विकासक्रम

जीवशास्त्रय **विकासक्रम** वा इभोलुसन धाःगु छगू प्रजातिया प्राणीया छगू पुस्तां मेगु पुस्ताय जैविक गुणय वइगु वंशानुगत हिउपा ख। थन्यागु हिउपा स्वंगु मू प्रक्रियां जुइगु या : भेरियसन (भिन्नता), प्रजनन व चयन। प्राणीइ दइगु जिन प्राणीतयेसं थःगु मस्तेत हस्तान्तरण याइ व थन्यागु जिनया अभिव्यक्तिं विकासक्रम न्ह्यथनि। थन्याःगु वंशानुगत गुण छगू जनसंख्याया दुने पाइ व प्राणीतयेगु गुणय् वंशानुगत भिन्नता दयाच्वनि। प्राणीतयेसं प्रजनन याइबिले इमिगु मचाय् मां-अबु स्वया पाःगु वा न्हुगु गुण दयेफु। थन्यागु वंशानुगत गुणय् भिन्नता निगु प्रक्रियां वयेफु- प्राणीया जिनय् म्युटेसन जुया वा जनसंख्या व प्रजातिया दथुइ जिनया बियेकाये ज्यां। मैथुनिय प्रजनन याइगु प्राणीइ जीनया न्हुगु मिश्रण आनुवंशीय पुनर्संयोजन (जेनेटिक रिकम्बिनेसनं) जुइफु गुकिलिं प्राणीतयेगु भिन्नता अप्वयका बी। सामान्य कथं थन्यागु प्रक्रियाया आवृत्ति अप्वसा वा म्हो जुसा विकासक्रम न्ह्यथनि। 

प्राणीतयेगु विकासक्रम निगु मू पद्धतिं जुइ। प्रथम पद्धति प्राकृतिक चयन ख। थ्व प्रक्रियां याना वातावरणय् म्वायेत माःगु वंशानुगत गुण दुपिं प्राणी थःगु वातावरणय् बांलाक्क म्वाइ व अप्व मचा-खाचा दयेकि। थ्व कथं थन्यागु वंशानुगत गुण दुपिं प्राणीतयेगु जनसंख्या अप्वइ धाःसा वातावरणनाप मेल मजूगु गुण दुपिं प्राणीत बांलाक्क म्वायेखनि मखु, यक्व ल्वचं कइ व प्रजनन सापेक्षिक कथं म्हो जुइगुलिं बिस्तारं लोप जुजुं वनि व इमिगु वंशानुगत गुण बिस्तार म्हो जुया वनि।  थ्व प्रक्रिया न्थ्यथंगु यक्व ई व पुस्ता धुंका अनूकुलन जुयावनि। अनूकुलन प्रक्रिया गुणय् लगातार चिधंगु अनिश्चित हिउपां न्ह्यथनि व थन्याःगु ह्युपाय् प्राकृतिक चयन जुया वातावरणय् झन् बांलाक्क म्वायेफूगु प्राणीतयेगु पुचःतयेगु जनसंख्या अप्वइ। 

विकासक्रमया मेगु मू प्रक्रिया आनुवंशिक प्रवाह (जेनेटिक ड्रिफ्ट) ख। थ्व छगू स्वतन्त्र प्रक्रिया ख गुकिलिं छगू जनसंख्याय् गुणया आवृत्तिइ अनिश्चित हिउपा हइ। प्राणीत म्वाना प्रजनन याइबिले छगू निश्चित वंशानुगत गुण इमिगु सन्ततिइ हस्तान्तरण जुइगु संभावताया लिच्वया कथं आनुवंशिक प्रवाह जुइ। छगू पुस्ताय् प्रहावय् वैगु हिउपा म्हो जुसां पुस्तौं-पुस्ताय् थन्यागु हे म्हो हिउपा मुना वं-वं यक्व समय धुंका प्राणीइ तधंगु हिउपा वै। थन्यागु हिउपाया मंकां प्राणीया न्हुगु प्रजाति दया वै।  आ तक्क लूगु सकल प्राणीतयेगु संरचनाया अध्ययनं सकल प्राणी छगू हे प्रकारया प्राणीं उत्त्पत्ति जूया आनुवंशिक प्रवाहया कारणं भिन्नता दयावःगु धैगु खं क्यं।   

विकासक्रमीय जीवविज्ञानया विज्ञतयेसं विकासक्रमया तथ्यात्मक दस्ताबेज दयेकिगु व थन्यागु तथ्यया कारण व्याख्या याइगु सिद्धान्ततयेगु परिक्षण याइगु ज्या याइ। विकासक्रमीय जीवशास्त्रया सीकेज्या मध्य-१९गु शताब्दीइ न्ह्यथंगु ख। थ्व ईले जीवाश्म (fossil)तयेगु अध्ययन व जैविक विविधताया सीकेज्यां यक्व वैज्ञानिकतयेत प्रजातित समयनाप हिलावनि धैगु खंया छुमा बिल।  तर प्रजातितयेगु भिन्नता व हिलावनिगु प्रक्रियाया बारेय् छुं विचा न्ह्यने वःसां मूलप्रवाह वैज्ञानिकतयेगु तर्क व प्रयोगं अन्यागु विचातयेत गलत प्रमाणित यानाबिल। सन् १८५९इ स्व॰ चार्ल्स डार्विनया अन द ओरिजिन अफ स्पिसिज नांया सफू पिथन व् अथ्व सफूलि विकासक्रम प्राकृतिक चयनं जुइगु विचा न्ह्यथन।  डार्विनया विचायात तत्कालीन वैज्ञानिक समुदायं अत्याधिक रुपय् समर्थन यात।  सन् १९३०या दशकय् डार्विनया प्राकृतिक चयनया सिद्धान्तयात ग्रेगर मेण्डलया इन्हेरिटेन्स सिद्धान्त नाप स्वाना आधुनिक विकासक्रमीय सिन्थेसिस दयेकेज्या जुल गुकिलिं विकासक्रमया इकाइ (जीन)यात विकासक्रमया प्रकृया (प्राकृतिक चयन)नाप स्वानाबिल। थ्व शक्तिशाली व्याख्यात्मक व नि‍र्धारात्मक सिद्धान्तं विकासक्रमया मालेज्यायात निर्देशित याइगु जक मखुसें आधुनिक जीवशास्त्रया जैविक विविधताया केन्द्रिय संयोजक सिद्धान्तया रुपय् पलिस्था जुया च्वंगुदु। 

## वंशानुगत
[thumb|right|200px|डिएनए संरचना। दथुइ [न्युक्लियोबेस|बेस व फस्फेट सुगर चेन सिथय् जाना डबल हेलिक्स दयेकी।]]
प्राणीइ इभोलुसन वंशानुगत गुणय् वइगु हिउपां जुइ। दसुया निंतिं मनुया मिखाया रंग छगू वंशानुगत गुण ख। थ्व गुण मनुतयेसं थःगु मां-अबुया गुणं इन्हेरिट याइ। इन्हेरिट याःगु गुणत जीनया नियन्त्रणय् दै व छगू प्राणीय् दइगु सकल जीनोमया सेटयात प्राणीया जिनोटाइप धाइ। 

