Editing Genetics (section)

=== Natural selection and evolution ===

{{Main|Evolution}}
{{further|Natural selection}}

Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Most mutations have little effect on an organism's phenotype, health, or reproductive [[fitness (biology)|fitness]].<ref>{{cite book | vauthors = Schaechter M |title=Encyclopedia of Microbiology |url=https://books.google.com/books?id=rLhdW5YzuO4C&pg=RA1-PA551 |year=2009 |publisher=Academic Press |isbn=978-0-12-373944-5 |page=551}}</ref> Mutations that do have an effect are usually detrimental, but occasionally some can be beneficial.<ref name="CalverLymbery2009">{{cite book | vauthors = Calver M, Lymbery A, McComb J, Bamford M |title=Environmental Biology |url=https://books.google.com/books?id=HemnRxzdiFQC&pg=PA118 |year=2009 |publisher=Cambridge University Press |isbn=978-0-521-67982-4 |page=118}}</ref> Studies in the fly ''[[Drosophila melanogaster]]'' suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations are harmful with the remainder being either neutral or weakly beneficial.<ref>{{cite journal | vauthors = Sawyer SA, Parsch J, Zhang Z, Hartl DL | title = Prevalence of positive selection among nearly neutral amino acid replacements in Drosophila | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 104 | issue = 16 | pages = 6504–6510 | date = April 2007 | pmid = 17409186 | pmc = 1871816 | doi = 10.1073/pnas.0701572104 | doi-access = free | bibcode = 2007PNAS..104.6504S }}</ref>

[[File:Eukaryote tree.svg|thumb|left|An [[evolutionary tree]] of [[Eukaryote|eukaryotic]] organisms, constructed by the comparison of several [[orthologous gene]] sequences]]

[[Population genetics]] studies the distribution of genetic differences within populations and how these distributions change over time.<ref name="griffiths2000sect3842">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3842 |chapter=Variation and its modulation}}</ref> Changes in the [[Allele frequency|frequency of an allele]] in a population are mainly influenced by [[natural selection]], where a given allele provides a selective or reproductive advantage to the organism,<ref name="griffiths2000sect3886">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3886 |chapter=Selection}}</ref> as well as other factors such as [[mutation]], [[genetic drift]], [[genetic hitchhiking]],<ref>{{cite journal | vauthors = Gillespie JH | title = Is the population size of a species relevant to its evolution? | journal = Evolution; International Journal of Organic Evolution | volume = 55 | issue = 11 | pages = 2161–2169 | date = November 2001 | pmid = 11794777 | doi = 10.1111/j.0014-3820.2001.tb00732.x | s2cid = 221735887 | doi-access = free }}</ref> [[artificial selection]] and [[Gene flow|migration]].<ref name="griffiths2000sect3906">{{cite book | veditors = Griffiths AJ, Miller JH, Suzuki DT, Lewontin RC, Gelbart|title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W.H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.3906 |chapter=Random events}}</ref>

Over many generations, the genomes of organisms can change significantly, resulting in evolution. In the process called [[adaptation]], selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment.<ref name="Darwin">{{cite book | vauthors = Darwin C |author-link=Charles Darwin |year=1859 |title=On the Origin of Species |place=London |publisher=John Murray |edition= |page=1 |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 |isbn=978-0-8014-1319-3 |url-status=live |archive-url=http://archive.wikiwix.com/cache/20061212020054/http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=16 |archive-date=12 December 2006 }}<br />Earlier related ideas were acknowledged in {{cite book | vauthors = Darwin C |author-link=Charles Darwin |year=1861 |title=On the Origin of Species |place=London |publisher=John Murray |edition=3rd |page=xiii |url=http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20 |no-pp=true |isbn=978-0-8014-1319-3 |url-status=live |archive-url=http://archive.wikiwix.com/cache/20110223145332/http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20 |archive-date=23 February 2011 }}</ref> New species are formed through the process of [[speciation]], often caused by geographical separations that prevent populations from exchanging genes with each other.<ref name="Gavrilets">{{cite journal | vauthors = Gavrilets S | title = Perspective: models of speciation: what have we learned in 40 years? | journal = Evolution; International Journal of Organic Evolution | volume = 57 | issue = 10 | pages = 2197–2215 | date = October 2003 | pmid = 14628909 | doi = 10.1554/02-727 | s2cid = 198158082 }}</ref>

By comparing the [[Sequence homology|homology]] between different species' genomes, it is possible to calculate the evolutionary distance between them and [[Molecular clock|when they may have diverged]]. Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. The evolutionary distances between species can be used to form [[evolutionary tree]]s; these trees represent the [[common descent]] and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as [[horizontal gene transfer]] and most common in bacteria).<ref>{{cite journal | vauthors = Wolf YI, Rogozin IB, Grishin NV, Koonin EV | title = Genome trees and the tree of life | journal = Trends in Genetics | volume = 18 | issue = 9 | pages = 472–479 | date = September 2002 | pmid = 12175808 | doi = 10.1016/S0168-9525(02)02744-0 }}</ref>