,o Factsbm @o

Isolation MechanismsThis Factsheet wi l l explain:o the nature of isolat ion mechanisms. including both pre-zvgotic

and post-zygotic types:. some roles and actions of isolation mechanisms in evolution and

speciat ion.

The overal l role of isolat ion mechanismsGenetic variation arises fron-r five sources. These are:o polygenic (mult iple al lele) inheritance.o random assortment of chromosomes in meiosis I and ol'chrclmatids in

meiosis I I .r chiasmata formation in meiosis I.o t-ertilisation, particularly enhancecl by outbreedin-u.. gene mutations and chromosome mutations.The sources of variation are covered in detail in Factsheet 50. Sources ofgenetic variation.

When all of the individuals of a population can interbreed. these variationswill not produce new species. The variation may allow the tbrmatictn of anumber of diffbrent races or varieties of the species but these could stillinterbreed with other members o1'the species. even when of diffbrent races.

Remember: - A species is a populotiort oJ sinilar tt'sarisnts all ofu'lticlt c'art irtterbt'eecl to Jcn'm fet'tile oftsprittg. Mentbers of a speciescatutot pt'oduce fertile oJfspt'ittg u'ith mentbcrs ofartother specit,s.

Befbre a new species can develop a barrier must lbrm that restricts breedineand gene llow between populations. Such a barrier. which prevents geneexchange, is cal led an isolat ing mechanism.

Once such a barrier is in place then the isolated populations can continueto vary independently b1, the usual mechanisms. They eventually mavbecome so dif'ferent lrom each other that they can no longer interbreedsuccesslully - they have become separate species.

Barriers to interbreedin-e may be:. prezygotic - these prevent l-ertilisation happening.

. postzygotic - f-ertilisation mav occur but any offspring are either notviable or are themselves sterile.

Fig I shows a classiflcation of isolation mechanisms.

Fig 1. Isolation mechanisms

Remember: -Tlrc gerrctic'yat'iatiotts tlrut occ'ur arc actecl uport bt trutut'olselectiotr.Tlis errubles tlte ctrgarisnts u'itlt tlrc ntost fcn'ow'able yariatiortsto surt'iw urrcl reprocluce nlot'e cffectivelv.Tlis nay ertlwtrcc diJJerencesbeM'eerr isolateclpopulatiotts ofa species. espec'iallt'iJ'cliJferat selectit'ept'(ssures opet'ate irt tlte diJlerent populatiorts.

SpeciationIsolation mcchanisms can operate to cause the emergence of ne w species. inone of two possible ways:o A l lopat r ic spec ia t ion occurs between geographica l ly iso la ted

populations. The sene pool of the speciesis physically seperated, sothe seperate populations can then evolve independently of each other.

. Sl'mpatric speciation occurs when organisms inhabiting the samearea become seperated into two or more reproductively isolated groups.

Simple model of speciat ionl. Initially an existing geneticalll, var_ving population is physically divided

into two or more sub-populations so that the tree exchange of genes canno lon-9er occur. This could be caused by geographical isolation, torinstance . when part of a land mass becomes detached as an island, or amountain range produced by earthquakes.

2. Once the sub-populat ions are spatial ly separated they can diver-eegenetically. Their environments will be diflerent so difl-erent geneticalvariations will be selected.

3. Eventually the genetic divergence will be so extensive that the groupswould no lon-ser be able to interbreed. even if they were brought togethera-uain. At this stage they iue reproductively isolated and can be consideredto be dit fbrent species.

This model of speciation is illustrated in Fig 2.