प्राणीया संरचना व व्यवहार दयेकिगु व खनेदैगु गुणतयेगु छगू पूर्ण सेटयात प्राणीया फेनोटाइप धाइ। थन्याःगु गुण प्राणीया वारावरण नापया अन्तरक्रियां निर्माण जुइ।  मेगु कथं धायेगु खःसा प्राणीया एक्स्प्रेस जूगु जीनतयेगु पुचःया पूवंगु सेट हे प्राणीया फेनोटाइप ख। फेनोटाइप वातावरण कथं पाइगु जूगुलिं सकल प्राणीतयेगु फेनोटाइप पाइ। दसुया निंतिं निभाले च्वना हाकुइगु धाःगु निभाया जलं मनुया छ्यंगुलि लाइबिले मनुया छ्यंगुलि दइगु मेलानोसाइटतयेसं मेलानिन धाःगु हाकूगु पदार्थ पिकाना जुइगु प्रक्रिया ख। छगू हे जीन दुपिं मनुत (दसु आइडेन्टिकल ट्विन्स)नं छम्ह निभाले जक्क च्वन व मेम्ह निभा मदूगु थासय् च्वन धाःसा (मेलानिन दयेकेज्याय् छुं व्याधि मदुसा) निभाले च्वनिम्ह जुडुवा, छगू हे जीन दसां मेम्ह स्वया हाकुइ। अथे जुसां हानं निभाया गुलि जलं मनु गुलि हाकुइगु धइगु परिमाण धाःसा हानं मनुया जीनं हे निर्धारण याइ। दसुया निंतिं छुं मनुइ मेलानोसाइटं मेलानिन हे दयेकिमख्। थन्यापिं मनुतयेसं अल्बिनिजम धाःगु गुण इन्हेरिट याइ। थन्याःगु गुण इन्हेरिट यापिं मनुतयेगु छ्यंगु न्ह्याक्वः निभाले च्वंसा भुइसे हे च्वनि। मेलानिन मदूगुलिं जःया विकिरणं याइगु विध्वंसात्मक प्रक्रियां थन्यापिं मनुतयेत अप्व असर याइ। 

वंशानुगत गुण छगू पुस्तां मेगु पुस्ताय् डिएनए धाःगु छगू मोलेक्युलया मार्फतं थ्यनि। थ्व मोलेक्युलय् जेनेटिक सूचंया सूत्र स्वथनाच्वंगु दै।  डिएनए धाःगु छगू पोलिमर ख। थ्व पोलिमरय् प्यंगु प्रकारया बेस दै। डिएनएय् दैगु बेसतयेगु सिक्वेन्सं जेनेटिक सूचं निर्धारण याइ। थ्व प्रक्रिया छगू खँग्वल्य् आखःया सिक्वेन्सं खँग्वःया अर्थ निर्धारण याइगु थें हे ख। छगू फंक्सनल युनिट दयेकिगु डिएनए मोलेक्युलया सिक्वेन्सयात जीन धाइ। थी-थी जीनया थी-थी बेस सिक्वेन्स दै। सेल बायोलोजीइ डिएनएया छगू ताःहाकःगु कन्डेन्स्ड संरचनायात क्रोमोजम धाइ। छगू क्रोमोजमया निश्चित थाय्‌यात लोकस धाइ। छगू लोकसय् डिएनएया सिक्वेन्स मनुदथुइ पाःसा उकियात एलिल धाइ। अतः, एलिल धाःगु क्रोमोजमया छगू हे निश्चित थासय् दइगु व छगू हे ज्या याइगु जीन (दसु मिखाया रंग बीगु जीन)या थी-थी प्रकार ख। डिएनए श्रृङ्खलाय् म्युटेसन जुया एलिलय् हिउपा वयेफु। थथे म्युटेसन वःसा न्हुगु एलिलं प्राणीया फेनोटाइप वंशानुगत फेनोटाइप स्वया हिलाछ्वये फु।  

एलिल व गुणया थ्व साधारण अन्तरक्रिया छुं थासय् पाय्‌छि जुसां यक्व गुण थ्व साधारण अन्तरक्रिया स्वया जटिल प्रक्रियां यक्व जीनया अन्तरक्रियां निर्धारण जुइ व सकल जीनया छुं भूमिका दःसां अन्ततः वैगु गुण धाःसा सकलया अन्तरक्रियाया लिच्वः जुइ।  थन्याःगु जटिल गुणतयेगु सीकेज्या धाःसा आःया समयया जेनेटिक मालेज्याया छगू मू ख्यः ख। जेनेटिक्सया छगू मेगु आजुचाइगु ख्यः इपिजेनेटिक्स ख गुकिलि वंशानुगत गुणय् जीनय् हिउपा मवेकःहे प्राणीया वंशानुगत गुणय् हिउपा वै। थ्व ख्यः नं जेनेटिक्सया छगू मू मालेज्याया ख्यः ख। 

## भिन्नता
छम्ह प्राणीया फेनोटाइप प्राणीया जेनोटाइप व वातावरणया अन्तरक्रियाया लिच्वः ख। फेनोटाइपया भिन्नता आपालं जेनोटाइपया भिन्नताया लिच्व ख।  आधुनिक विकासक्रम संश्लेषणया परिभाषा कथं विकासक्रम वा इभोलुसन धाःगु जेनेटिक भिन्नताय् समयनाप वइगु हिउपा ख। छगू प्रकारया एलिलया फ्रिक्वेन्सी समय नाप पाना वनि व मेमेगु एलिलया तुलनाय् सापेक्षिक रुपय् वातावरणनापया अन्तरक्रियाया लिच्वःया कथं पाइ। विकासक्रम थ्व हे एलिलया फ्रिक्वेन्सीया हिउपा व म्युटेसनया आधारय् न्ह्यने बढेजुइ। भिन्नता सामान्यतया उपस्थित जुसां छगू एलिल फिक्सेसनया फुतिइ थ्यन वा छगू एलिल बाहेक मेगु एलिल दक्वं नास जुयावन धाःसा तनावनि। 

