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.