Fig 2. A model of speciation.

orisinal population with single aenepool I

Y

/6-\^ o.t-\ ffiffi'iil,:il'i::JJi:,[ffi",\3_-/ ( Av

^ \-----l^\:-/ ::HtJTJ::J:i"in ranges 1 i\

V

fi} -----'-------*- ,.[;, : -:,:,,,,)

\uDpoputattons Deeome qenetlcall'

L( A )^ \ � d iverseandreproduct ivel r isolared

t-_______-/A (neu speetes) 1

6 f, l�'J i: ::* iffiFr *:'f."I#i\---------lA are thus srmpatric species. C remains

:',",fr'"1.,:"-i.il1.t'"0 B,nd c iue

Exam Hint :- Questions will often be of data interpretation type. Readthe data given carefully and then apply your knowledge to give reasonedanswers. lf you are asked to write a continuous prose essay about' isolat ion mechanisms' make sure you explain their roles andimportance Do not be tempted to describe too many examples at theexpense of omitting some types of isolation mechanism.

,lsolrtion mechanrsm

r'Reproductive isolation

IPostzygotic barriers:(a) Hybrid inviability(b) Hybrid srerility

Prezygotic barriers:(a) Seasonal isolationtbt Ecological isolat ion(c) Behavioural (Ethological) isolation(d) Mechanical isolat ion(e) Physiological incompatibility

Isolation Mechanisms Bio Factsbeet

Examples of particular isolation mechanisms

P rez.v got ic b a rr i e r s ( pre v e nt in_s f-e rt i I i s at io n )

1. Geographical isolationGeographical barriers to _senc llow mav be tbrmed bv mountain ranses.glaciers. canyons. seas ancl r ivers. enabling speciat ion to occur.. Grand Canvon in Colorado, USA. is no barrier to l1ying birds hut it is

to rodents. The north and south rims have the same identical birdspecies but dift-erent species of rodents.

. The Galapagos islands lie 600 miles west of Ecuaclor in South America.The islands are volcanic in origin ancl have never becn attachecl to themainland. The llnches inhabiting them must have arisen lrom a singlemainland species and come across thc sea. possibil'carrieclon clritiwooclor blown by wind currents.

The tlnches became established on several of the islands and since eachisland populat ion was separated lrom the others b;- 'sea (f inches wil lnot l ly across wide stretches of u,ater). thev were geographical ly 'isolated and thus divereed and evoived into dist inct species (al lopatr icspeciat ion).

2. Ecological isolationThis occurs when species inhabit the same geographical area but occupydift-erent habitats or ecological niches within the area.. The Wild Pansy (lliola n'icolor') qrows on acid soils whereas the Field

Pansy (\'iola an'utsis) grows on calcareous soils.. In the Galapasos islands. the llnches within one population on an

island diverge and become adapted enabling them to occupy dilterentecological niches (fig 3).

Fig 3. Beak shapes of Galapagos finches

Because these species overlap in terr i torv they have evolved bysympatric speciation. Altogether there are l3 ditf erent species of flnchon the Galapagos Islands.

3. Seasonal isolat ionThis occurs when populations exist in the same area but are sexuallymature at diff-erent times of the year.. In California, Pinus radiata cones mature in February but Pirlas

attetutato cones mature in Aoril. So these oine trees cannot hvbridisetogether.

. In Britain the common marbled carpet moth (Drsstroma trurtcata)mates at different times of the year to the closely related dark marbledcarpet moth (Dt'sstromI citrata\.

4. Behavioural isolationThis occurs when animals exhibit species-specific courtship patterns.Mating only occurs if the courtship display by one sex results in acceptanceby the other sex.. Many species of fish, bird and insect exhibit highly specitic courtship

colourations, movements and dances to attract the opposite sex.

. Birds also use speci l ic bird song to communicate between the sexes.

. Male spiders of many' species approach the 1-emale lbr mating by'semaphore' signals with the palps and legs - the male spider thargives the wrong signals r ' , , i l l probabh'get eatenl

. Dift'erent species o1'firelly have difterent courtship tlashing patterns(1 ig 4) .

Fig 4. The flashing patterns of males of three species of fire flies

species A sPecies B species C

Key-)r)

tlight path

light flash

The f'emales only recognise the tlashingpattern of males of their own species andrespond to them.

