Evolution lecture 2
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1. Microevolution is changes in a species' genome or gene pool over a short period of time due to alterations in successfully reproducing individuals. 2. Hardy-Weinberg equilibrium describes how allele and genotype frequencies remain stable in a population not experiencing evolutionary pressures. 3. Five factors can disrupt Hardy-Weinberg equilibrium and drive microevolution: mutation, immigration, genetic drift, non-random mating, and natural selection.
Evolution lecture 2
- 1. Evolution lecture 2 Factors initiating elementary evolutionary changes (Microevolution) Microevolution Microevolution isthe change inthe genome,orgene pool,foragivenspeciesinarelativelyshortperiodof geologic time bythe alterationsof successfullyreproducingindividualswithinapopulation. Some environmental conditionsare harsherthanothers,andorganismsmayhave to adapt more to survive inthat conditions. Areaswhere the environmentalpressuresare stable,orthe organismshave adaptedtoit,exhibitnon-evolving populations. In a non-evolvingpopulation,the allele frequency, genotype frequency,and phenotype frequency remainin genetic equilibrium. ThisphenomenonwasillustratedbyaGerman physician,Weinberg,andaBritishmathematician,Hardy,both workingindependentlyin1908. Theircombinedeffortsare now knownasthe Hardy-Weinbergequilibriummodel. Hardy-WeinbergEquilibrium To understandthe Hardy-Weinbergequilibrium, assumeGand g are the dominantandrecessive allelesforatrait where GG = green,gg= yellow,andGg= orange. In our imaginarypopulationof 1,000 individuals,assume that600 have the GG genotype,300are Gg, and 100 are gg. The allele andgenotype frequencyforeachalleleiscalculatedbydividingthe total populationintothe number for eachgenotype: GG = 600/1,000 = 0.6 Gg = 300/1,000 = 0.3 gg = 100/1,000 = 0.1 The frequencyof the allele inthe firstgenerationof offspring. First,determinethe total numberof allelespossiblein the firstgeneration.Inthisimaginarycase,because eachorganismhas2 allelesandthere are 1,000 organisms,the numberof possible allelesinthe firstgenerationof offspringis: 2× 1,000 = 2,000 For the G allele,bothGG andGg individualsmustbe considered. Takenseparately, GG = 2 × 600 = 1,200 + Gg = 300 total = 1,500 The letterp isusedto identifythe allele frequencyforthe dominantallele(0.75) andq forthe recessive allele(0.25). Note that p + q = 1. The frequencyforthe G allele istherefore: 1,500/2,000 = 0.75 For the g allele,the calculationissimilar: Gg= 300 + gg = 2 × 100 = 200 total = 500 The frequencyforthe g allele istherefore: 500/2,000 = 0.25 Hardy-Weinbergcanalsopredictsecond-generationgenotype frequencies. Fromthe previousexample,the allele frequenciesforthe onlypossible allelesare p = 0.75(G) and q = 0.25(g) aftermeiosis.Therefore,the probabilityof a GG offspringis p × p = p2 or (0.75) × (0.75) = 55 percent.For the gg possibility,the allele frequenciesare q× q or (0.25) × (0.25) = 6 percent.For the heterozygousgenotype,the dominantallele cancome fromeitherparent,so there are two possibilities: Gg = 2pq = 2(0.75) (0.25) = 39 percent. Note that the percentagesequal 100,and the allele frequencies(p andq) are identical tothe genotype frequencyin the firstgeneration!Because there isnovariationinthishypothetical situation,itisinHardy-Weinbergequilibrium, and boththe gene and allele frequencieswill remainunchanged until acteduponbyan outside force(s).Therefore, the populationisina stable equilibriumwithnochange inphenotypiccharacteristics. The Hardy-Weinbergequationhighlightsthe factthatsexual reproductiondoesnotalterthe allele frequenciesina gene pool. Five factorsimpactthe Hardy-Weinbergequilibriumandcreate theirownmethodfor microevolution. 1. Mutationpressure 2. Immigration
- 2. 3. Geneticdrift 4. Crossbreeding 5. Selectionpressure 1. Mutationpressure A mutation isan inheritablechange of agene by one of several differentmechanismsthatalterthe DNA sequencing of an existingallele tocreate anewallele forthatgene A primarymechanismformicroevolutionisthe formationof new allelesby mutation.Spontaneouserrorsinthe replicationof DNA create newallelesinstantlywhile physical andchemical mutagens,suchasultravioletlight,create mutationsconstantlyata lowerrate. Mutationsaffectthe geneticequilibriumbyalteringthe DNA,thuscreatingnew allelesthatmaythenbecome part of the reproductive genepool forapopulation.Whenanew allele createsanadvantage forthe offspring,the numberof individualswiththe newallele mayincrease dramaticallythroughsuccessive generations.This phenomenonisnotcausedbythe mutationsomehow over manufacturingthe allele,butbythe successful reproductionof individualswhopossessthe new allele.Because mutationsare the onlyprocessthatcreatesnew alleles,itisthe onlymechanismthatultimatelyincreasesgeneticvariation 2. Immigration Gene migration isthe movementof allelesintooroutof a populationeitherbythe immigrationoremigrationby individualsorgroups.Whengenesflowfromone populationtoanother,thatflow mayincrease the geneticvariation for the individual populations. 3. GeneticDrift Geneticdriftisthe phenomenonwherebychance orrandomeventschange the allele frequenciesinapopulation. Geneticdrifthasa tremendouseffectonsmall populationswherethe gene pool issosmall thatminorchance events greatlyinfluence the Hardy-Weinbergarithmetic.The failure of asingle organismorsmall groupsof organismsto reproduce createsalarge geneticdriftinasmall populationbecauseof the lossof genesthatwere notconveyedto the nextgeneration Conversely,large populations,statisticallydefinedasgreaterthan100 reproducingindividuals,are proportionally lessaffectedbyisolatedrandomeventsandretainmore stable allele frequencywithlow geneticdrift. 4. Cross breeding The Hardy-Weinbergequationassumesthatall maleshave anequal chance to fertilizeall females.However,in nature,thisseldomistrue.Infact, the ultimate nonrandommatingisthe actof self-fertilizationthatiscommonin some plants.Inothercases,as the reproductive seasonapproaches,the numberof desirable matesislimitedby theirpresence (orabsence) aswell asbytheircompetitive prematingrituals.Finally,botanistsandzoologists practice nonrandommatingas theyattempttobreedmore and betterorganismsforeconomicbenefit. 5. Selectionpressure The processby whichcomparativelybetteradaptedindividualsoutof a heterogeneouspopulationare favoredby the Nature overthe lessadaptedindividualsiscallednatural selection. The processof natural selectionoperatesthroughdifferential reproduction. It meansthat those individuals,whichare bestadaptedtothe environment,survive longerandreproduce ata higherrate and produce more offspringsthanthose whichare lessadapted. So the formerscontribute proportionatelygreaterpercentage of genestothe gene pool of nextgenerationwhile lessadaptedindividualsproduce feweroffsprings.If differential reproductioncontinuesforanumberof generations, thenthe genesof those individualswhichproduce more offspringswill become predominantinthe gene pool of the population.