Pemilihan semula jadi: Perbezaan antara semakan
Kandungan dihapus Kandungan ditambah
Mencipta laman baru dengan kandungan ''''Pemilihan semula jadi''' ialah proses yang mana sifat-sifat yang terwariskan oleh sesejenis organisme untuk bermandiri dan berjaya [...' |
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Inilah hubungan antara pemilihan semula jadi dan genetik, seperti yang diterangka oleh [[sintesis evolusi moden]]. Walaupun [[teori evolusi]] yang lengkap juga perlu menerangkan bagaimana variasi genetik timbul pada mulanya (seperti melalui [[mutasi]] dan [[pembiakan seks]]) serta merangkumi mekanisme evolusi yang lain (seperti [[aliran gen]]), namun pemilihan semula jadi masih diakui sebagai mekanisme asas evolusi.
==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.
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===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 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.)
Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can produce a continuum of possible phenotypic values.<ref>Falconer DS & Mackay TFC (1996) ''Introduction to Quantitative Genetics'' Addison Wesley Longman, Harlow, Essex, UK ISBN 0-582-24302-5</ref>
===Directionality of selection===
When some component of a trait is heritable, selection will alter the frequencies of the different alleles, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies.<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>
[[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]].
Far more common is [[stabilizing selection]] (which is commonly '''confused''' with ''purifying selection''<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>), which lowers the frequency of alleles that have a deleterious effect on the phenotype - that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Purifying selection results in functional genetic features, such as [[protein biosynthesis|protein-coding genes]] or [[regulatory sequence]]s, being [[conservation (genetics)|conserved]] over time due to selective pressure against deleterious variants.
Finally, a number of forms of [[balancing selection]] exist, which do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in [[diploid]] species (that is, those that have two pairs of [[chromosome]]s) when [[heterozygote]] individuals, who have different alleles on each chromosome at a single [[Locus (genetics)|genetic locus]], have a higher fitness than [[homozygote]] individuals that have two of the same alleles. This is called [[heterozygote advantage]] or overdominance, of which the best-known example is the [[malaria]]l resistance observed in heterozygous humans who carry only one copy of the gene for [[sickle cell anemia]]. Maintenance of allelic variation can also occur through [[disruptive selection|disruptive or diversifying selection]], which favors genotypes that depart from the average in either direction (that is, the opposite of overdominance), and can result in a [[bimodal distribution]] of trait values. Finally, balancing selection can occur through [[frequency-dependent selection]], where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of [[game theory]] have been applied to understand the fitness distributions in these situations, particularly in the study of [[kin selection]] and the evolution of [[reciprocal altruism]].<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>
===Selection and genetic variation===
A portion of all [[genetic variation]] is functionally neutral in that it produces no phenotypic effect or significant difference in fitness; the hypothesis 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====
Natural selection results in the reduction of genetic variation through the elimination of maladapted individuals and consequently of the mutations that caused the maladaptation. At the same time, new mutations occur, resulting in a [[mutation-selection balance]]. The exact outcome of the two processes depends both on the rate at which new mutations occur and on the strength of the natural selection, which is a function of how unfavorable the mutation proves to be. Consequently, changes in the mutation rate or the selection pressure will result in a different mutation-selection balance.
====Genetic linkage====
[[Genetic linkage]] occurs when the [[locus (genetics)|loci]] of two alleles are ''linked'', or in close proximity to each other on the chromosome. During the formation of [[gametes]], [[genetic recombination|recombination]] of the genetic material results in reshuffling of the alleles. However, the chance that such a reshuffle occurs between two alleles depends on the distance between those alleles; the closer the alleles are to each other, the less likely it is that such a reshuffle will occur. Consequently, when selection targets one allele, this automatically results in selection of the other allele as well; through this mechanism, selection can have a strong influence on patterns of variation in the genome.
[[Selective sweep]]s occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, linked alleles can also become more common, whether they are neutral or even slightly deleterious. This is called ''[[genetic hitchhiking]]''. A strong selective sweep results in a region of the genome where the positively selected [[haplotype]] (the allele and its neighbours) are essentially the only ones that exist in the population.
Whether a selective sweep has occurred or not can be investigated by measuring [[linkage disequilibrium]], or whether a given haplotype is overrepresented in the population. Normally, [[genetic recombination]] results in a reshuffling of the different alleles within a haplotype, and none of the haplotypes will dominate the population. However, during a selective sweep, selection for a specific allele will also result in selection of neighbouring alleles. Therefore, the presence of strong linkage disequilibrium might indicate that there has been a 'recent' selective sweep, and this can be used to identify sites recently under selection.
[[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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