The Theory of Evolution

MechanismsofEvolution

NotesPt.4

Population Genetics & Evolution

– IMPORTANTTOREMEMBER:Populations,notindividuals,evolve.

– Population=agroupofindividualsofthesamespecieslivinginaparticularareaatthesametime.

– Allallelesofapopulation’sgenes=itsGENEPOOL.

– Thepercentageofanyonespecificalleleisknownastheallelicfrequency.

Variations in the Same Species

– Anyvariations(differencesamongmembersofaspecies)withinthesamespecies,likepeoplehavingdifferentheights,arecausedbydifferentallelesinthegenepool.

Changes in Allelic Frequency

– Iftheallelicfrequencystaysthesameovermanygenerations,itisingeneticequilibrium,andisnotevolving.

– But,changesintheallelicfrequencycanbecausedbymutationsorbychance,knownasgeneticdrift.Thisresultsinevolution.

Genetics Review

–Genes:segmentsofDNAonachromosome

Alleles

– Allelesaredifferentversionsofagene

– ex)Theeyecoloralleles:blue,brown,etc

– Individualshave2allelesforeverygene

BlueAllelefromDad

BrownAllelefromMom

Genotype vs. Phenotype

– Genotype:Anindividualsgeneticmakeup

– Phenotype:Anindividual’sappearance

– Genotypedeterminesphenotype

Evolution

– Evolution:Achangeinapopulationovertime

–Geneticevolution:Achangeingenefrequencyovertime

– Genefrequency:Whatpercentofanalleleispresentinapopulation

Mechanisms of Evolution

– Anythingthatchangesthefrequencyofallelesinapopulationisamechanismofevolution

1. NaturalSelection

2. Mutation

3. GeneticDrift

4. GeneFlow(Migration)

5. Non– RandomMating

1. Natural Selection

– Individualswiththebestadaptedphenotypeswillsurviveandpassontheirgenestothenextgeneration.

– Inthenextgenerationthereshouldbeahigherfrequencyofallelesforthebetteradaptedphenotype

Natural Selection Example

Generation 1

Mating!

Generation2

GreenAlleleFrequency

2/5=0.4 1/8 =0.125

BrownAlleleFrequency

3/5=0.6 7/8=0.875

Types of Natural Selection

– Stabilizing:Removesextremes

– Directional:Increasestheoccurrenceofoneextreme

– Disruptive/diversifying:Removesaveragetraits,favorsbothextremes

Artificial Selection

– Selectivebreedingofdomesticatedplantsandanimalstoproduceoffspringwithdesiredgenetictraits

2. Mutation

– Amutationinaparent’sDNAcouldresultinoffspringwithdifferentalleles,changingtheallelefrequencyofthepopulation

Generation 1

Mating!

Generation2

GreenAlleleFrequency

2/2=1 2/3 =0.67

BrownAlleleFrequency

0/2=0 1/3=0.33

3. Gene Flow (Migration)

– Geneflowisanymovementofgenesfromonepopulationtoanother.

– Wind,habitatchange,moving,etc.canallcauseorganismstomigrate,causinggeneflow

4. Non- Random Mating

– Iforganismscanchoosetheirmatingpartners,thenthemoredesirablealleleswillhaveahigherfrequencyinfuturepopulations

– Thisiscalledsexualselection.Sexualselectioncanresultinsomebizarretraits.

5. Genetic Drift

– Geneticdriftoccurswhentheallelefrequencyofapopulationchangesbychance.

Generation 1

DIE!

Generation2

GreenAlleleFrequency

3/9=0.33 1/7 =0.14

BrownAlleleFrequency

6/9=0.67 6/7=0.86

5. Genetic Drift

– Verycommoninsmallpopulations

– FounderEffect:Smallpopulationsettlesanewlocation(oftenislands)

– Bottleneck:Occursafteramajorpopulationdecline(manyendangeredspecies)

The Evolution of Species

– Speciationistheevolutionofnewspecies.

– Thisoccurswhenaspecies(agroupoforganismsthatcaninterbreedtoproducefertileoffspring)cannolongerinterbreedintheircurrentenvironment.

Speciation

– Speciationcanoccurwhenpopulationsaresplitapartbygeographicisolation,suchasvolcanoes,fires,sealevelchangesandotherenvironmentalinfluences.

– Eachindividualpopulationadaptstoits“new”environment,eventuallyforminganew,differentspecies.

– REMEMBER:thishappensoverLONGperiodsoftime,likemanygenerations!

Adaptive radiation

– Thistheorysaysthatonespeciescanmigrate,ormovebytransplantation,tonewhabitatsandchangetofitthatnewenvironment.

– Whenthishappens,severalnewspeciescanarise.

Adaptive Radiation (Cont)

– Examples:

– TheHawaiianHoneycreeperformed14newspecies.

– Darwin’sFinches

Divergent Evolution

– Adaptiveradiationcancausedivergentevolution.

– Thisiswhencloselyrelatedorganismsdevelopbodypartsthathavedifferentfunctions,likethebackbonesofsnakesandlizards.

– ThisresultsinalargeamountofBIODIVERSITYallthroughouttheworld!

Hardy- Weinberg Equilibrium

– Apopulationthatisnotevolvingisconsideredtobein“Hardy- WeinbergEquilibrium”becausetheallelefrequenciesdonotchange.

– Thefollowingsituationsdisrupt“Hardy-WeinbergEquilibrium”andcauseevolution

1. NaturalSelection

2. Mutation

3. GeneticDrift

4. GeneFlow(Migration)

5. Non- RandomMating