speciation chapter 18. barriers to gene flow whether or not a physical barrier deters gene flow...
TRANSCRIPT
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