Speciation Speciation

Chapter 18Chapter 18

Barriers to Gene FlowBarriers to Gene Flow

• Whether or not a physical barrier Whether or not a physical barrier

deters gene flow depends upon:deters gene flow depends upon: Organism’s mode of dispersal or Organism’s mode of dispersal or

locomotionlocomotion

Duration of time organism can move Duration of time organism can move

Speciation & Natural Speciation & Natural SelectionSelection

Natural selection Natural selection cancan lead to lead to speciationspeciation

Speciation can also occur as a result Speciation can also occur as a result of other microevolutionary processesof other microevolutionary processes Genetic driftGenetic drift MutationMutation

Morphology & SpeciesMorphology & Species

Morphological traits may not be Morphological traits may not be

useful in distinguishing speciesuseful in distinguishing species Members of same species may appear Members of same species may appear

different because of environmental different because of environmental

conditionsconditions

Morphology can vary with age and sexMorphology can vary with age and sex

Different species can appear identicalDifferent species can appear identical

Variable Variable MorphologyMorphology

Grown in water Grown on land

Figure 18.2Page 294

Biological Species Biological Species ConceptConcept

“ “Species are groups of interbreeding Species are groups of interbreeding natural populations that are natural populations that are reproductively isolated from other reproductively isolated from other such groups.”such groups.”

Ernst Ernst MayrMayr

Reproductive IsolationReproductive Isolation

Cornerstone of the biological species Cornerstone of the biological species conceptconcept

Speciation is the attainment of Speciation is the attainment of reproductive isolationreproductive isolation

Reproductive isolation arises as a Reproductive isolation arises as a by-product of genetic changeby-product of genetic change

Genetic DivergenceGenetic Divergence

Gradual accumulation of differences Gradual accumulation of differences in the gene pools of populationsin the gene pools of populations

Natural selection, genetic drift, and Natural selection, genetic drift, and mutation can contribute to mutation can contribute to divergencedivergence

Gene flow counters divergenceGene flow counters divergence

Genetic DivergenceGenetic Divergence

time A time B time C time D

daughter species

Figure Figure 18.318.3

Page 294Page 294

parent species

Reproductive Isolating Reproductive Isolating MechanismsMechanisms

Prezygotic isolationPrezygotic isolation

Mating or zygote formation is preventedMating or zygote formation is prevented

Postzygotic isolationPostzygotic isolation

Takes effect after hybrid zygotes formTakes effect after hybrid zygotes form

Zygotes may die early, be weak, or be sterileZygotes may die early, be weak, or be sterile

Prezygotic IsolationPrezygotic Isolation

Ecological IsolationEcological Isolation

Temporal IsolationTemporal Isolation

Behavioral IsolationBehavioral Isolation

Mechanical Isolation Mechanical Isolation

Gametic MortalityGametic Mortality

Postzygotic MechanismsPostzygotic Mechanisms

Zygotic mortalityZygotic mortality

Hybrid inviabilityHybrid inviability

Hybrid sterilityHybrid sterility

Mechanisms of Mechanisms of SpeciationSpeciation

Allopatric speciationAllopatric speciation

Sympatric speciationSympatric speciation

Parapatric speciationParapatric speciation

Allopatric SpeciationAllopatric Speciation

Speciation in geographically Speciation in geographically

isolated populations isolated populations

Some sort of barrier arises and Some sort of barrier arises and

prevents gene flowprevents gene flow

Effectiveness of barrier varies with Effectiveness of barrier varies with

speciesspecies

Extensive Divergence Extensive Divergence Prevents InbreedingPrevents Inbreeding

Species separated by geographic Species separated by geographic barriers will diverge geneticallybarriers will diverge genetically

If divergence is great enough it If divergence is great enough it will prevent inbreeding even if the will prevent inbreeding even if the barrier later disappears barrier later disappears

ArchipelagosArchipelagos

Island chains some distance from Island chains some distance from continentscontinents Galapagos IslandsGalapagos Islands Hawaiian IslandsHawaiian Islands

Colonization of islands followed by Colonization of islands followed by genetic divergence sets the stage for genetic divergence sets the stage for speciationspeciation

SpeciatioSpeciation on an n on an

ArchipelaArchipelagogo

1

23

4

1

23

4

1

2

A few individuals of a species on the mainland reach isolated island 1. Speciation follows genetic divergence in a new habitat.

