Speciation

biological species concept.

1. Chapter six
Speciation
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2. Outlines
Introduction
Speciation
Patterns of speciation
Species concepts
Modes of speciation
Rates of species
Isolating mechanism of speciation
Types of evolution
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3. Introduction
Species-agroupoforganismswiththeabilitytointerbreed
andproducefertileoffspringinthenaturalenvironment.
Aspeciesisasinglelineageoforganismsthatmaintainsits
identityfromothersuchlineagesandhasitsown
evolutionarytendenciesandhistoricalfate(Wiley1978)
Aspeciesisagroupoforganismsthatmaintainsa
distinctivesetofattributesinnature.
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4. Speciation
Speciationistheevolutionofoneormorespeciesfromasingle
ancestorspecies.
Itcanoccurwhenmembersofapopulationbecomeisolatedfromeach
other.
Oncetwopopulationsarereproductivelyisolated,theyareconsidered
separatespecies.
Speciationistheprocesswherebynewspeciesareformed.
Speciationleadstomacroevolution
Macroevolutionrefers to the evolutionary changes that create new
species and groups of species.
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5. Speciation
Creates multiple “daughter” species from
one ancestor
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6. Cont…
Theprocessbywhichonegenetically-cohesive
populationsplitsintotwoormorereproductively-
isolatedpopulations.
Requiresthedisruptionofgeneflowandtheevolution
ofreproductiveisolatingmechanisms(RIMs).
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7. Speciation involves barriers to reproduction
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8. Patterns of speciation
Therearetwopatternsofspeciationasevidencedbythefossilrecord
•Anagenesis
Itistheaccumulationofchangesassociatedwiththetransformationofone
speciesintoanother.
Evolutiontakingplaceinasinglegroup (alineage)withthepassageof
time.
Doesnotpromotebiologicaldiversity
•Cladogenesis
The splitting of one lineage into two new species arise through
cladogenesis.
It is also called branching evolution, is the budding of one or more new
species from a parent species.
promotes biological diversity by increasing the number of species.
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10. Cont…
Anagenesis
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Cladogenesis

11. Species concept
Biologicalspeciesconcept:
Thebiologicalspeciesconceptdefinesaspeciesasmembersof
populationsthatactuallyorpotentiallyinterbreedinnaturenot
accordingtosimilarityofappearance.
Speciesaregroupsofactuallyorpotentiallyinterbreeding
naturalpopulations,whicharereproductivelyisolatedfromother
suchgroups(Mayer,1940).
Aspeciesisareproductivecommunityofpopulations(
reproductivelyisolatedfromothers)thatoccupiesaspecific
nicheinnature(Mayer,1982)
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12. Cont…
Speciesarethemembersinaggregateofagroupof
populationsthatbreedorpotentiallyinterbreedwitheach
otherundernaturalconditions(Futuyma).
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13. Cont…
EcologicalSpeciesConcept:
Aspeciesisalineagethatoccupiesanicheminimally
differentfromthatofanyotherlineageinitsrangeand
whichevolvesseparatelyfromalllineagesoutsideits
range.
Evolutionary species concept:
A species is a single lineage of ancestor-descendant
populations which maintain its identity from other such
lineages and which has it own evolutionary tendencies
and historical fate.
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14. Cont…
Pheneticspecies concept:
Aspeciesisasetoforganismsthatlooksimilartoeach
otheranddistinctfromothersets.
Phenotypicsimilarityisallthatmattersinrecognizing
separatespecies.
Phylogenetic species concept:
Aspeciesisthesmallestdiagnosableclusterofindividual
organismswithinwhichthereisaparentalpatternof
ancestryanddescent.
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15. What causes speciation?
Speciation,ortheevolutionofreproductiveisolation,
occursasaby-productofgeneticchangesthataccumulate
betweentwopreviouslyinterbreedingpopulationsofthe
samespecies.
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16. How does speciation occur?
Thereareseveraldifferentwaysinwhichtheevolutionof
reproductiveisolationisthoughttooccur.
Thesecanbegeneralizedintoaseriesofevents,orsteps.
The“Steps”inaspeciationevent:
Step1:geneflowbetweentwopopulationsisinterrupted
(populationsbecomegeneticallyisolatedfromeachother)
Step2:geneticdifferencesgraduallyaccumulatebetweenthe
twopopulations(populationsdivergegenetically)
Step3:reproductiveisolationevolvesasaconsequenceof
thisdivergence(areproductiveisolatingmechanismevolves
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17. The Rate of Speciation
Acomparisonofgradualismandpunctuatedequilibrium.
