Download presentation
Presentation is loading. Please wait.
Published byChloe Rodgers Modified over 9 years ago
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) time morph species rest are intermediates
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
Phenotypic space Adaptation to new conditions more likely if many spp. rather than one species occupy more “phenotypic space”: character x character y 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
t 2 t 1 body size time Differential Speciation
18
t 2 t 1 body size time Differential Extinction
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.
Similar presentations
© 2024 SlidePlayer.com Inc.
All rights reserved.