भिन्नता आनुवंशिक पदार्थय् (genetic material) म्युटेसन, प्राणीया जनसंख्याया स्थानान्तरणं जुइगु जीन बहाव (gene flow) व मैथुनिक प्रजननं जुइगु जिनया फेरबदल (reshuffling)या लिच्वःया कथं वै। भिन्नता, विशेषयाना एककोशिक प्राणीइ थी-थी प्राणीतयेगु दथुइ जुइगु जीन हिजेज्यां नं वै। दसु ब्याक्टेरियाय् जुइगु क्षैतिज जिन प्रदान (horizontal gene transfer) व स्वांमाय् याइगु वर्ण संकरन प्रक्रिया (hybridization)।  थन्यागु प्रविधिं निरन्तर रुपय् भिन्नता हयातःसां छगू प्रजातिया आपालं जेनोम व हे प्रजातिया मेमेपिं प्राणीतनाप मेल न।  नापं, जेनोटाइपय् म्हो जक्क हिउपा वःसां फेनोटाइपय् नाटकीय रुपान्तरण जुइ। दसुया निंतिं चिम्पाञ्जी व मनुया जेनोम ५% जक्क पा तर चिम्पाञ्जी व मनुतयेगु फेनोटाइप यक्व हे पा। 

### म्युतेसन
thumb|100px|left|क्रोमोजमया छगू भागया दुप्लिकेसन।
जेनेतिक भिन्न्ता प्राणीतयेगु जेनोमय् जुइगु अक्रमिक (random) म्युतेसनया लिच्वः ख। म्युतेसन धाःगु थी-थी कारणं (दसु- विकिरण, भाइरस, त्रान्स्पोजोन, म्युताजेनिक रसायन, मायोसिस व दिएनए रेप्लिकेसनय् जुइगु गलती आदि) प्राणीया दिएनए सिक्वेन्सय् वैगु परिवर्तन ख।  म्युतेसन याकिगु तत्त्वयात म्युताजेन धाइ। थन्यागु म्युतेसनं दिएनए सिक्वेन्सय् थी-थी कथंया परिवर्तन हयेफु। म्युताजेन्सं छुं असर मयायेगु, जीनया उत्पादयात स्यंकिगु, जीनयात ज्या यायेमबिगु आदि हिउपा हयेफु। म्युतेसनं आपालं ध्वंसात्मक हिउपा हैगुलिं प्राणीतयेसं म्युतेसन म्हो यायेत थी-थी विकासक्रमात्मक पद्दति दयेकावःगु दु।  अतः, म्युतेसन दर म्युतेसन याइगु तत्त्व व म्युतेसन पनिगु तत्त्वया लिच्वलं निर्धारण याइ।  छुं प्रजाति दसु रेर्तोभाइरसय् यक्व म्युतेसन खनेदु व अन्यागु प्राणीतयेगु यक्व मचातयेके म्युतेतेद जिन दैगु या।  थन्यागु तच्वः गतिया म्युतेसन थ्व प्राणीया निंतिं लाभात्मक जु छाय्धाःसा थ्व प्राणीं त्च्वःगु गतिं विकशित जुया मनुया प्रतिरोधात्मक व्यवस्थाया कारवाहीयात पनेफु व थन्यागु त्च्वःगु म्युतेसन मदूगु जुसा थ्व प्राणी लोप जुइत थाकू मजु।

म्युतेसन दुप्लिकेसन जुयाच्वंगु दिएनएया तःधंगु खण्डय् जुइफु गुकिलिं विकासक्रमय् न्हुगु जिन उत्पादन यायेत ग्वहालि याइ। Most genes belong to larger families of genes of shared ancestry. Novel genes are produced by several methods, commonly through the duplication and mutation of an ancestral gene, or by recombining parts of different genes to form new combinations with new functions. Here, domains act as modules, each with a particular and independent function, that can be mixed together to produce genes encoding new proteins with novel properties. For example, the human eye uses four genes to make structures that sense light: three for color vision and one for night vision; all four arose from a single ancestral gene. Another advantage of duplicating a gene (or even an entire genome) is that overlapping or redundant functions in multiple genes allows alleles to be retained that would otherwise be harmful, thus increasing genetic diversity.

Changes in chromosome number may involve even larger mutations, where segments of the DNA within chromosomes break and then rearrange. For example, two chromosomes in the ''Homo'' genus fused to produce human chromosome 2; this fusion did not occur in the lineage of the other apes, and they retain these separate chromosomes. In evolution, the most important role of such chromosomal rearrangements may be to accelerate the divergence of a population into new species by making populations less likely to interbreed, and thereby preserving genetic differences between these populations.

Sequences of DNA that can move about the genome, such as transposons, make up a major fraction of the genetic material of plants and animals, and may have been important in the evolution of genomes. For example, more than a million copies of the Alu sequence are present in the human genome, and these sequences have now been recruited to perform functions such as regulating gene expression. Another effect of these mobile DNA sequences is that when they move within a genome, they can mutate or delete existing genes and thereby produce genetic diversity.

### Sex and recombination

In asexual organisms, genes are inherited together, or *linked*, as they cannot mix with genes in other organisms during reproduction. However, the offspring of sexual organisms contain random mixtures of their parents' chromosomes that are produced through independent assortment. In the related process of genetic recombination, sexual organisms can also exchange DNA between two matching chromosomes. Recombination and reassortment do not alter allele frequencies, but instead change which alleles are associated with each other, producing offspring with new combinations of alleles. While this process increases the variation in any individual's offspring, genetic mixing can be predicted to either have no effect, increase, or decrease the genetic variation in the population, depending on how the various alleles in the population are distributed. For example, if two alleles are randomly distributed in a population, then sex will have no effect on variation; however, if two alleles tend to be found as a pair, then genetic mixing will even out this non-random distribution and over time make the organisms in the population more similar to each other. The overall effect of sex on natural variation remains unclear, but recent research suggests that sex usually increases genetic variation and may increase the rate of evolution.

Recombination allows even alleles that are close together in a strand of DNA to be inherited independently. However, the rate of recombination is low, since in humans in a stretch of DNA one million base pairs long there is about a one in a hundred chance of a recombination event occurring per generation. As a result, genes close together on a chromosome may not always be shuffled away from each other, and genes that are close together tend to be inherited together. This tendency is measured by finding how often two alleles of different genes occur together, which is called their linkage disequilibrium. A set of alleles that is usually inherited in a group is called a haplotype.

Sexual reproduction helps to remove harmful mutations and retain beneficial mutations. Consequently, when alleles cannot be separated by recombination – such as in mammalian Y chromosomes, which pass intact from fathers to sons – harmful mutations accumulate. In addition, recombination and reassortment can produce individuals with new and advantageous gene combinations. These positive effects are balanced by the fact that this process can cause mutations and separate beneficial combinations of genes.

### Population genetics

From a genetic viewpoint, evolution is a *generation-to-generation change in the frequencies of alleles within a population that shares a common gene pool*. A population is a localized group of individuals belonging to the same species. For example, all of the moths of the same species living in an isolated forest represent a population. A single gene in this population may have several alternate forms, which account for variations between the phenotypes of the organisms. An example might be a gene for coloration in moths that has two alleles: black and white. A gene pool is the complete set of alleles in a single population, so each allele occurs a certain number of times in a gene pool. The fraction of genes within the gene pool that are a particular allele is called the allele frequency. Evolution occurs when there are changes in the frequencies of alleles within a population of interbreeding organisms; for example the allele for black color in a population of moths becoming more common.

