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 Equilibrium1) neontology informs paleontology

(understand past via present)2) speciation is cladogenesis, not anagenesis

“real speciation” vs. “phyletic speciation”3) speciation by peripheral isolates4) widespread pop’ns change slowly, if at all5) sp. usually develop in geog. limited regions6) sp. develop in stratigraphically limited extent7) abrupt appearance of new spp. (fossil record)8) adaptive change mostly during speciation9) 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)

time

mor

ph

species

rest are intermediates

Bias in LiteratureEvidence supporting gradualism• e.g. microscopic protoctists: radiolarians,

diatoms, foramsBut 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”

Neontology1) Modern spp: mostly cladogenesis:

multiplicat’n & diversificat’n2) Most common speciation: allopatric isolates3) Speciation rare (prob extinction > prob speciation)4) Parent to daughter transition time: short5) 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 + Selection(Local Pop’ns) (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 x

char

acte

r y new

character optima

Evidence• “Living Fossils”: clades with low spp. diversity( speciation; anagenesis)But:

Minnows (many spp.) Sunfish (few spp.)

Arose at similar timeNo diff’n in morph divergence

Speciation Morph Evol’nConclusion: do not need cladogenesis for anagenetic changeBut, 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

t 2

t 1

body size

tim

eDifferential Speciation

t 2

t 1

body size

tim

eDifferential Extinction

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.