Gradualspeciationisthegradualdivergenceofpopulations
untiltheyhavereachedthelevelsofspecificdistinctness.
Duringpunctuatedequilibrium,longperiodsofequilibrium
inwhichspeciesexistessentiallyunchangedare;
punctuatedbyrelativelyshortperiodsofevolutionary
changeduringwhichphenotypiccharacteristicsmaychange
rapidly.
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18. Cont…
Comparison between gradualism and punctuated equilibrium
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20. Modes of Speciation
Speciation means the formation of species.
The alternative methods of speciation have been proposed
as follow:
Allopatric speciation,
Sympatric speciation,
Parapatricspeciation, and
Peripatricspeciation (altationalspeciation).
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21. Modes of Speciation
Allopatric Speciation:
Speciationwithgeographicisolation.
Evolutionofreproductiveisolatingmechanismsbetweenpopulations
thataregeographicallyseparated.
Geographicseparationisdefinedbyspatialrestrictionofgeneflow,
notjustphysicaldistance.
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22. MODEL OF ALLOPATRIC SPECIATION
ANCESTRAL
POPULATION
GEOGRAPHIC
BARRIER
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23. .
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24. Cont…
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25. PeripatricSpeciation:
Speciation in a peripheral isolate.
Due to rare dispersal or colonization events.
Also referred to as Founder Effectspeciation.
In small populations DRIFTand NATURAL SELECTION
may cause rapid divergence from the parental population
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ANCESTRAL
POPULATION
PERIPHERAL ISOLATE

26. Cont…
.
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27. Sympatric speciation
Sympatricspeciationinvolvestheformationofdifferent
speciesthatarenotgeographicallyisolatedfromone
another.
Patterninwhichspeciationoccursintheabsenceofa
physicalbarrier
Adaptationtolocalenvironments,andsexualselection
aremechanismsthatcanpromotesympatricspeciation.
Aspeciesachievesreproductiveisolationandevolvesin
thesamegeographiclocationasitsancestralspecies
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28. Cont…
 polyploidy(extrasetsofchromosomes)isamajor
factorinsympatricspeciationinplants
autopolypoidy–multiplesetsfromoneparent
species;example–plantsaroundAgentOrangesites
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29. Primulakewensisarosevia
allopolyploidyinaninterbreedingeven
ofP.floribundaandP.verticiliatain
1898attheRoyalBotanicGardensin
Kew,England
Eachoftheparentalstockshavea2n=
18.
P.kewensisis2n=36becauseof
chromosomalduplicationeventsafter
thehybridcross–wasobserved3times
P.kewensisnowapopulargardenplant.
Example, Primrose Speciation
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31. Parapatricspeciation
Itoccurswhenpopulationsareseparatednotbya
geographicalbarrier,suchasabodyofwater,butbyan
extremechangeinhabitat.
Whilepopulationsintheseareasmayinterbreed,they
oftendevelopdistinctcharacteristicsandlifestyles.
Itisextremelyrare.
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33. Summery
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34. Isolation Mechanisms
Isolationisanimportantfactorcontributingtothe
processofevolution.
Itisthesegregationofthepopulationofaparticular
speciesintosmallerunitswhichpreventinterbreeding
betweenthem.
Isolationaidsinsplittingofthespeciesintoseparate
groups.
Inconsequenceofbeingseparatedfromoneanother,
organismshavedevelopeddifferentcharacteristics,dueto
whichtheyhavebecomeseparatespecies.
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35. Types of Isolation
Evolutionarybiologistshaverecognizedthefollowing
typesofisolationswhichcausespeciation.
1.IsolationbyTime:
AlsoknownasSeasonalortemporalisolation
Thepopulationsexistinthesameregionsbutaresexually
matureatdifferenttimes,sothatpotentialmatesremain
unabletomate.
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36. 2. Isolation by Distance (Spatial Isolation)
Sheerdistanceapartmayalsoactasanisolatingfactorfora
specieswhichoccupiesagreatrangeofarea,whichis
unbrokenbyeffectivebarriers.
Itsexampleiswrens(birds)ofSouthAmerica.
Wrensarefoundalloverthecontinentbutthewrensofone
regiondifferfromthoseoftheotherincolourpatterns,size,
proportionsandhabits.
Itshowsthatwithoutanybarrier,sheerdistanceaparttends
toproducelocalraces(subspecies).
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37. Cont…
. Variation among wrens due to Spatial Isolation
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38. 3. Geographical Isolation
Itisthemostcommontypeofisolationandoccurswhenan
originalpopulationisdividedintotwoormoregroupsby
geographicalbarrierssuchasariver,desert,glacier,mountainor
ocean.
