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 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)

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 env’t)

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 pop’ns: 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: 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

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” envt’l changes in “adaptive landscape”

10. 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)

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. var’n w/o speciation

13. 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

14. 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

15. 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

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 pop’ns 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 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.