Lecture 19: Punctuated Equilibrium - PowerPoint PPT Presentation

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Lecture 19: Punctuated Equilibrium

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Application to paleontology REVOLUTION! Tenets of Punctuated ... Minnows (many spp.) Sunfish (few spp.) Arose at similar time. No diff'n in morph divergence ... – PowerPoint PPT presentation

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Title: Lecture 19: Punctuated Equilibrium


1
Lecture 19 Punctuated Equilibrium
  • Background
  • paleontology idiographic ? nomothetic
  • (descriptive ? theoretical)
  • Punctuated Equilibrium first introduced by
  • Mayr (1954) allopatric speciation model
  • Application to paleontology ? REVOLUTION!

2
Tenets of Punctuated Equilibrium
  • 1) neontology informs paleontology
  • (understand past via present)
  • 2) speciation is cladogenesis, not anagenesis
  • real speciation vs. phyletic speciation
  • 3) speciation by peripheral isolates
  • 4) widespread popns change slowly, if at all
  • 5) sp. usually develop in geog. limited regions
  • 6) sp. develop in stratigraphically limited
    extent
  • 7) abrupt appearance of new spp. (fossil record)
  • 8) adaptive change mostly during speciation
  • 9) trends in adaptation sp. selection (sp.
    sorting)

3
Fossil Record
  • punctuated stasis real, not an artifact of
    preservation
  • Taphonomy how orgs are preserved as fossils
  • Geological Processes
  • sediment deposition (varies in time space)
  • erosion
  • compression
  • hard vs. soft parts
  • niches - skewed samples (characteristics of
    envt)

4
Paleontology
  • phyletic gradualism
  • anagenesis is most important
  • Species problem (chronospecies - only morph)

species
rest are intermediates
morph
time
5
Bias in Literature
  • Evidence supporting gradualism
  • e.g. microscopic protoctists radiolarians,
    diatoms, forams
  • But characteristics of organisms
  • asexual or alternation of generations
  • no genetic exchange among lineages
  • enormous popns no local isolation
  • ecophenotypic variation no changes in gene
    freq. responses to environment

6
Problems...
  • Large studies of bryozoans, molluscs, mammals do
    not fit gradualist model
  • Stasis with sudden appearance of new forms
  • Punctuated Equilibrium

7
Neontology
  • 1) Modern spp mostly cladogenesis
  • multiplicatn diversificatn
  • 2) Most common speciation allopatric isolates
  • 3) Speciation rare (prob extinction gt prob
    speciation)
  • 4) Parent to daughter transition time short
  • 5) Sig. changes in daughter popn (founder
    effect)
  • 6) Adapns in daughter popns excluded from
    parental popns
  • (RIMs)
  • 7) Gene flow in parent popn inhibits directnal
    change (genetic homeostasis)
  • 8) Most changes in morph restricted to speciation
    events

8
Implications of P.E. for Paleontology
  • Speciation
  • Short timeline
  • Small area
  • Sudden appearance in fossil record with no
    transitional forms

9
Why controversial?
  • Panselectionist view (adaptationist program)
  • Each feature under constant selection
  • Species tracking envtl changes in adaptive
    landscape

10
Explanations for stasis
  • Fossil record incomplete
  • Stabilizing selection
  • But evidence for spp. stability in spite of
    envtl change
  • e.g. glaciations some gradual changes but many
    spp. unchanged (migration)

11
Current Ideas
  • Stasis maintained by
  • integrated gene complexes
  • developmental constraints
  • gene flow in large populations
  • polyhedron vs. rolling ball

12
Is rapid change always linked with Speciation?
  • Cladogenesis w/o Anagenesis
  • e.g. Plethodon (salamanders)
  • - diverged 60 mya (molecular)
  • - little morph change
  • Anagenesis w/o Cladogenesis
  • e.g. mimetic butterflies ring species
  • -much geog. varn w/o speciation

13
Population Genetics
  • Wright (1977)
  • Shifting Balance
  • Drift - Gene Flow Selection
  • (Local Popns) (Among Popns)
  • may lead to large, adaptive changes in a
    widespread popn
  • rapid evolutionary changes with changes in
    ecological conditions

14
Phenotypic space
  • Adaptation to new conditions more likely if many
    spp. rather than one species
  • occupy more phenotypic space

new character optima
15
Evidence
  • Living Fossils clades with low spp. diversity
  • (? speciation ? anagenesis)
  • But
  • Minnows (many spp.) Sunfish (few spp.)
  • Arose at similar time
  • No diffn in morph divergence
  • Speciation ? Morph Evoln
  • Conclusion do not need cladogenesis for
    anagenetic change
  • But, speciation is necessary for adaptive
    radiation into sympatric niches

16
Species Selection (Species Sorting)
  • Major tenet of P.E.
  • concept related spp. overlap in niche space
  • competition displacement extinction
  • 2 main processes leading to anagenetic change
  • Differential Speciation
  • Differential Extinction

17
Differential Speciation
t 2
time
t 1
body size
18
Differential Extinction
t 2
time
t 1
body size
19
Implications
  • Very different views about properties of spp.
  • P.E. survival of a sp. depends on
    species-specific properties
  • Species are units with integrity
  • (not just collections of popns or individuals)
  • e.g. tendency for rapid speciation a property of
    sp., not of individuals

20
Comparisons of Speciation Rates
  • African antelopes impalas slow
  • wildebeest fast
  • Marine Snails planktonic larva slow
  • non-planktonic fast
  • b/c of Genetic Isolation
  • same in
  • Burrowing rodents coruros slow
  • tuco-tuco fast

21
  • Traditional Neo-Darwinists no diffn b/w sp.
    selection NS
  • Reductionist viewpoint orgs are gene vehicles
    (e.g. Dawkins)
  • Dont see spp. as entities
  • Implication If species selection is real
  • Macroevolution is not Microevolution writ large!
  • Phenomena at microevolutionary scale may have
  • little to say about major evolutionary events.
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