To understand the mechanisms that cause a population to evolve, it is useful to consider what conditions are required for a population not to evolve. The *[[Hardy-Weinberg principle]]* states that the frequencies of alleles (variations in a gene) in a sufficiently large population will remain constant if the only forces acting on that population are the random reshuffling of alleles during the formation of the sperm or egg, and the random combination of the alleles in these sex cells during fertilization. Such a population is said to be in *Hardy-Weinberg equilibrium* - it is not evolving.

## Mechanisms
The three main mechanisms that produce evolution are natural selection, genetic drift, and gene flow. Natural selection favors genes that improve capacity for survival and reproduction. Genetic drift is random change in the frequency of alleles, caused by the random sampling of a generation's genes during reproduction. Gene flow is the transfer of genes within and between populations. The relative importance of natural selection and genetic drift in a population varies depending on the strength of the selection and the effective population size, which is the number of individuals capable of breeding. Natural selection usually predominates in large populations, while genetic drift dominates in small populations. The dominance of genetic drift in small populations can even lead to the fixation of slightly deleterious mutations. As a result, changing population size can dramatically influence the course of evolution. Population bottlenecks, where the population shrinks temporarily and therefore loses genetic variation, result in a more uniform population. Bottlenecks also result from alterations in gene flow such as decreased migration, expansions into new habitats, or population subdivision.

### Natural selection

[thumb|right|300px|[Natural selection of a population for dark coloration.]]

Natural selection is the process by which genetic mutations that enhance reproduction become, and remain, more common in successive generations of a population. It has often been called a "self-evident" mechanism because it necessarily follows from three simple facts:
* Heritable variation exists within populations of organisms.
* Organisms produce more offspring than can survive.
* These offspring vary in their ability to survive and reproduce.

These conditions produce competition between organisms for survival and reproduction. Consequently, organisms with traits that give them an advantage over their competitors pass these advantageous traits on, while traits that do not confer an advantage are not passed on to the next generation.

The central concept of natural selection is the evolutionary fitness of an organism. This measures the organism's genetic contribution to the next generation. However, this is not the same as the total number of offspring: instead fitness measures the proportion of subsequent generations that carry an organism's genes. Consequently, if an allele increases fitness more than the other alleles of that gene, then with each generation this allele will become more common within the population. These traits are said to be "selected *for*". Examples of traits that can increase fitness are enhanced survival, and increased fecundity. Conversely, the lower fitness caused by having a less beneficial or deleterious allele results in this allele becoming rarer — they are "selected *against*". Importantly, the fitness of an allele is not a fixed characteristic, if the environment changes, previously neutral or harmful traits may become beneficial and previously beneficial traits become harmful.

Natural selection within a population for a trait that can vary across a range of values, such as height, can be categorized into three different types. The first is directional selection, which is a shift in the average value of a trait over time — for example organisms slowly getting taller. Secondly, disruptive selection is selection for extreme trait values and often results in two different values becoming most common, with selection against the average value. This would be when either short or tall organisms had an advantage, but not those of medium height. Finally, in stabilizing selection there is selection against extreme trait values on both ends, which causes a decrease in variance around the average value and less diversity. This would, for example, cause organisms to slowly become all the same height.

A special case of natural selection is sexual selection, which is selection for any trait that increases mating success by increasing the attractiveness of an organism to potential mates. Traits that evolved through sexual selection are particularly prominent in males of some animal species, despite traits such as cumbersome antlers, mating calls or bright colors that attract predators, decreasing the survival of individual males. This survival disadvantage is balanced by higher reproductive success in males that show these hard to fake, sexually selected traits.

Natural selection most generally makes nature the measure against which individuals, and individual traits, are more or less likely to survive.  "Nature" in this sense refers to an ecosystem, that is, a system in which organisms interact with every other element, physical as well as biological, in their local environment. Eugene Odum, a founder of ecology, defined an ecosystem as: "Any unit that includes all of the organisms...in a given area interacting with the physical environment so that a flow of energy leads to clearly defined trophic structure, biotic diversity, and material cycles (ie: exchange of materials between living and nonliving parts) within the system."  Each population within an ecosystem occupies a distinct niche, or position, with distinct relationships to other parts of the system.  These relationships involve the life history of the organism, its position in the food chain, and its geographic range. This broad understanding of nature enables scientists to delineate specific forces which, together, comprise natural selection. 

An active area of research is the unit of selection, with natural selection being proposed to work at the level of genes, cells, individual organisms, groups of organisms and even species. None of these are mutually exclusive and selection may act on multiple levels simultaneously. An example of selection occurring below the level of the individual organism are genes called transposons, which can replicate and spread throughout a genome. Selection at a level above the individual, such as group selection, may allow the evolution of co-operation, as discussed below.

### Genetic drift

[thumb|right|250px| Simulation of [genetic drift of 20 unlinked alleles in populations of 10 (top) and 100 (bottom). Drift to fixation is more rapid in the smaller population.]]
Genetic drift is the change in allele frequency from one generation to the next that occurs because alleles in offspring are a random sample of those in the parents, as well as from the role that chance plays in determining whether a given individual will survive and reproduce. In mathematical terms, alleles are subject to sampling error. As a result, when selective forces are absent or relatively weak, allele frequencies tend to "drift" upward or downward randomly (in a random walk). This drift halts when an allele eventually becomes fixed, either by disappearing from the population, or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone. Even in the absence of selective forces, genetic drift can cause two separate populations that began with the same genetic structure to drift apart into two divergent populations with different sets of alleles. 

The time for an allele to become fixed by genetic drift depends on population size, with fixation occurring more rapidly in smaller populations. The precise measure of population that is important is called the effective population size. The effective population is always smaller than the total population since it takes into account factors such as the level of inbreeding, the number of animals that are too old or young to breed, and the lower probability of animals that live far apart managing to mate with each other.

Although natural selection is responsible for adaptation, the relative importance of the two forces of natural selection and genetic drift in driving evolutionary change in general is an area of current research in evolutionary biology. These investigations were prompted by the neutral theory of molecular evolution, which proposed that most evolutionary changes are the result of the fixation of neutral mutations that do not have any immediate effects on the fitness of an organism. Hence, in this model, most genetic changes in a population are the result of constant mutation pressure and genetic drift. This form of the neutral theory is now largely abandoned, since it does not seem to fit the genetic variation seen in nature. However, a more recent and better-supported version of this model is the nearly-neutral theory, where most mutations only have small effects on fitness.

### Gene flow

[250px|thumb|left|Male [lions leave the pride where they are born and take over a new pride to mate. This results in gene flow between prides.]]
Gene flow is the exchange of genes between populations, which are usually of the same species. Examples of gene flow within a species include the migration and then breeding of organisms, or the exchange of pollen. Gene transfer between species includes the formation of hybrid organisms and horizontal gene transfer.