Allofwhichpreventinterbreedingbetweenthem,theninthe
courseoftimedifferentmutationsmaybecomeincorporatedin
thegenepoolsofthedifferentgroups.
Certainadditionalkindsofgeographicalbarriersarethevolcanic
formationofamountainonland,mountainrangeswithdeep
valleysbetweenandlandmassesasislandsinsea.
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39. Cont…
.
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40. Cont…
.
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41. Cont….

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42. 4. Reproductive Isolation
Insexuallyreproducingorganismsspeciescanbedefinedas
Mendelianpopulationsbetweenwhichthegeneexchangeisprevented
byreproductiveisolation..
Insimpletermsisolatingmechanismsarethosewhichprevent
successfulreproductionbetweenmembersoftwoormorepopulations
(viz.,closelyrelatedspecies)thathavedescendedfromthesame
originalpopulation.
Evolutionofreproductiveisolatingmechanismspreventsnascent
speciesfrominterbreeding.
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43. Types of reproductive isolating mechanisms
MostmodernevolutionistssuchasMecham(1961),Mayr
(1948,1970),Stebbins(1966,1971),etc.,haveclassified
thereproductiveisolatingmechanismsintotwoclasses.
Theseare:
premating or prezygoticisolating mechanisms and
Postmatingor postzygoticisolating mechanisms.
PrezygoticMechanismsprevent mating formation of
viable or zygotes.
PostzygoticMechanismsprevent hybrids from passing on
their genes.
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44. Cont…
prematingorprezygoticisolatingmechanismsprevent
wastageofgametes(germcells)andsoarehighly
susceptibletoimprovementbynaturalselection;
postmatingorpostzygoticisolatingmechanismsdonot
preventwastageofgametesandtheirimprovementby
naturalselectionisindirect
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45. A.Prematingorprezygoticisolatingmechanisms
Mechanismsthatpreventinterspecificcrosses(i.e.,
fertilizationandzygoteformation).
Causesofprezygoticisolatingmechanism:
1. Habitat isolation :
Two closely related species may occupy different ecosystems
within a geographical region.
The populations live in the same regions but occupy different
habitats, so that potential mates do not meet.
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46. Cont…
Example1:CentralCaliforniapopulationsofRanaspp.
◦TheRed-leggedFrog(Ranadraytonii)tendstobreedinlarge
ponds.
◦TheYellow-leggedFrog(Ranaboylii)breedsalmostexclusivelyin
fast-movingstreams.
Theirhabitatpreferencescontributetotheirreproductiveisolation.
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47. Cont…
Example 2: European populations ofTurdusspp.
TheCommonBlackbird(Turdusmerula)livesandbreedsinforest.
TheRingOuzel(Turdustorquatus),aclosecousin,livesandbreedson
moors.
Evenwhenforestandmoorabut,thetwospeciesdonotinterbreed.
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48. 2. Seasonal or temporal isolation
Thepopulationsexistinthesameregionsbutaresexuallymatureatdifferenttimes,
sothatpotentialmatesremainunabletomate.
Example1:CloselyrelatedRanaspeciesinCaliforniaCoastal
Ecosystems
◦TheRed-leggedFrog(Ranadraytonii)breedingseasonlastsfrom~
November-lateApril.
◦TheYellow-leggedFrog(Ranaboylii)breedingseasonlastsfrom~
lateApril-June.
◦Thebreedingseasonsmayoverlapinsomeareas.
◦Thecombinationofecologicalandtemporalisolationprevents
hybridization.
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49. Cont…
Example2:CloselyrelatedFruitFliesinHawaii
Drosophilapersimilisbreedsinearlymorning.
CloselyrelatedDrosophilapseudoobscurabreedsin
theafternoon.
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50. 3. Ethological (Behavioral)isolation :
Specieswithcomplexcourtshipbehaviorsusuallyexhibit
stereotyped"callandresponse"signalsbetweenmaleand
femalebeforeactualmatingtakesplace.
Theseritualspreventwastedmatingeffortthatwouldhalt
genetransmissionbyinviableorinfertilehybrids.
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51. Cont…
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52. 4. Mechanical Isolation
Crossfertilizationorpollinationispreventedorrestricted
bydifferencesinstructureofreproductiveorgans
(genitaliainanimals,flowersinplants).
Sothat,copulationisattemptedbutnotransferofsperm
takesplace.
Morphologicaldifferencesbetweenspeciesprevent
hybridization.
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53. Cont…
Example1:SnailShellCoiling
Insomesnailspecies,thedirectionofshellcoiling
iscontrolledbyasingle(maternaleffect)gene.
Left-coilingsnailscannotmatewithright-coilingsnails.
