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==Asas genetik pemilihan semuka jadi==
Gagasan pemilihan semula jadi mendahului pemahaman genetik. Kini, kita lebih memahami ilmu biologi yang berdasarkan keterwarisan, iaitu asas kepada pemilihan semula jadi.
===Genotip dan Fenotip===
{{see also|Perbezaan genotip dan fenotip}}
Pemilihan semula jadi bertindak pada fenotip organisme, atau sifat-sifat fizikalnya. Fenotip ditentukan oleh kandungan genetik organisme itu (genotip) dan persekitaran yang didiami oleh organisme itu. Selalunya, pemilihan semula jadi bertindak pada sifat-sifat tertentu pada individu, maka istilah fenotip dan genotip digunakan dengan teliti untuk menyatakan sifat-sifat tertentu ini.
Apabila organisme berbeza dalam sesebuah populasi memiliki versi berlainan bagi sesuatu gen yang membawa satu sifat tertentu; setiap versi ini dikenali sebagai [[alel]]. Variasi genetik inilah yang menyiratkan sifat-sifat fenotip. Sebagai contoh, kombinasi gen tertentu untuk warna mata manusia yang menghasilkan fenotip mata biru atau mata perang. (Sebaliknya, apabila kesemua organisme dalam sesebuah populasi berkongsi alel yang sama untuk satu sifat yang tertentu, dan keadaan ini kekal stabil sepanjang masa, maka alel ini dikatakan berkeadaan ''[[Pengikatan (genetik populasi)|terikat]]'' dalam populasi itu.)
Apabila organisme berbeza dalam sesebuah populasi memiliki different organisms in a population possess different versions of a gene for a certain trait, each of these versions is known as an allele. It is this genetic variation that underlies phenotypic traits. A typical example is that certain combinations of genes for [[eye color]] in humans which, for instance, give rise to the phenotype of blue eyes. (On the other hand, when all the organisms in a population share the same allele for a particular trait, and this state is stable over time, the allele is said to be ''[[fixation (population genetics)|fixed]]'' in that population.)
SomeSesetengah traitssifat arehanya governeddikuasai byoleh onlysatu agen single genetunggal, buttetapi mostkebanyakan traitssifat aredipengaruhi influencedoleh byinteraksi thesesama interactionsberbilang of many genesgen. ASatu variationvariasi indalam onesatu ofdaripada theberbanyak manygen genesini thatyang contributesmenyumbang topada asuatu traitsifat, mayhanya havemeninggalkan onlykesan akecil smallpada effect on the phenotypefenotip; together, thesegen-gen genesini canbersama-sama produceboleh amenghasilkan continuumsatu ofkeselanjaran possiblekemungkinan phenotypicnilai-nilai valuesfenotip.<ref>Falconer DS & Mackay TFC (1996) ''Introduction to Quantitative Genetics'' Addison Wesley Longman, Harlow, Essex, UK ISBN 0-582-24302-5</ref>
===DirectionalityKearahan of selectionpemilihan===
WhenDi somemana componentadanya ofsuatu akomponen traitdalam issuatu heritable,sifat selectionyang willboleh alterdiwarisi, thepemilihan frequenciesakan ofmengubah thekekerapan differentalel-alel allelesberlainan itu, oratau variantsvarian-varian ofgen theitu geneyang thatmenghasilkan producesvarian-varian thesifat variants of the traititu. SelectionPemilihan canboleh beterbahagi dividedkepada intotiga three classeskelas, on the basis of itsberdasarkan effectkesannya onpada allelekekerapan frequenciesalel.<ref name="Rice">Rice SH. (2004). Evolutionary Theory: Mathematical and Conceptual Foundations. Sinauer Associates: Sunderland, Massachusetts, USA. ISBN 0-87893-702-1 See esp. ch. 5 and 6 for a quantitative treatment.</ref>
[[Pemilihan berarah]] berlaku apabila suatu alel tertentu lebih tegap berbanding alel-alel lain, sehingga menyebabkan kekerapannya meningkat. Proses ini boleh berterusan sehingga alel itu [[Pengikatan (genetik populasi)|terikat]] dan seluruh populasi berkongsi fenotip yang lebih tegap itu. Pemilihan berarah inilah yang digambarkan dalam contoh ketahanan antibiotik [[#Contoh: ketahanan antibiotik|di atas]].
[[Directional selection]] occurs when a certain allele has a greater fitness than others, resulting in an increase of its frequency. This process can continue until the allele is [[fixation (population genetics)|fixed]] and the entire population shares the fitter phenotype. It is directional selection that is illustrated in the antibiotic resistance example [[#An example: antibiotic resistance|above]].