Migration into or out of a population can change allele frequencies, as well as introducing genetic variation into a population. Immigration may add new genetic material to the established gene pool of a population. Conversely, emigration may remove genetic material. As barriers to reproduction between two diverging populations are required for the populations to become new species, gene flow may slow this process by spreading genetic differences between the populations. Gene flow is hindered by mountain ranges, oceans and deserts or even man-made structures such as the Great Wall of China, which has hindered the flow of plant genes.

Depending on how far two species have diverged since their most recent common ancestor, it may still be possible for them to produce offspring, as with horses and donkeys mating to produce mules. Such hybrids are generally infertile, due to the two different sets of chromosomes being unable to pair up during meiosis. In this case, closely related species may regularly interbreed, but hybrids will be selected against and the species will remain distinct. However, viable hybrids are occasionally formed and these new species can either have properties intermediate between their parent species, or possess a totally new phenotype. The importance of hybridization in creating new species of animals is unclear, although cases have been seen in many types of animals, with the gray tree frog being a particularly well-studied example. 

Hybridization is, however, an important means of speciation in plants, since polyploidy (having more than two copies of each chromosome) is tolerated in plants more readily than in animals. Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis. Polyploids also have more genetic diversity, which allows them to avoid inbreeding depression in small populations.

Horizontal gene transfer is the transfer of genetic material from one organism to another organism that is not its offspring; this is most common among bacteria. In medicine, this contributes to the spread of antibiotic resistance, as when one bacteria acquires resistance genes it can rapidly transfer them to other species. Horizontal transfer of genes from bacteria to eukaryotes such as the yeast *[[Saccharomyces cerevisiae]]* and the adzuki bean beetle *Callosobruchus chinensis* may also have occurred. An example of larger-scale transfers are the eukaryotic bdelloid rotifers, which appear to have  received a range of genes from bacteria, fungi, and plants. Viruses can also carry DNA between organisms, allowing transfer of genes even across biological domains. Large-scale gene transfer has also occurred between the ancestors of eukaryotic cells and prokaryotes, during the acquisition of chloroplasts and mitochondria.

## Outcomes
Evolution influences every aspect of the form and behavior of organisms. Most prominent are the specific behavioral and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by co-operating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.

These outcomes of evolution are sometimes divided into macroevolution, which is evolution that occurs at or above the level of species, such as extinction and speciation, and microevolution, which is smaller evolutionary changes, such as adaptations, within a species or population. In general, macroevolution is regarded as the outcome of long periods of microevolution. Thus, the distinction between micro- and macroevolution is not a fundamental one - the difference is simply the time involved. However, in macroevolution, the traits of the entire species may be important. For instance, a large amount of variation among individuals allows a species to rapidly adapt to new habitats, lessening the chance of it going extinct, while a wide geographic range increases the chance of speciation, by making it more likely that part of the population will become isolated. In this sense, microevolution and macroevolution might involve selection at different levels - with microevolution acting on genes and organisms, versus macroevolutionary processes acting on entire species and affecting the rate of speciation and extinction.

A common misconception is that evolution is "progressive," but natural selection has no long-term goal and does not necessarily produce greater complexity. Although complex species have evolved, this occurs as a side effect of the overall number of organisms increasing, and simple forms of life remain more common. For example, the overwhelming majority of species are microscopic prokaryotes, which form about half the world's biomass despite their small size, and constitute the vast majority of Earth's biodiversity. Simple organisms have therefore been the dominant form of life on Earth throughout its history and continue to be the main form of life up to the present day, with complex life only appearing more diverse because it is more noticeable.

### Adaptation

Adaptations are structures or behaviors that enhance a specific function, causing organisms to become better at surviving and reproducing. They are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment. This process can cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with genetic changes causing antibiotic resistance by both modifying the target of the drug, or increasing the activity of transporters that pump the drug out of the cell. Other striking examples are the bacteria *[[Escherichia coli]]* evolving the ability to use citric acid as a nutrient in a long-term laboratory experiment, *[[Flavobacterium]]* evolving a novel enzyme that allows these bacteria to grow on the by-products of nylon manufacturing, and the soil bacterium *[[Sphingobium]]* evolving an entirely new metabolic pathway that degrades the synthetic pesticide pentachlorophenol.

However, many traits that appear to be simple adaptations are in fact exaptations: structures originally adapted for one function, but which coincidentally became somewhat useful for some other function in the process. One example is the African lizard *Holaspis guentheri*, which developed an extremely flat head for hiding in crevices, as can be seen by looking at its near relatives. However, in this species, the head has become so flattened that it assists in gliding from tree to tree—an exaptation. Another is the recruitment of enzymes from glycolysis and xenobiotic metabolism to serve as structural proteins called crystallins within the lenses of organisms' eyes.

[350px|thumb|right|A [baleen whale skeleton, *a* and *b* label flipper bones, which were adapted from front leg bones: while *c* indicates vestigial leg bones.]]
As adaptation occurs through the gradual modification of existing structures, structures with similar internal organization may have very different functions in related organisms. This is the result of a single ancestral structure being adapted to function in different ways. The bones within bat wings, for example, are structurally similar to both human hands and seal flippers, due to the common descent of these structures from an ancestor that also had five digits at the end of each forelimb. Other idiosyncratic anatomical features, such as bones in the wrist of the panda being formed into a false "thumb," indicate that an organism's evolutionary lineage can limit what adaptations are possible.

During adaptation, some structures may lose their original function and become vestigial structures. Such structures may have little or no function in a current species, yet have a clear function in ancestral species, or other closely related species. Examples include pseudogenes, the non-functional remains of eyes in blind cave-dwelling fish, wings in flightless birds, and the presence of hip bones in whales and snakes. Examples of vestigial structures in humans include wisdom teeth, the coccyx, and the vermiform appendix.

A critical principle of ecology is that of competitive exclusion: no two species can occupy the same niche in the same environment for a long time. Consequently, natural selection will tend to force species to adapt to different ecological niches. This may mean that, for example, two species of cichlid fish adapt to live in different habitats, which will minimize the competition between them for food. 

An area of current investigation in evolutionary developmental biology is the developmental basis of adaptations and exaptations. This research addresses the origin and evolution of embryonic development and how modifications of development and developmental processes produce novel features. These studies have shown that evolution can alter development to create new structures, such as embryonic bone structures that develop into the jaw in other animals instead forming part of the middle ear in mammals. It is also possible for structures that have been lost in evolution to reappear due to changes in developmental genes, such as a mutation in chickens causing embryos to grow teeth similar to those of crocodiles. It is now becoming clear that most alterations in the form of organisms are due to changes in the level and timing of the expression of a small set of conserved genes.

### Co-evolution

Interactions between organisms can produce both conflict and co-operation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. Here, the evolution of one species causes adaptations in a second species. These changes in the second species then, in turn, cause new adaptations in the first species. This cycle of selection and response is called co-evolution. An example is the production of tetrodotoxin in the rough-skinned newt and the evolution of tetrodotoxin resistance in its predator, the common garter snake. In this predator-prey pair, an evolutionary arms race has produced high levels of toxin in the newt and correspondingly high levels of resistance in the snake.