Suchmutationscouldquicklyleadtofurtherdifferentiationand,
possibly,speciation.
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54. 5. GameticIsolation
Inthiscase,spermandovaofthetwospeciesare
chemicallyincompatible,andwillnotjointoforma
zygote.
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55. B. Postmatingor postzygoticisolating
mechanisms
Fertilizationtakesplace,hybridzygotesareformed,but
theseareinviable,orgiverisetoweakorsterilehybrids.
1.Hybridinviabilityorweakness:
Spermandeggfromthetwospeciesmaycombine,butthe
geneticinformationisinsufficienttocarrytheorganism
throughnormaldevelopment.
Theembryodiesafterafewcleavages,orsometime
beforebirth/hatching.
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56. Cont…
Example:Drosophilaspp.
◦Despite their superficially similar appearance,D.
melanogasterandD. simulanshave incompatible alleles
for nuclear pore proteins.
◦Dysfunction of this vital gene results in inviablehybrids.
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57. 2. Developmental hybrid sterility:
Hybridsaresterilebecausegonadsdevelopabnormally,or
meiosisbreaksdownbeforeitiscompleted.
Example 1: Tigers (Pantheratigris) and Lions (Pantheraleo)
◦Tigers and Lions are sister taxa, but separate for millions of
years.
◦Their hybrid offspring are viable and robust, but sterile.
◦Chromosomes are not homologous, so do not migrate
normally at meiosis.
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58. Cont…
male tiger x lioness --> tigon
male lion x tigress --> liger
reciprocal cross offspring are somewhat different
◦maternal mitochondria input.
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59. Cont…
Example 2: Horse (Equuscaballus) and
Donkey (Equusasinus)
◦Horses and donkeys have been separate
species for millions of years.
◦Their hybrid offspring are viable and
robust, but sterile.
◦male horse x female donkey --> hinny
◦male donkey x female horse --> mule
◦As above, reciprocal cross offspring are
somewhat different
maternal mitochondrial input
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60. 3. Hybrid Breakdown
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Tworelatedspeciescanhybridize,andtheirF1offspringarefertile.
Butsuccessivegenerations(F2andbeyond)sufferlowerviabilityor
fecundity.
Thus,theycannotbecomeanestablishedpopulation.
Example:Ricecultivars
◦Cultivarsofdomesticricehavebeenartificiallyselectedforcenturies.
◦Somearecloselyrelatedenoughtohybridize.
◦F1hybridsarefertileandviable.
F2generationisstuntedandsterile.

61. Cont…
Hybridization between closely related species can have one of several
possible outcomes.
◦Species Reinforcement
Hybrids have lower fitness than either parent species.
Reproductive isolation is maintained due to lack of hybrid
survival/reproduction.
◦Species Fusion
Two species in a hybrid zone may have weak reproductive isolating
barriers.
The two species may, over time, eventually share a common gene
pool.
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62. Cont…
◦Species Stability/Hybrid Equilibrium
Hybrids are continually produced by the two parent
populations in a hybrid zone.
A narrow hybrid zone can foster constant hybridizationwith
reduced hybrid survival..
◦Hybrid Speciation
Hybrids may be reproductively superiorto parent
populations.
Positive assortativemating among hybrids can lead tohybrid
speciation.
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63. Patterns of Evolution
Evolutionovertimecanfollowseveral
differentpatterns.
Factorssuchasenvironmentandpredationpressurescan
havedifferenteffectsonthewaysinwhichspecies
exposedtoevolve.
Asaresulttherearethreemaintypesofevolution:
1.Divergentevolution,
2.Convergentevolution,and
3.Parallelevolution.
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64. 1. Divergent evolution
processwherespeciesbecomeincreasinglydifferentovertime.
adaptiveradiationisaformofdivergentevolutionwhereanumber
ofdiversespeciesformfromasingleancestralone.
eg.GalapagosfinchesobservedbyDarwin
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65. Cont…
Homologousstructuresalsoshowsthatdivergent
evolution
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66. 2. Convergent evolution
Theappearanceofapparentlysimilarstructuresin
organismsofdifferentlinesofdescent.
Thedevelopmentthroughevolutionofsimilarfeaturesby
organismswithdistinctlydifferentancestors.
Acommonexampleofthisistheevolutionofwingsin
insectsandbirds.
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67. Cont…
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68. 3. Parallel Evolution
istheindependentevolutionofsimilartraits,starting
fromasimilarancestralcondition.
Frequentlythisisthesituationinmorecloselyrelated
lineages,whereseveralspeciesrespondtosimilar
challengesinasimilarway.
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69. Summery
,
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