Lecture%2022:%20Coevolution

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Lecture 22 Coevolution

• reciprocally induced evolutionary ?s in 2 spp.
  or popns
• Mutualistic vs. Antagonistic

type species 1 species 2
commensalism 0
competition - -
predation -
parasitism -
mutualism
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Mutualism

• e.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 coevoln
• carrying capacity of island is a function of body
  size

best body size for invading spp
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After Invasion - invader selected for smaller
body size - competition displaces residents
body size ?
frequency
body size

• Later
• invader evolves to
• optimum body size
• - eventually, resident
• driven to extinction

frequency
X
body size
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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 insect
• OR
• 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 interactionshost evolves defenses
• should parasite ? or ? virulence?
• depends!

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Virulence

• 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

1. Ungulate
2. Carnivore

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Conclusions

• Relative brain size ? through time
• Carnivores are smarter than ungulates
• Evidence for coevolution?
• Less evidence for coevoln of running speed
• Why? costs of adaptation
• resistance to 1 pred. may ? vulnerability to
  others
• e.g. Cucurbitacinsprotect from mites attract
  beetles

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Generally

• Specialist predator Single prey ? coevoln
  probable
• Multiple Interactions ? coevoln 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 2 1 1000 1000 2
900 740 3 810 600 4 729 580 5
656 570 6 590 560 7 531 550 8
478 540 9 430 460
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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Extinction

• Probability of Extinction New Taxa Old Taxa
• What causes extinctions?
• Biotic factors antagonistic interactions
• (predn, parasitism, competn)

lag load L
Diffn b/w mean optimum genotype L ? rate of
evolution ? Why? selection coefficient ? L ?
probability of extinction ? Why? falling behind
in the arms race
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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
• Envt constantly deteriorating
• due 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. interns
• 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 envt !

• 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 specn extinction
• gt 99 of all species are extinct
• Because of
• Background extinctions
• genlly due to biotic factors
• e.g. competition, predation etc.

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Background Rate

• marine families ? relatively constant
• 5 - 10 families / my

mass extinctions
e.g. Sepkoski Raup (1982)
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Ecological Significance of Mass Extinctions

• Open up vast niche spaces
• 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 hard
• Cause 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 gt oceanic
• Devastation (245 mya)
• 96 marine spp 75 terrestrial spp
• Causes
• a) formation of Pangea?
• b) vulcanism? - basaltic flows in Siberia
• - sulphates in atmosphere ? ash clouds
• c) glaciation at both poles major climatic flux
• d) ? salinity of oceans?