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Lecture 22: Coevolution
• reciprocally induced evolutionary Δ’s in 2 + spp. or pop’ns
• Mutualistic vs. Antagonistic
type species 1 species 2
commensalism + 0
competition - -
predation + -
parasitism + -
mutualism + +
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Mutualisme.g. C. Am. Acacias & Ants: Herbivory: growth; permits competition
from fast growing spp. • 90% acacia spp: bitter alkaloids → prevent
insect/mammal browsing • 10% spp: lack alkaloids; have symbiotic
ants
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Acacias Ants• swollen thorns
(nest sites)
• petioles (nectaries)
• Beltian bodies (protein)
• attack herbivores
• remove fungal spores
• attack shading plants
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Competition
Anolis spp.
• spp. turnover (Caribbean islands) due to coevol’n• carrying capacity of island is a function of body
size:
best body size for invading spp
body size
freq
uenc
y
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body size
freq
uenc
y
body size
After Invasion:- invader selected for smaller body size- competition displaces residents : body size ↓
Later:-invader evolves to optimum body size- eventually, residentdriven to extinction
freq
uenc
y
X
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Sequential Evolution
“tit for tat”
e.g. plants & herbivorous insects (predation):
plants : 2° metabolites to repel insects
insects: detoxification (mixed function oxidases)
e.g. nicotine: from a.a. or sugar pathway
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Erlich & Raven (1964):
2° metabolites → new adaptive zones
MFOs → new adaptive zones
• leads to cycle of adaptive radiations
& ↑ diversity
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speciation of plant → speciation of insectOR
speciation of insect → speciation of plant
Phylogenetic analysis of sequential evolution:e.g. pinworm parasites of primates:
congruent phylogenies divergence in host → divergence of parasite
not the other way around• parasite/host interactions:host evolves defenses
should parasite ↑ or ↓ virulence?depends!
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Virulence1) Transmission:• Correlated w repro rate: NS ↑ virulence • Requires live host: NS ↓ virulence (trade-off)e.g. Myxoma virus of rabbits
2) Coinfection• 1 parasite : all offspring related
kin selection: → ↓ virulence• multiple infection : competition
selection for ↑ repro rate → ↑ virulence
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3) Type of Transmission:
• Horizontal: ↑ virulence
• Vertical: ↓ virulence
“Arms Race” : adaptive advances must be countered or face extinction!
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e.g. “Brain Size Race” b/w Ungulates & Carnivores:
a) Ungulateb) Carnivore
archaic
paleogene
neogenerecent
Pop
ulat
ion
dist
’n
Brain:Body size ratio
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Conclusions
• Relative brain size ↑ through time
• Carnivores are “smarter” than ungulates
• Evidence for coevolution?
• Less evidence for coevol’n of running speed
Why? costs of adaptation
• resistance to 1 pred. may ↑ vulnerability to others
e.g. Cucurbitacins:protect from mites; attract beetles
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Generally:
Specialist predator; Single prey → coevol’n probable
Multiple Interactions → coevol’n slow; sporadic
How important is coevolution to pattern of diversity?
• taxonomic survival curves: used to determine if survival of taxon is age-independent
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Taxonomic Survival Curves
• Does mortality (extinction) depend on age ?
age species 1 species 21 1000 10002 900 7403 810 6004 729 5805 656 5706 590 5607 531 5508 478 5409 430 460
Sp. 1: 10% die yearly, regardless of age
Sp. 2: mortality high for young & old; mortality low in middle age
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Log - linear analysis : Age - independent mortality is linear
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Taxonomic Survival Curves• log (# of taxa surviving) vs. age of taxon
• for most taxa: linear → age - independent
• 2 interpretations:
time time
a) constant rate of extinction b) variable rate of extinction independent of age
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ExtinctionProbability of Extinction: New Taxa = Old Taxa
• What causes extinctions?
• Biotic factors: antagonistic interactions
(pred’n, parasitism, compet’n) lag load: L =
Diff’n b/w mean & optimum genotypeL ↑ : rate of evolution ↑
Why? selection coefficient ↑L ↑ : probability of extinction ↑
Why? falling behind in the “arms race”
opt - opt
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Lag-Load Models
1. Contractionary
• sp. w ↑ L : falls behind, goes extinct
2. Expansionary
• sp. w ↓ L : outcompetes; increases
these 2 models are unstable
may fluctuate between 1 & 2
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3. Stationary: • all spp. L = 0• no change; no extinction• perturbations; back to equilibrium• extinctions not due to biotic factors• 4. Dynamic Equilibrium: “Red Queen” hypothesis• all spp. have ↑ L• Env’t constantly deterioratingdue to arms race• “running as fast as they can
to stay in the same place!”
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Implications of Red Queen to TSCs• older taxa same prob. of extinction as newer taxa
• log - linear survival curves are evidence for RQ
Why?: “zero - sum game” : means L stays constant
2 versions of RQ:
1. Strong •Abiotic factors negligible•Extinctions due to spp. inter’ns•improbable, but testable
2. Weak•Abiotic & Biotic factors imp.•likely true, but untestable
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Testing RQ using TSCs:
Evidence for Strong RQ:•constant chance of going extinct b/c of spp. interactions- extinctions even in constant physical env’t !
Evidence for weak RQ?:-other mechanisms b/c extinction rates fluctuate over time
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Lecture 23: Mass Extinctions
• Biodiversity: balance b/w spec’n & extinction
• > 99% of all species are extinct
• Because of:
1) Background extinctions:
• gen’lly due to biotic factors
• e.g. competition, predation etc.
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Background Rate• marine families: → relatively constant
• ~ 5 - 10 families / my
massextinctions
e.g. Sepkoski & Raup (1982)
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Ecological Significance of Mass Extinctions
1. Open up vast niche spaces2. Lead to adaptive radiations
e.g. mammals diversify after extinction of dinosaurs
3. Taxa can recover: e.g. ammonites decimated in Permian extinction; came back & diversified in Triassic
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Mass Extinctions of the Phanerozoic: “The Big 5”
1.) Cambrian (540 - 510 mya):• Explosion of diversification• Marine; soft-bodied (few fossils)• Evidence for ~ 4 separate events• Trilobites, conodonts, brachiopods hit hardCause: Glaciation:
- sea level ↓ (locked in ice)
- cold H2O upwelling & spread
- ↓ O2 levels?
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2.) Ordovician (510 - 438 mya)
• 2nd most devastating to marine organisms• Echinoderms, nautiloids, trilobites, reef - building
corals Causes: Glaciation of Gondwanaland• evidence in Saharan deposits• drifted over N. pole (cooling)• sea level ↓• losses correspond to start & retreat of glaciers
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3.) Devonian (408 - 360 mya)
• Terrestrial life starts & diversifies
• Extinctions over 0.5 - 15 my (peak ~ 365 mya)
• Marine more than terrestrial
• Brachiopods, ammonites, placoderms
Causes: Glaciation of Gondwanaland
• evidence in Brazil
• Meteor impact?
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4.) Permian (286 - 245 mya)
• formation of Pangea: continental area > oceanic• Devastation (~245 mya):
~96% marine spp; 75% terrestrial sppCauses: a) formation of Pangea?b) vulcanism? - basaltic flows in Siberia
- sulphates in atmosphere → ash cloudsc) glaciation at both poles: major climatic flux d) ↓ salinity of oceans?