5. Mechanical isolat ionThis occurs in animals where dif-lerence s in the shane of senitalia Dreventmating between closely related species.. The male palps of spiders. which are used to insert sperm into the

epigyne (f'emale -genital opening)are extremely complex in shape andcan only fit the complex cpigyne of the specitic f-emale by a 'lock andkey'mechanism.

. In milkweeds the pollen is released in small sacs which stick to the legsof insects. The st igmas have small sl i ts into which the sacs must beinserted lbr pollination to occur. The sacs of pollen ancl stigma slits areof complementarv shape in each species and will not fit together withdit lerent species.

6. Physiological incompatibi l i tyThis is common in ,erasses and clovers. The stigma produces genetically-determined proteins which inhibit or retard the germination and growth offbreign pollen, even lrom closely relatecl species.

Postzygotic barriers (prevent the development/f-efiility of hybrids)

l. H.vbrid inviabilitl 'Although hybrids are formed. the-v are usually weatri and mallormed anddie befbre they can reproduce.. When diff-erent species of tobacco plant hybridise. the h1'brids develop

tumours in their vegetative parts and die befbre llowering occurs.. In North America. hl,brids formed by interbreeding between the

nonhern and southern races of the leopard trog(Rarru pipiens) invariablydie in the tadpole stage, preventing gene flow between these two racesof lrogs.

2. Hybrid sterilityIn this case the hybrid may be visorous and grow to adult size. It willhowever be sterile because meiosis will lail to produce gametes. This isbecause the ditterent parent species have different chromosome shapes(and possibly difterent chromosome numbers). Thus pairing of homologouschromosomes (synapsis) cannot occur in meiosis.. The sterile mule (2n = 63 ) results fiom a cross between a horse (2n = 60)

and a donkey (2n = 66).o d horse and a zebra can interbreed to fbrm a sterile zebroid.

C

s p l n e

E

A: fruit or bud-eating linches, with a panot-like beakB: insect-eating tree llnchesC: cactus-eatin-s -qround llnchesD: seed-eating finchesE: woodpecker-like flnches which use cactus spines as tools to

dislodge insects lrom crevices in the bark.

Isolation Mechanisms Bio Factsbeet

Tip: - Obviously in a Factsheet of this length the number of examplesquoted must be very limited. lf you wish to know more examples eitherrefer textbooks on evolution or if you have internet access, search for'isolation mechanisms' - you will find a wealth of examples described.

Practice Questions1. Read through the fol lowing passage abour speciat ion and isolat ionmechanisms and then complete it by tilling in the spaces with the mostappropriate worcl or words.

Isolat ion mechanisms (barr iers) restr ict . . . . . . f low betweenpopulations thus allowing the separatecl populations to diver-ee geneticall)'from one another, leading to the lbrmation of new separate species.Speciation occurrin-e due to an isolation mechanism operating w,ithin a -senepool . in a s ing le -eeographica l reg ion. is known as . . . . . . . . . . . . .speciat ion.Speciation due to isolation mechanisms operatins on genc pools in dif-fbrentgeographical re-eions is known as .. . . . . . . . . . . . . speciat ion. The typeof iso la t ion mechanism which resu l ts in th is tvpe of socc ia t ion is

isolat ion.Reproductive isolation is classed into two main t1,pes. The first tirpe is

.. . . . isolat ion in which populat ions may be prevented lromcompleting t 'ert i l isat ion and zygote lbrmation. This type of isolat ionmechanism includes .. . . . . . . . . . . . isolat ion. when populat ions l ive in the sameloca l i t y bu t ma tu re a t d i f f e ren t t imes o f t he yea r and

... . . . . . . . isolat ion when populat ions I ive in the same arca but indiff'erent habitats.The second type of reproductive isolat ion is . . . . . . . . . . . . . .wh i ch f e r t i l i sa t i on occu rs bu t t he hvb r i ds f o rmed

""" " 'or

2. (a) Explain the meaning of each of the fbllowing terrns:( i) Species(ii) Isolatin-e mechanism

(b) Dist inguish between each of the lbl lowing pairs:(i) Allopatric and sympatric speciation(ii) Prezygotic reproductive isolation and postzygotic reproductive

isolation 4

3. The diaeram below shows pluma-ee variation in tbur subspecies ol theflycatcher, Monarcha castaneoventris of the Solomon Islands in theSouth Pacillc Ocean, and their distribution throu-ehout the islands.These subspecies of flycatcher can still interbreed successlullv.

