Mechanisms*of*Evolution**!Learning*Objectives:*Upon!finishing!this!lab!you!should!be!able!to:!1. Identify!different!mechanisms!of!evolution!and!their!effect!on!allele!frequencies!in!a!population.!2. Evaluate!how!different!circumstances!in!a!population!can!change!allele!frequencies!and!predict!the!direction!and!magnitude!of!the!changes!in!allele!frequency.!!Introduction:!Evolution!is!the!process!whereby!allele!frequencies!in!populations!of!organisms!change!over!time.!This!change!occurs!on!small!or!large!scales!resulting!in!changes!from!modest!to!massive.!The!changes!range!from!the!development!antibiotic!resistance!in!bacteria,!observable!over!the!course!of!bacteria,!to!the!diversity!of!life!we!see!today,!evolved!over!millennia.!Evolution!is!such!an!integral!part!to!so!many!aspects!of!biology,!it!prompted!the!illustrious!scientist,!Theodozius!Dobzhansky!(1900G1975),!to!famously!remark,!“Nothing!in!biology!makes!sense!except!in!the!light!of!evolution.”!!Most!simply,!evolution!can!be!defined!as!a!change!in!allele!frequencies!in!a!population!over!time.!Although!the!word!“evolution”!often!evokes!images!of!large,!observable!change!G!such!as!the!evolution!of!the!long!neck!in!the!giraffe!–!any!significant!change!in!its!allele!frequencies!can!be!considered!evolution.!After!enough!time!and!the!right!conditions,!these!changes!in!allele!frequency!can!produce!morphological,!physiological,!or!behavioral!changes!in!the!population!as!long!as!there!is!a!genetic!contribution!to!those!traits.!An!individual’s!genotype!is!its!genetic!constitution,!the!sum!total!of!its!genetic!material.!This!genotype!is!translated!into!the!phenotype!of!an!organism,!which!is!its!physical!and!behavioral!makeup.!The!phenotype!is!the!product!of!their!genetic!makeup!(genotype),!development,!and!environmental!conditions.!!Natural*Selection*Although!the!name!most!often!associated!with!evolution!is!Charles!Darwin!(1809G1882),!many!biologists!and!naturalists!before!Darwin’s!time!recognized!that!species!were!not!static,!i.e.!they!evolved.!Darwin’s!own!grandfather,!Erasmus!Darwin!(1731G1802),!a!noted!naturalist!himself,!theorized!that!species!changed!over!time.!JeanGBaptiste!Lamarck!(1744G1829)!posited!that!morphological!changes!occurred!over!time!due!to!the!relative!use!or!disuse!of!features.!Charles!Darwin’s!theory!of!evolution!by!natural!select ion!put!forth!a!satisfactory!mechanism!explaining!how!these!changes!had!occurred.!Darwin’s!explanation!of!how!populations!change!over!time!is!now!accepted!as!the!mechanism!for!evolution!via!natural!selection.!!!!!!Briefly!summarized,!the!mechanism!of!natural!selection!as!framed!by!Darwin!has!four!key!components:!1. More!individuals!in!a!population!are!born!than!survive!and!reproduce!2. There!is!inherent!variation!of!traits!in!any!population!3. Traits!are!heritable!4. Survival!and!reproductive!success!is!not!random.!It!is!determined!by!the!traits!exhibited!by!the!organism.!!Darwin!deduced!that!the!variation!in!the!traits!determined!which!individuals!reproduced!more!than!others!(natural!selection);!and!that!because!these!traits!are!passed!from!parent!to!offspring,!subsequent!generations!of!that!population!would!differ!from!their!ancestors!in!the!traits!they!possessed!(evolution).!!Darwin’s!theory!of!evolution!by!natural ! selection!has!been!popularly!referred!to!as,!“Survival!of!the!fittest.”!This,!however,!is!an!inaccurate!statement!for!two!reasons.!First,!natural!selection!does!not!necessarily!maximize!those!traits!that!enhance!individual!survival,!but!rather!favors!any!trait!tha t !leads!to!the!greatest!propagation!of!genes!into!following!generations.!A!trait!causing!great!longevity!but!poor!fecundity!would!not!be!favored!by!natural!selection.!Similarly,!natural!selection!would!not!favor!a!trait!producing!many!offspring,!if!those!offspring!die!before!adulthood.!Survival!is!only!favored!if!it!increases!fitness,!which!brings!us!to!our!second!reason.!The!term!“fitness”!has!a!specific!meaning!in!biological!terms.!Simply!put,!fitness!refers!to!the!number!of!surviving!offspring!produced!by!an!organism,!relative!to!the!number!of!surviving!offspring!produced!by!others!in!the!population.!Note!that!it!is!always!relative!to!others!in!the!population,!and!that!the!offspring!must!survive!and!go!on!to!reproduce!themselves.!!One!important!aspect!of!evolution!by!natural!selection!is!that!selection!itself!acts!on!the!individual,!while!populations!as!a!whole!evolve.!An!individual!cannot!itself!evolve;!it!either!lives!or!dies,!breeds!or!doesn’t!breed.!The!phenotype!(influenced!by!genotype)!influences!whether!an!organism!survives!and!reproduces.!Selection!determines!which!individuals!reproduce,!and!it!is!this!differential!success!in! rep roduction!that!causes!allele!frequency!changes!in!the!population!as!a!whole.!It!is!the!change!in!population!level!allele!composition!that!we!refer!to!as!evolution.!Since!populations!evolve,!the!characteristics!of!a!population!such!as!population!size!and!variation!are!of!critical!importance.!!Evolution!is!an!ongoing!process!in!response!to!the!current!environment.!Natural!selection!favors!variants!that!do!well!in!a!particular!environment,!but!this!does!not!imply!a!direction!in!evolution!towards!a!“perfect”!form.!There!are!two!important!reasons!this!is!the!case.!First,!natural!selection!acts!solely!on ! the!variation!present!in!the!population . !Natural!selection!cannot!simply!conjure!up!the!traits!it!finds!most!desirable.!The!second!reason!there!is!no!direction!to!a!perfect!morphology!is!that!environmental!conditions!are!variable!in!space!and!time.!The!traits!that!were!highly!adaptive!in!the!ice!age!may!no!longer!be!advantageous!in!the!present!day.!Similarly,!a!single!trait!might!be!favorable!in!Paris,!France!while!being!unfavorable!in!Paris,!Texas.!!Another!barrier!to!“perfect”!forms!lies!in!tradeGoffs.!Often!a!gain!of!function!in!one!area!entails!a!lowering!of!function!in!another.!For!example,!a!higher!metabolic!rate!might!enable!an!organism!to!move!more!quickly.!A!faster!animal!can!reach!and!consume!food!sources!before!competitors.!However,!a!faster!metabolism!also!requires!more!food,!and!the!organism!may!have!to!devote!more!time!searching!for!and!consuming!food.!Thus!higher!metabolism!presents!a!tradeGoff.!!!How!strong!does!selection!have!to!be!to!chan
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