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

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lecture 19 punctuated equilibrium
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
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
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
Paleontology
  • phyletic gradualism
  • anagenesis is most important
  • “Species problem” (chronospecies - only morph)

species

rest are

intermediates

morph

time

bias in literature
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
Problems...
  • Large studies of bryozoans, molluscs, mammals do not fit gradualist model
  • Stasis with sudden appearance of new forms

“Punctuated Equilibrium”

neontology
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
Implications of P.E. for Paleontology
  • Speciation:

Short timeline

Small area

  • Sudden appearance in fossil record with no transitional forms
why controversial
Why controversial?
  • Panselectionist view (adaptationist program)
  • Each feature under constant selection
  • Species “tracking” envt’l changes in “adaptive landscape”
explanations for stasis
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
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
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
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

character y

character x

Phenotypic space
  • Adaptation to new conditions more likely if manyspp. rather than one species
  • occupy more “phenotypic space”:

new character optima

evidence
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
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

slide17

Differential Speciation

t 2

time

t 1

body size

slide18

Differential Extinction

t 2

time

t 1

body size

implications
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
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

slide21
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.