(a) Suggest and explain a mechanism by which the different subspeciesmay have arisen. 5

(b) What lurther events must occur belbre the subspecies become fullindependent species? Explain your answer. 4

4. Three distinct populations of birds, X. Y and zlive separatelv on tlreeisolated oceanic islands. The birds me all omnivorous. eating insects. tin1,'seeds and nectar. but have slightly diflerent beaks and 1'eather colourinss.The islands are frequentlv swept by strong south-westerly galcs.Populations X aurd Y can interbreed and lbrm f-ertile ofTspring. PopulaticlnY can interbreed with populat ion Z but the offspring are sreri le.Populat ion X wil i not even mate with populat ion z t . i r t t l art i f ic ialinseminatirn of Z birds with X bird sDenn is unsuccessful.

(a) Suggest an explanation why populations X and Y can still interbreeclancl fbrm fertile olfspring. 4

(b) Suggest an explanation why matins between Y and Z producessterile offsprin-e. 5

(c) Suggest an explanationmate.

fbr the inability of populations X and Z to-)

-5. Read through the lbllowin-e passage and then answer the questionsbelow.

AIthou-sh evolution is usually considered to be a very slow processi t can somet imes occu-r qu ick ly . For example. sa lmon wereintroduced into Lake Washin-eton. (in Washin-uton State, USA). int 9 3 1 .

l ine 5 Since then thel 'have evolved into two dist inct populat ions whichiire reluctant to breed with each other.

I ine 7 One popula t ion has become adapted to breeding in a r iverenvironment. swimming up the rivers and streams which enterLake Washington. The other population has become adapted to

l ine l0 lav their eggs in the shal lows near the lake's beaches.line I I The male river fish have developed shallower-shaped bodies that

enable them to swim against strons river currents easilSr becauseshallow bodies ofler less resistance to water tlow. The 1-emale rivertish have become much bigger than the lake fish and consequentlydig much deeper nests ir.t the river bed in which to lay their e-egs.Because the eggs are deeply covered they are less likely to be

l ine l7 washed away or eaten.

(a) Do you consider the two popularions (line 5) to be distinct species'?Explain your answer. z

(b) Classify the type of isolation mechanism described in tliis passage( l ines 7 to 10) . 3

(c) Outline the process which enabled to the two fbrms of salmon toevolve ( l ines l l ro l7) . 5

isolation ina re e i t he r

l 0

21

q\qq EBlack

Chestnut

White

Isolation Mechanrsms Bio Factslteet

Ansu'ersl . qene: sympatr ic: a l lopatr ic: geographical : prezl ,got ic: seasonal :

ecological: postzygotic: inviable: steri le: (these two points cnn be giveneither way round) i 0

2. (a) (i) a population of similar organisms that are able to interbreed totbrm f-ertile olfspring: they are reproductivelv isolatecl trornothcr such populations/cannot interbreed u,ith other speciesto torm f'ertilc offspring:

(ii) an obstacle to interbreeding:thus limiting gene llow betwecn parts of the -eene pool:thus enabling diver-eence: max 2

(b) ( i) Al lopatr ic: speciat ion due to populat ions occupyine dit terentgeographical iueas:thus there is no gene llow between the populations:Sympatric: speciat ion where the populat ions occupl, ' the samegeographical locality :but gene llow is restricted between the populations/demes:

4

(ii) Prezygotic: prevents l-ertilisation and the tormation of zl,gotes:Any two examples:geographical isolation/seasonal/mature at dillerent times/ecological/live in difterent habitats but in same region/behavioural/incompatible mrting rituals/incompatibility/cannotf-ertilise due to physiological incompatibility::Postzygotic: f-ertilisation can occur but hybrids are either notlbrmed or are sterile: 4

