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→Natural selection: grammar — a common ancestor has met with objections from some religious groups. Their — been met with or simply met? |
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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 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 generations alleles, and chance plays a role in whether an individual survives to reproduce and to pass a sample of their generation onward to the next. The allelic frequency of a population is the ratio of the copies of one specific allele that share the same form compared to the number of all forms of the allele present in the population.<ref>{{harvnb|Futuyma|1998|p=Glossary}}</ref>
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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