/cat03.html | title =Evolution Library| publisher =WGBH Educational Foundation| url =http://www.pbs.org/wgbh/evolution/library/faq/cat03.html|accessdate=2008-01-23 }}</ref> For example, [[flea]]s (wingless parasites) are descended from a winged, ancestral [[scorpionfly]],<ref>{{cite web |url= http://www.roberth.u-net.com/Fleas.htm |title= Fleas |accessdate=2007-09-03 |last= Hutchinson |first= Robert|date=1999 |publisher= Veterinary Entomology }}</ref> and snakes are lizards that no longer require limbs. Organisms are merely the outcome of variations that succeed or fail, dependent upon the environmental conditions at the time. Environmental changes that occur over short periods typically lead to [[extinction]].<ref name= wgbh/> Of all species that have existed on Earth, 99.9 percent are now extinct.<ref name= PBS4>{{citation| year =2001 | publication-date = | contribution =Roundtable: Mass Extinction | contribution-url =http://www.pbs.org/wgbh/evolution/extinction/massext/index.html | title =Evolution: a jouney into where we're from and where we're going| publisher =WGBH Educational Foundation| url =http://www.pbs.org/wgbh/evolution/index.html |accessdate=2008-01-23 }}.</ref> Since life began on Earth, five major mass extinctions have lead to significant reduction in species diversity. The most recent, the [[Cretaceous–Tertiary extinction event]], occurred 65 million years ago, and has attracted more attention than all others because it killed the [[dinosaur]]s.<ref>{{Citation | last=Bambach | first=R.K. | last2=Knoll | first2=A.H. | last3=Wang | first3=S.C. | title=Origination, extinction, and mass depletions of marine diversity | journal=Paleobiology | volume=30 | issue=4 | pages=522–42 | date=December 2004 | url=http://findarticles.com/p/articles/mi_qa4067/is_200410/ai_n9458414/pg_1 |accessdate=2008-01-24}}</ref>
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A very large number of fossils have now been discovered and identified. These fossils serve as a chronological record of evolution. The fossil record provides examples of [[Transitional fossil|transitional species]] that demonstrate ancestral links between past and present life forms.<ref>{{cite web |url= http://books.nap.edu/openbook.php?record_id=11876&page=22 |title=Science, Evolution, and Creationism|accessdate=2008-01-06 |last=Committee on Revising Science and Creationism: A View from the National Academy of Sciences, National Academy of Sciences and Institute of Medicine of the National Academies|date=2008 |publisher=National Academy of Sciences}}</ref>
One such transitional fossil is ''[[Archaeopteryx]]'', an ancient organism that had the distinct characteristics of a reptile, yet had the feathers of a bird. The implication from such a find is that modern reptiles and birds arose from a common ancestor.<ref>{{Harv |Gould (b)|1995| p=360
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