3. (a) t iom the distr ibution map it is clear that) the populat ions areisolated on diffbrent islands/groups ol island;even though they are not separated by huge distances/may not liketlying over water/psychological barrier to crossing water:sene mutations/genetic variation may cause difl-erent plumagepatterns/colours:which are selected fbr/of survival/camoutlage value on dillerentislands:since interbreedins is restr icted these variat ions can becomeestablished:but if they do interbreecl chromosomes can still pair in meiosis scrgametes can be made (by otfspring)/ nclt reproductively isolated:also courtship rituals are still compatible/not behaviouralll, isolated:

max )3. (b) continuedrestr ict ion on interbreeding/isolat ion:

continued mutation/genetic variation:this must be selected for and become establ ished:until behavioural/mating rituals become incompatible/behaviouralisolat ion:and chromosomes become so difl-erent that meiosis cannot occur inanv hyhrids:since pairing/synapsis of chromosomes cannot occur: max 4

4. (a) populations/islands X and Y are relatively close/not -eeographicallyisolated:thus birds can still come into contact and breed together:no chance for any mutations to become genetically isolated/becomeestablished:so little diver-eence occurs between X and Y/still reproductivelycompatible:populat ion Y probably arose lrom populat ion X because ofprevailing winds:chromosomes of hybrids will still pair in meiosis (so gametes canform): max 4

(b) populat ions/islands Y and Z arc -ueographical lr , isolated:thus will not normally interbreed:thus mutations/genetic variation in the two populations u'ill occurindependentlr,:thus become isolated by pctstz-vgotic isolation/chromosomes of Ydifler liom those oI ZlwiII not pair in meiosis (to torm s.unetes):Z probabll, arose trom Y as blown bv winds rather than originatingliom X:not diversed suff iciently ' to have dif ferent courtship r i tuals/behavioural patterns:some Y may st i l l be blown to Z al lowin-e occasiunal interbreeding(although this has nou'become ir"reff-ective): max -5

(c) population/islancl X is geographicall"v isolated liom population Z:bv ocean and island Y:thus mutat ions/genet ic var ia t ion in thc two popula t ions hascontinuecl independently:they are now reproductivel5r isolated because their courting/matin-ebehaviours difler:incompatible mating rituals/courtship dances/plumage colours/breeding times:this is prezygotic isolat ion:ref to chromosomes of X will no lonqer match with those of Z evenif thev could mate: max 5

no: 'reluctant to breed with each other' inf-ers that the-y still caninterbreed (and so are still the same species/are races of the samespecies): 2

reproductive isolation: prez),-sotic isolation: ecological/behaviouralisolation: 3

ref to genetic variation gave rise to diff'erent body shapes/sizesbehavioural patterns:any tluee of:variat ion arising from polygenic (mult iple al lele) inheritance/variation clue to random assortment in meiosis /variation due tochiasmata tormation in meiosis/variat ion arising lrom cross-f-ertilisation/variation due to gene mutations/due to chromosomemutations:::

ref to natural selection acting on these variations: 5

Acknowledgements:Tltis Fuctsltcct wus resaorcltatl utrd urittcn hy Murtin GriffinCuniculum Press,Unit 3058,The Big Peg, I20Vyse Street,Birmingham- BI8 6NF Bio Factsheetsnru.t bt'copied Jicc oJ clturgc bt teaching sta.fl or stltdcnts, prtn'it)etl thut their sc'fuxtl is ureyistercd substribcr. 1y"7t pur"t oJ thesc Fut'tshect.t ntuy be repruiluced. storad irt u rt'trieyal.r |J/cir l , ()r trunsmittcd, in unv ot l tcr. forn or by uttt othcr ntettt ts,v' i t l t r tut t l tc pri t) t 'p(rmis.\ i t tnrt r lrc nubl isher: /SSN l35l-5136

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