Evolution lecture 3
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1) Isolation prevents interbreeding between species and allows their gene pools to diverge over time, leading to speciation. There are prezygotic and postzygotic isolating mechanisms. 2) Prezygotic mechanisms like ecological, temporal, behavioral, and mechanical isolation prevent mating and fertilization between species. Postzygotic mechanisms prevent hybrid offspring from surviving or reproducing. 3) Speciation can occur with or without geographic barriers separating populations. Allopatric speciation occurs when barriers lead to reproductive isolation, while sympatric speciation involves mechanisms like balanced polymorphisms, polyploidy, or hybridization.
Evolution lecture 3
- 1. Evolution lecture 3 Role of isolationin evolution Usingthe (older) typologicaldefinitionof species,these twogroupsof frogswere classifiedasone. Uponfurther investigation,itwasdiscoveredthatthere are twogroupsthat differonlyintheir mating calls. Theycall for matesat differentfrequencies. Reproductive IsolatingMechanisms Biologistsrelyonmorphological featuresfromfossil recordstodistinguishthousandsof differentspecies. For livingpopulationsthatare morphologically similar,behavioral orotherbiological methodsare neededto distinguishspecies. One methodusedisto identifythe species’reproductiveisolatingmechanisms(anybehavioral,structural or biochemical traitsthatpreventindividualsof differentspeciesfromreproducingsuccessfullytogether). There are twotypesof isolatingmechanisms prezygoticisolatingmechanisms postzygoticisolatingmechanisms. Prezygoticmechanisms Prezygoticmechanismspreventinterspeciesmatingandfertilization. There are fourtypesof isolationthatprevent matingfromoccurring,thus maintainingspeciesisolation. 1) Ecological isolation 2) Temporal isolation 3) Behavioural isolation 4) Mechanical isolation 5) Gameticisolation Ecological isolationiswhenspeciesoccupy separate habitatsornichesanddo not encounter one anotherto reproduce due tosome geographicorecological barrier. Example – groundsquirrel speciesoccupy differenthabitats. Temporal isolationiswhentwospeciesare foundinthe same area,but are incapable of matingdue todifferent reproductive cyclesformating. Example – Redand blackseaurchinslive inthe same location,butrelease theirgametesatdifferenttimesof the year. e.g. Drosophila pseudobscura and D.persimilisare sympatricovera large area inNorth America.Theybreed duringthe same seasonof the yearbut at different timesof the day. D.Pseudobscura issexually activein the evening while D. persimilis in the morningand hence don’tformhybrids. Behavioural isolationis whendistinctmatingritualsbyone speciesmaypreventmembersof anotherspeciesfrom recognizingorselectingamate. Example:male jumpingspidersdance (shake theirlegsandwave theirpalps). Femalesof differentspeciesdonot respondtothe dance. Differentspeciesof firefliesdonotrecognize eachothers'matingsignals,andasa resultdo not generallyinterbreed. Mechanical isolationis whenstructural differencesinreproductive organspreventsuccessful fertilization.This isolationcomesaboutbecause of differentandcomplex structure of genitalorgans,indifferentorganismswhich,in turn,doesn’tallowcopulationtooccurbetweendifferentspecies.
- 2. Thisis particularlytrue of insects,where accordingtoDufour,genitaliaare suppposedtoworkon “lockand key” principle,like enzymes. Example - . The male and female genital organsare exactlyfittedtoeachotherandcopulationdoesn’toccurevenif there isslightdifference inthe structure of eitherorgans.Inter-specificcrossesbetweendifferentspeciesof Drosophila can causeinjury or even deathto the female .Copulationbetweenfemale Drosophila pseudobscura and male D.melanogasterissimply impossible. Gameticisolation– maypreventreproductionata molecularlevel. Example – incoral reefs,manyspecieswithexternal fertilizationmayrelease gametessimultaneously,sotrillionsof spermand eggsmay be inthe shallowwateratone time. Spermandeggsof the same speciesrecognize eachother by molecularmarkers. PostzygoticIsolating mechanisms Postzygoticmechanisms preventthe hybridzygote fromdevelopingintohealthyandfertileadults. There are three likelycasesthatwill occurtoensure that the hybriddoesnotreproduce: 1) Zygote mortality isa resultof chromosomal incompatibility. 2) Hybridinviability iswhenthe embryodoesdevelopbutthe hybridexperiencesreducedfitnessandoftenan earlydeath. 3) Hybridinfertility occurswhenahybriddevelopsintoamature adultbutis unable toundergosuccessful meioticdivision,andisunable toproduce offspring.(seenindonkey-horsehybrids:mules) AllopatricSpeciation Occurs whena populationisdividedbyageographicbarrier(Barriers:mountains,rivers,regionsexcludingvital resources(water,food) areascoveredwithvolcanic lava) Interbreedingbetweenpopulationsnotpossible (reproductive isolation) Gene frequenciescandiverge due tonatural selection,mutation,andgeneticdrift Sympatric Speciation The formationof newspecieswithoutthe presence of ageographicbarrier Occurs by one of 3 ways: BalancedPolymorphism Polyploidy Hybridization BalancedPolymorphism Suppose apopulationof insects possessesapolymorphismforcolor. Each color providesacamouflage toa different substrate (rock,tree stump,etc. Whennot camouflaged,theyare eaten.Thus,onlyinsectswiththe same colorcan associate andmate. Similarlycoloredinsectsare reproductivelyisolated…sogenepoolscandiverge. Polyploidy More than 2 setsof chromosomesfoundindiploid(2n) cells. Oftenoccursinplants(occasionallyanimals) where triploid(3n),tetraploid(4n) andhigherchromosome numbersexist. Causedby nondisjunctioninmeiosis.Tetraploid individualswill continue toproduce diploidgametes –makingthemreproductivelyisolatedveryquickly. Hybridization Occurs when2 differentformsof aspeciesmate andproduce offspringalongageographicboundarycalledahybrid zone. The geneticvariationof the hybridsisgreaterthanthatof eitherparent.Thispermitshybridstoadaptto environmental conditionsbeyondthe range of eitherparent. Hybridscaneventuallydivergefromparentforms whenfacedwithselective pressures