Lecture 19: Punctuated
Equilibrium
Background:
• paleontology: idiographic → nomothetic
(descriptive → theoretical)
Punctuated Equilibrium first introduced by:
• Mayr (1954): allopatric speciation model
Application to paleontology → REVOLUTION!
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 pop’ns 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)
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 env’t)
Paleontology
• phyletic gradualism
• anagenesis is most important
• “Species problem” (chronospecies - only morph)
species
morph
rest are
intermediates
time
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 pop’ns: no local isolation
• ecophenotypic variation: no changes in gene
freq.; responses to environment
Problems...
• Large studies of bryozoans, molluscs,
mammals do not fit gradualist model
• Stasis with sudden appearance of new forms
“Punctuated Equilibrium”
Neontology
1) Modern spp: mostly cladogenesis:
multiplicat’n & diversificat’n
2) Most common speciation: allopatric isolates
3) Speciation rare (prob extinction > prob speciation)
4) Parent to daughter transition time: short
5) Sig. changes in daughter pop’n (founder effect)
6) Adap’ns in daughter pop’ns excluded from parental pop’ns
(RIMs)
7) Gene flow in parent pop’n inhibits direct’nal change
(genetic homeostasis)
8) Most changes in morph restricted to speciation events
Implications of P.E. for
Paleontology
• Speciation:
Short timeline
Small area
• Sudden appearance in fossil record with no
transitional forms
Why controversial?
• Panselectionist view (adaptationist program)
• Each feature under constant selection
• Species “tracking” envt’l changes in “adaptive
landscape”
Explanations for stasis
• Fossil record incomplete
• Stabilizing selection
But: evidence for spp. stability in spite of envt’l
change
e.g. glaciations: some gradual changes but many
spp. unchanged (migration)
Current Ideas:
Stasis maintained by :
• integrated gene complexes
• developmental constraints
• gene flow in large populations
“polyhedron” vs. “rolling ball”
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. var’n w/o speciation
Population Genetics:
• Wright (1977):
“Shifting Balance”:
Drift
- Gene Flow
(Local Pop’ns)
+ Selection
(Among Pop’ns)
• may lead to large, adaptive changes in a
widespread pop’n
• rapid evolutionary changes with changes in
ecological conditions
Phenotypic space
• Adaptation to new conditions more likely if many
spp. rather than one species
• occupy more “phenotypic space”:
character y
new
character
optima
character x
Evidence
• “Living Fossils”: clades with low spp. diversity
( speciation;  anagenesis)
But:
Minnows (many spp.)
Sunfish (few spp.)
Arose at similar time
No diff’n in morph divergence
Speciation  Morph Evol’n
Conclusion: do not need cladogenesis for anagenetic
change
But, speciation is necessary for adaptive radiation into
sympatric niches
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
Differential Speciation
time
t2
t1
body size
Differential Extinction
time
t2
t1
body size
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 pop’ns or individuals)
e.g. tendency for rapid speciation a property of sp.,
not of individuals
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
• Traditional Neo-Darwinists: no diff’n b/w sp.
selection & NS
• Reductionist viewpoint : orgs are “gene vehicles”
(e.g. Dawkins)
• Don’t 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.