### Co-operation

However, not all interactions between species involve conflict. Many cases of mutually beneficial interactions have evolved. For instance, an extreme cooperation exists between plants and the mycorrhizal fungi that grow on their roots and aid the plant in absorbing nutrients from the soil. This is a reciprocal relationship as the plants provide the fungi with sugars from photosynthesis. Here, the fungi actually grow inside plant cells, allowing them to exchange nutrients with their hosts, while sending signals that suppress the plant immune system.

Coalitions between organisms of the same species have also evolved. An extreme case is the eusociality found in social insects, such as bees, termites and ants, where sterile insects feed and guard the small number of organisms in a colony that are able to reproduce. On an even smaller scale, the somatic cells that make up the body of an animal limit their reproduction so they can maintain a stable organism, which then supports a small number of the animal's germ cells to produce offspring. Here, somatic cells respond to specific signals that instruct them whether to grow, remain as they are, or die. If cells ignore these signals and multiply inappropriately, their uncontrolled growth causes cancer.

These examples of cooperation within species are thought to have evolved through the process of kin selection, which is where one organism acts to help raise a relative's offspring. This activity is selected for because if the *helping* individual contains alleles which promote the helping activity, it is likely that its kin will *also* contain these alleles and thus those alleles will be passed on. Other processes that may promote cooperation include group selection, where cooperation provides benefits to a group of organisms.

### Speciation

[left|thumb|350px|The four mechanisms of [speciation.]]
Speciation is the process where a species diverges into two or more descendant species.  Evolutionary biologists view species as statistical phenomena and not categories or types. This view is counterintuitive since the classical idea of species is still widely-held, with a species seen as a class of organisms exemplified by a "type specimen" that bears all the traits common to this species. Instead, a species is now defined as a separately evolving lineage that forms a single gene pool. Although properties such as genetics and morphology are used to help separate closely-related lineages, this definition has fuzzy boundaries. However, the exact definition of the term "species" is still controversial, particularly in prokaryotes, and this is called the species problem. Biologists have proposed a range of more precise definitions, but the definition used is a pragmatic choice that depends on the particularities of the species concerned. Typically the actual focus on biological study is the population, an observable *interacting* group of organisms, rather than a species, an observable *similar* group of individuals.

Speciation has been observed multiple times under both controlled laboratory conditions and in nature. In sexually reproducing organisms, speciation results from reproductive isolation followed by genealogical divergence. There are four mechanisms for speciation. The most common in animals is allopatric speciation, which occurs in populations initially isolated geographically, such as by habitat fragmentation or migration. Selection under these conditions can produce very rapid changes in the appearance and behaviour of organisms. As selection and drift act independently on populations isolated from the rest of their species, separation may eventually produce organisms that cannot interbreed. 

The second mechanism of speciation is peripatric speciation, which occurs when small populations of organisms become isolated in a new environment. This differs from allopatric speciation in that the isolated populations are numerically much smaller than the parental population. Here, the founder effect causes rapid speciation through both rapid genetic drift and selection on a small gene pool.

The third mechanism of speciation is parapatric speciation. This is similar to peripatric speciation in that a small population enters a new habitat, but differs in that there is no physical separation between these two populations. Instead, speciation results from the evolution of mechanisms that reduce gene flow between the two populations. Generally this occurs when there has been a drastic change in the environment within the parental species' habitat. One example is the grass *[[Anthoxanthum|Anthoxanthum odoratum]]*, which can undergo parapatric speciation in response to localized metal pollution from mines. Here, plants evolve that have resistance to high levels of metals in the soil. Selection against interbreeding with the metal-sensitive parental population produces a change in flowering time of the metal-resistant plants, causing reproductive isolation. Selection against hybrids between the two populations may cause *reinforcement*, which is the evolution of traits that promote mating within a species, as well as character displacement, which is when two species become more distinct in appearance.
[frame|right|[Geographical isolation of finches on the Galápagos Islands produced over a dozen new species.]]

Finally, in sympatric speciation species diverge without geographic isolation or changes in habitat. This form is rare since even a small amount of gene flow may remove genetic differences between parts of a population. Generally, sympatric speciation in animals requires the evolution of both genetic differences and non-random mating, to allow reproductive isolation to evolve.

One type of sympatric speciation involves cross-breeding of two related species to produce a new hybrid species. This is not common in animals as animal hybrids are usually sterile. This is because during meiosis the homologous chromosomes from each parent are from different species and cannot successfully pair. However, it is more common in plants because plants often double their number of chromosomes, to form polyploids. This allows the chromosomes from each parental species to form a matching pair during meiosis, since as each parent's chromosomes is represented by a pair already. An example of such a speciation event is when the plant species *[[Arabidopsis thaliana]]* and *Arabidopsis arenosa* cross-bred to give the new species *Arabidopsis suecica*. This happened about 20,000 years ago, and the speciation process has been repeated in the laboratory, which allows the study of the genetic mechanisms involved in this process. Indeed, chromosome doubling within a species may be a common cause of reproductive isolation, as half the doubled chromosomes will be unmatched when breeding with undoubled organisms.

Speciation events are important in the theory of punctuated equilibrium, which accounts for the pattern in the fossil record of short "bursts" of evolution interspersed with relatively long periods of stasis, where species remain relatively unchanged. In this theory, speciation and rapid evolution are linked, with natural selection and genetic drift acting most strongly on organisms undergoing speciation in novel habitats or small populations. As a result, the periods of stasis in the fossil record correspond to the parental population, and the organisms undergoing speciation and rapid evolution are found in small populations or geographically restricted habitats, and therefore rarely being preserved as fossils.

### Extinction

[thumb|left|225px|''[Tarbosaurus* fossil. Non-[[bird|avian]] [[dinosaur]]s died out in the [[Cretaceous–Tertiary extinction event]] at the end of the [[Cretaceous]] period.]]
[[Extinction]] is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation, and disappear through extinction. Indeed, virtually all animal and plant species that have lived on earth are now extinct, and extinction appears to be the ultimate fate of all species. These extinctions have happened continuously throughout the history of life, although the rate of extinction spikes in occasional mass [[extinction event]]s. The [[Cretaceous–Tertiary extinction event]], during which the dinosaurs went extinct, is the most well-known, but the earlier [[Permian–Triassic extinction event]] was even more severe, with approximately 96 percent of species driven to extinction. The [[Holocene extinction event]] is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years. Present-day extinction rates are 100-1000 times greater than the background rate, and up to 30 percent of species may be extinct by the mid 21st century. Human activities are now the primary cause of the ongoing extinction event; [[global warming]] may further accelerate it in the future.