FarYang morelebih commonkerap isditemui pula ialah [[stabilizingpemilihan selectionstabil]] (which isyang commonlysering '''confusedterkeliru''' withdengan ''purifyingpemilihan selectionpembersihan''<ref>{{cite book |last= Lemey |first= Philippe |coauthors= Marco Salemi, Anne-Mieke Vandamme |title= [[The Phylogenetic Handbook]] |publisher= [[Cambridge University Press]] |year= 2009 |isbn= 978-0-521-73071}}</ref> <ref>http://www.nature.com/scitable/topicpage/Negative-Selection-1136</ref>), whichyang lowersmengurangkan thekekerapan frequencyalel ofyang allelesmembawa thatkesan havemudarat akepada deleteriousfenotip, effect on the phenotype - that is,iaitu producemenghasilkan organismsorganisme ofyang lowerkurang fitnesstegap. ThisProses processini canboleh continueberterusan untilsehingga thealel alleleitu isdihapuskan eliminateddari from the populationpopulasi. PurifyingPemilihan selectionpembersihan resultsmenyebabkan inciri-ciri functionalgenetik geneticberfungsi features, such asseperti [[proteinBiosintesis biosynthesisprotein|proteingen-codinggen genespengekodan protein]] oratau [[regulatoryurutan sequencekawal atur]]s, being [[conservationPengabadian (geneticsgenetik)|conserveddiabadikan]] oversepanjang timemasa duehasil totekanan selectivepemilihan pressureterhadap againstvarian-varian deleteriousyang variantsmemudaratkan.
FinallyAkhir sekali, awujudnya numbersebilangan of forms ofbentuk [[balancingpemilihan selectionpengimbang]] exist,yang whichtidak domenyebabkan not result inapa-apa fixationpengikatan, butsebaliknya maintainmengekalkan ansuatu allelealel atpada intermediatekekerapan frequenciesperantaraan indalam asesebuah populationpopulasi. ThisIni canboleh occurberlaku indalam spesies [[diploid]] species (that isiaitu, thosehidupan thatyang havememiliki twodua pairs ofpasang [[chromosomekromosom]]s) whenapabila individu [[heterozygoteheterozigot]] individuals,yang whomemiliki havealel-alel differentberlainan allelespada onsetiap eachkromosom chromosomepada at a singlesuatu [[LocusLokus (geneticsgenetik)|geneticlokus locusgenetik]], havetunggal, alebih highertegap fitnessberbanding thanindividu [[homozygotehomozigot]] individualsyang thatmemiliki havedua twoalel ofyang the same allelessama. ThisInilah isyang calleddipanggil [[heterozygotekelebihan advantageheterozigot]] oratau overdominancekedominanan lampau, ofseperti which the best-knownyang exampledigambarkan isoleh theketahanan [[malaria]]l resistanceyang observeddiperhatikan inpada heterozygousmanusia humansheterozigot whoyang carryhanya onlymembawa onesatu copysalinan ofgen the gene foruntuk [[sickleanemia cellsel anemiasabit]]. MaintenancePengekalan ofvariasi allelicalel variationjuga canboleh alsoterjadi occur throughmelalui [[disruptivepemilihan selectionberporak-peranda|disruptivepemilihan orberporak-peranda diversifyingatau selectionmempelbagai]], whichyang favorsmengutamakan genotypesgenotip thatyang departmenyimpang fromdari thekebiasaan averageke inmana-mana either directionarah (thatiaitu is,berlawanan thedengan oppositekedominanan of overdominancelampau), and can resultsehingga inboleh amenyebabkan [[bimodaltaburan distributionbimod]] of traitnilai-nilai valuessifatnya. FinallyAkhir sekali, balancingpemilihan selectionpengimbang canboleh occurberlaku throughmelalui [[frequency-dependentpemilihan bersandarkan selectionkekerapan]], whereyang themana fitnessketegapan ofsuatu onefenotip particulartertentu phenotypebergantung dependspada ontaburan the distribution of other phenotypes infenotip-fenotip thedalam populationpopulasi. The principles ofPrinsip-prinsip [[gameteori theorypermainan]] havediterapkan beendemi appliedpemahaman totaburan understandketegapan thedalam fitnesskeadaan-keadaan distributions in these situationssedemikian, particularlykhususnya indalam the study ofkajian [[kinpemilihan selectionsaudara]] and the evolutionserta ofevolusi [[reciprocalaltruisme altruismsalingan]].<ref name="Hamilton">Hamilton WD. (1964). The genetical evolution of social behaviour I and II. ''Journal of Theoretical Biology'' 7: 1-16 and 17-52. PMID 5875341 PMID 5875340</ref><ref name="Trivers">Trivers RL. (1971). The evolution of reciprocal altruism. ''Q Rev Biol'' 46: 35-57.</ref>
===SelectionPemilihan anddan geneticvariasi variationgenetik===
ASebahagian portiondaripada of allsegala [[geneticvariasi variationgenetik]] is functionallyadalah neutral indari thatsegi itfungsinya, producesiaitu notidak phenotypicmenghasilkan effectapa-apa orkesan significantfenotip differenceatau inperbezaan fitnessketaran dalam ketegapan; thehipotesis bahawa variasi hypothesisini that this variation accounts for a large fraction of observed [[genetic diversity]] is known as the [[neutral theory of molecular evolution]] and was originated by [[Motoo Kimura]]. When genetic variation does not result in differences in fitness, selection cannot ''directly'' affect the frequency of such variation. As a result, the genetic variation at those sites will be higher than at sites where variation does influence fitness.<ref name="Rice" />
====Mutation selection balance====
[[Background selection]] is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection, linked variation will tend to be weeded out along with it, producing a region in the genome of low overall variability. Because background selection is a result of deleterious new mutations, which can occur randomly in any haplotype, it produces no linkage disequilibrium.
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==Lihat juga==
* [[Pemilihan buatan]]
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