Later in time, a few individuals of the new species colonize nearby island 2. In this new habitat, speciation follows genetic divergence.

Speciation may also follow colonization of islands 3 and 4. And it may follow invasion of island 1 by genetically different descendents of the ancestral species.

Figure 18.6Page 297

Hawaiian IslandsHawaiian Islands

Volcanic origins, variety of habitats Volcanic origins, variety of habitats

Adaptive radiations:Adaptive radiations: Honeycreepers - In absence of other Honeycreepers - In absence of other

bird species, they radiated to fill bird species, they radiated to fill

numerous nichesnumerous niches Fruit flies (Fruit flies (DrosophilaDrosophila) - 40% of fruit fly ) - 40% of fruit fly

species are found in Hawaiispecies are found in Hawaii

Hawaiian HoneycreepersHawaiian Honeycreepers

FOUNDER SPECIESFigure 18.7Page 297

Speciation without a Speciation without a BarrierBarrier

Sympatric speciationSympatric speciation Species forms within the home range Species forms within the home range

of the parent speciesof the parent species

Parapatric speciationParapatric speciation Neighboring populations become Neighboring populations become

distinct species while maintaining distinct species while maintaining

contact along a common bordercontact along a common border

Speciation by PolyploidySpeciation by Polyploidy

Change in chromosome number Change in chromosome number (3(3nn, 4, 4nn, etc.), etc.)

Offspring with altered chromosome Offspring with altered chromosome number cannot breed with parent number cannot breed with parent populationpopulation

Common mechanism of speciation in Common mechanism of speciation in flowering plantsflowering plants

Parapatric SpeciationParapatric Speciation

Adjacent Adjacent populations populations evolve into evolve into distinct species distinct species while while maintaining maintaining contact along a contact along a common common borderborder

BULLOCK’S ORIOLE

BALTIMORE ORIOLE

HYBRID ZONE

Figure 18.10Page 299

We’re All RelatedWe’re All Related

All species are related by descentAll species are related by descent

Share genetic connections that Share genetic connections that extend back in time to the extend back in time to the prototypical cellprototypical cell

Patterns of Change Patterns of Change in a Lineagein a Lineage

CladogenesisCladogenesis Branching patternBranching pattern Lineage splits, isolated populations Lineage splits, isolated populations

divergediverge

AnagenesisAnagenesis No branchingNo branching Changes occur within single lineageChanges occur within single lineage Gene flow throughout processGene flow throughout process

Evolutionary TreesEvolutionary Trees

new species

branch point (a time of divergence, speciation)

a single lineage

branch point (a time of divergence, speciation)

a new species

a single lineage

extinction (branch ended before present)

dashed line (only sketchy evidence of presumed evolutionary relationship)

Figure 18.11Page 300

Gradual ModelGradual Model

Speciation model in which species Speciation model in which species emerge through many small emerge through many small morphological changes that morphological changes that accumulate over a long time periodaccumulate over a long time period

Fits well with evidence from certain Fits well with evidence from certain lineages in fossil recordlineages in fossil record

Punctuation ModelPunctuation Model

Speciation model in which most Speciation model in which most changes in morphology are changes in morphology are compressed into brief period near compressed into brief period near onset of divergence onset of divergence

Supported by fossil evidence in Supported by fossil evidence in some lineagessome lineages

Adaptive RadiationAdaptive Radiation

Burst of divergence Burst of divergence Single lineage gives rise to Single lineage gives rise to

many new speciesmany new species New species fill vacant New species fill vacant

adaptive zone adaptive zone Adaptive zone is “way of life”Adaptive zone is “way of life”

Adaptive RadiationAdaptive Radiation

Figure 18.12Page 301

ExtinctionExtinction

Irrevocable loss of a speciesIrrevocable loss of a species Mass extinctions have played a Mass extinctions have played a

major role in evolutionary major role in evolutionary historyhistory

Fossil record shows 20 or Fossil record shows 20 or more large-scale extinctionsmore large-scale extinctions

Reduced diversity is followed Reduced diversity is followed by adaptive radiationby adaptive radiation

Who Survives?Who Survives?

Species survival is to some extent Species survival is to some extent randomrandom

Asteroids have repeatedly struck Asteroids have repeatedly struck Earth, destroying many lineagesEarth, destroying many lineages

Changes in global temperature favor Changes in global temperature favor lineages that are widely distributedlineages that are widely distributed