The role of extinction in evolution depends on which type is considered. The causes of the continuous "low-level" extinction events, which form the majority of extinctions, are not well understood and may be the result of competition between species for limited resources ([[competitive exclusion]]). If competition from other species does alter the probability that a species will become extinct, this could produce [[Unit of selection#Species selection and selection at higher taxonomic levels|species selection]] as a level of natural selection. The intermittent mass extinctions are also important, but instead of acting as a selective force, they drastically reduce diversity in a nonspecific manner and promote bursts of [[Adaptive radiation|rapid evolution]] and speciation in survivors.

== Evolutionary history of life ==


=== Origin of life ===

The origin of [[life]] is a necessary precursor for biological evolution, but understanding that evolution occurred once organisms appeared and investigating how this happens does not depend on understanding exactly how life began. The current [[scientific consensus]] is that the complex [[biochemistry]] that makes up life came from simpler chemical reactions, but it is unclear how this occurred. Not much is certain about the earliest developments in life, the structure of the first living things, or the identity and nature of any [[last universal ancestor|last universal common ancestor]] or ancestral gene pool. Consequently, there is no scientific consensus on how life began, but proposals include self-replicating molecules such as [[RNA]], and the assembly of simple cells.

=== Common descent ===

[[किपा:Ape skeletons.png|right|320px|thumbnail|The [[Ape|hominoids]] are descendants of a [[common descent|common ancestor]].]]
All [[organism]]s on [[Earth]] are descended from a common ancestor or ancestral gene pool. Current species are a stage in the process of evolution, with their diversity the product of a long series of speciation and extinction events. The [[common descent]] of organisms was first deduced from four simple facts about organisms: First, they have geographic distributions that cannot be explained by local adaptation. Second, the diversity of life is not a set of completely unique organisms, but organisms that share morphological similarities. Third, vestigial traits with no clear purpose resemble functional ancestral traits, and finally, that organisms can be classified using these similarities into a hierarchy of nested groups - similar to a family tree. However, modern research has suggested that, due to horizontal gene transfer, this "[[Tree of life (science)|tree of life]]" may be more complicated than a simple branching tree since some genes have spread independently between distantly-related species.

Past species have also left records of their evolutionary history. [[Fossil]]s, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record. By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as [[bacteria]] and [[archaea]] share a limited set of common morphologies, their fossils do not provide information on their ancestry.

More recently, evidence for common descent has come from the study of [[biochemistry|biochemical]] similarities between organisms. For example, all living cells use the same basic set of [[nucleotide]]s and [[amino acid]]s. The development of [[molecular genetics]] has revealed the record of evolution left in organisms' [[genome]]s: dating when species diverged through the [[molecular clock]] produced by mutations. For example, these DNA sequence comparisons have revealed the close genetic similarity between humans and chimpanzees and shed light on when the common ancestor of these species existed.

=== Evolution of life ===


Despite the uncertainty on how life began, it is generally accepted that [[prokaryote]]s inhabited the Earth from approximately 3–4 billion years ago. No obvious changes in [[morphology (biology)|morphology]] or cellular organization occurred in these organisms over the next few billion years.

The [[eukaryote]]s were the next major change in cell structure. These came from ancient bacteria being engulfed by the ancestors of eukaryotic cells, in a cooperative association called [[endosymbiont|endosymbiosis]]. The engulfed bacteria and the host cell then underwent co-evolution, with the bacteria evolving into either [[mitochondrion|mitochondria]] or [[hydrogenosome]]s. An independent second engulfment of [[cyanobacteria]]l-like organisms led to the formation of [[chloroplast]]s in algae and plants. It is unknown when the first eukaryotic cells appeared though they first emerged between 1.6 - 2.7 billion years ago.

The history of life was that of the unicellular eukaryotes, prokaryotes, and archaea until about 610 million years ago when multicellular organisms began to appear in the oceans in the [[Ediacara biota|Ediacaran]] period. The [[evolution of multicellularity]] occurred in multiple independent events, in organisms as diverse as [[sponge]]s, [[brown algae]], [[cyanobacteria]], [[slime mold|slime moulds]] and [[myxobacteria]].

Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over approximately 10 million years, in an event called the [[Cambrian explosion]]. Here, the majority of [[Phylum|types]] of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct. Various triggers for the Cambrian explosion have been proposed, including the accumulation of [[oxygen]] in the [[atmosphere]] from [[photosynthesis]]. About 500 million years ago, [[plant]]s and [[fungus|fungi]] colonized the land, and were soon followed by [[arthropod]]s and other animals. [[Amphibian]]s first appeared around 300 million years ago, followed by early [[amniote]]s, then [[mammal]]s around 200 million years ago and [[bird]]s around 100 million years ago (both from "[[reptile]]"-like lineages). However, despite the evolution of these large animals, smaller organisms similar to the types that evolved early in this process continue to be highly successful and dominate the Earth, with the majority of both [[Biomass (ecology)|biomass]] and species being prokaryotes.

== History of evolutionary thought ==

[[किपा:Charles Darwin aged 51 crop.jpg|right|thumb|150px|[[Charles Darwin]] at age 51, just after publishing *On the Origin of Species*.]]
Evolutionary ideas such as [[common descent]] and the [[transmutation of species]] have existed since at least the 6th century BC, when they were expounded by the [[Greek philosophy|Greek philosopher]] [[Anaximander]]. Others who considered such ideas included the Greek philosopher [[Empedocles]], the [[History of Western philosophy|Roman philosopher-poet]] [[Lucretius]], the [[Islamic science|Arab biologist]] [[Al-Jahiz]], the [[Early Islamic philosophy|Persian philosopher]] [[Ibn Miskawayh]], the [[Brethren of Purity]], and the Chinese philosopher [[Zhuangzi]].  As biological knowledge grew in the 18th century, evolutionary ideas were set out by a few natural philosophers including [[Pierre Louis Maupertuis|Pierre Maupertuis]] in 1745 and [[Erasmus Darwin]] in 1796. The ideas of the biologist [[Jean-Baptiste Lamarck]] about [[transmutation of species]] had wide influence. [[Charles Darwin]] formulated his idea of [[natural selection]] in 1838 and was still developing his theory in 1858 when [[Alfred Russel Wallace]] sent him a similar theory, and both were presented to the [[Linnean Society of London]] in [[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection|separate papers]]. At the end of 1859 Darwin's publication of *On the Origin of Species* explained natural selection in detail and presented evidence leading to increasingly wide acceptance of the occurrence of evolution.

Debate about the mechanisms of evolution continued, and Darwin could not explain the source of the heritable variations which would be acted on by natural selection. Like Lamarck, he thought that parents [[inheritance of acquired characters|passed on adaptations acquired]] during their lifetimes, a theory which was subsequently dubbed [[Lamarckism]]. In the 1880s [[August Weismann|August Weismann's]] experiments indicated that changes from use and disuse were not heritable, and Lamarckism gradually fell from favour. More significantly, Darwin could not account for how traits were passed down from generation to generation. In 1865 [[Gregor Mendel]] found that traits were [[Mendelian inheritance|inherited]] in a predictable manner. When Mendel's work was rediscovered in 1900s, disagreements over the rate of evolution predicted by early geneticists and [[biostatistics|biometricians]] led to a rift between the Mendelian and Darwinian models of evolution.

