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.

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