Yet it was the rediscovery of Gregor Mendel’s pioneering work on the fundamentals of genetics (of which Darwin and Wallace were unaware) by [[Hugo de Vries]] and others in the early 1900s that provided the impetus for a better understanding of how variation occurs in plant and animal traits. That variation is the main fuel used by natural selection to shape the wide variety of adaptive traits observed in organic life. Even though [[Hugo de Vries]] and other early geneticists were very critical of the theory of evolution, their rediscovery of and subsequent work on genetics eventually provided a solid basis on which the theory of evolution stood even more convincingly than when it was originally proposed.

The apparent contradiction between Darwin’s theory of evolution by natural selection and Mendel’s work was reconciled in the 1920s and 1930s by evolutionary biologists such as [[J.B.S. Haldane]], [[Sewall Wright]], and particularly [[Ronald Fisher]], who set the foundations for the establishment of the field of [[population genetics]]. The end result was a combination of evolution by natural selection and Mendelian inheritance, the [[modern evolutionary synthesis]]. In the 1940s, the identification of [[DNA]] as the genetic material by [[Oswald Avery]] and colleagues and the subsequent publication of the structure of DNA by [[James D. Watson|James Watson]] and [[Francis Crick]] in 1953, demonstrated the physical basis for inheritance. Since then, [[genetics]] and [[molecular biology]] have become core parts of [[evolutionary biology]] and have revolutionized the field of [[phylogenetics]].

In its early history, evolutionary biology primarily drew in scientists from traditional taxonomically oriented disciplines, whose specialist training in particular organisms addressed general questions in evolution. As evolutionary biology expanded as an academic discipline, particularly after the development of the modern evolutionary synthesis, it began to draw more widely from the biological sciences. Currently the study of evolutionary biology involves scientists from fields as diverse as [[biochemistry]], [[ecology]], [[genetics]] and [[physiology]], and evolutionary concepts are used in even more distant disciplines such as [[psychology]], [[medicine]], [[philosophy]] and [[computer science]].

== Social and cultural responses ==

[[किपा:Darwin ape.jpg|right|150px|thumb|As "[[Darwinism]]" became widely accepted in the 1870s, [[caricature]]s of [[Charles Darwin]] with a [[quadrupedal]] [[ape]] or [[monkey]] body symbolised evolution.]]
In the 19th century, particularly after the publication of *On the Origin of Species*, the idea that life had evolved was an active source of academic debate centered on the philosophical, social and religious implications of evolution. Nowadays, the fact that organisms evolve is uncontested in the [[scientific literature]] and the modern evolutionary synthesis is widely accepted by scientists. However, evolution remains a contentious concept for some religious groups.

While [[Level of support for evolution#Support for evolution by religious bodies|many religions and denominations]] have reconciled their beliefs with evolution through various concepts of [[theistic evolution]], there are many [[creationism|creationists]] who believe that evolution is contradicted by the [[creation myth]]s found in their respective religions. As Darwin recognized early on, the most controversial aspect of evolutionary biology is its implications for [[human evolution|human origins]]. In some countries—notably the United States—these tensions between science and religion have fueled the ongoing [[creation-evolution controversy|creation–evolution controversy]], a religious conflict focusing on [[politics of creationism|politics]] and [[creation and evolution in public education|public education]]. While other scientific fields such as [[physical cosmology|cosmology]] and [[earth science]] also conflict with literal interpretations of many religious texts, evolutionary biology experiences significantly more opposition from religious literalists.

Another example associated with evolutionary theory that is now widely regarded as unwarranted is misnamed "[[Social Darwinism]]," a term given to the 19th century [[British Whig Party|Whig]] [[Malthusianism|Malthusian]] theory developed by [[Herbert Spencer]] into ideas about "[[survival of the fittest]]" in commerce and human societies as a whole, and by others into claims that [[social inequality]], racism, and [[imperialism]] were justified. However, these ideas contradict [[Charles Darwin|Darwin]]'s own views, and contemporary scientists and philosophers consider these ideas to be neither mandated by evolutionary theory nor supported by data.

== Applications ==


A major technological application of evolution is [[artificial selection]], which is the intentional selection of certain traits in a population of organisms. Humans have used artificial selection for thousands of years in the [[domestication]] of plants and animals. More recently, such selection has become a vital part of [[genetic engineering]], with [[selectable marker]]s such as antibiotic resistance genes being used to manipulate DNA in [[molecular biology]].

As evolution can produce highly optimized processes and networks, it has many applications in [[computer science]]. Here, simulations of evolution using [[evolutionary algorithm]]s and [[artificial life]] started with the work of Nils Aall Barricelli in the 1960s, and was extended by [[Alex Fraser (scientist)|Alex Fraser]], who published a series of papers on simulation of [[artificial selection]]. [[Evolutionary algorithm|Artificial evolution]] became a widely recognized optimization method as a result of the work of [[Ingo Rechenberg]] in the 1960s and early 1970s, who used [[Evolution strategy|evolution strategies]] to solve complex engineering problems. [[Genetic algorithm]]s in particular became popular through the writing of [[John Henry Holland|John Holland]]. As academic interest grew, dramatic increases in the power of computers allowed practical applications, including the automatic evolution of computer programs. Evolutionary algorithms are now used to solve multi-dimensional problems more efficiently than software produced by human designers, and also to optimize the design of systems.

== लिधंसा ==


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**विकासक्रमीय विचारधाराया इतिहास**
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**विशिष्टिकृत ब्वनेज्या**
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== External links ==

 

**General information**
* [http://www.newscientist.com/channel/life/evolution Everything you wanted to know about evolution by ''New Scientist'']
* [http://science.howstuffworks.com/evolution.htm/printable Howstuffworks.com — How Evolution Works]
* [http://nationalacademies.org/evolution/ National Academies Evolution Resources]
* [https://web.archive.org/web/20090806034821/http://anthro.palomar.edu/synthetic/ Synthetic Theory Of Evolution: An Introduction to Modern Evolutionary Concepts and Theories]
* [http://evolution.berkeley.edu/ Understanding Evolution from University of California, Berkeley]

**History of evolutionary thought**
* [http://darwin-online.org.uk The Complete Work of Charles Darwin Online]
* [http://www.rationalrevolution.net/articles/understanding_evolution.htm Understanding Evolution: History, Theory, Evidence, and Implications]

**On-line lectures'''
*What Genomes Can Tell Us About the Past - lecture by Sydney Brenner
*The Origin of Vertebrates - lecture by Marc Kirschner


पुचः:Biology theories
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पुचः:विज्ञान