Transcript Lecture 22: Coevolution

Lecture 22: Coevolution
• reciprocally induced evolutionary Δ’s in 2 + spp.
or pop’ns
• Mutualistic vs. Antagonistic
type
commensalism
competition
predation
parasitism
mutualism
species 1
+
+
+
+
species 2
0
+
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
Acacias
• swollen thorns
(nest sites)
• petioles (nectaries)
• Beltian bodies (protein)
Ants
• attack herbivores
• remove fungal spores
• attack shading plants
Competition
frequency
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
frequency
After Invasion:
- invader selected for
smaller body size
- competition displaces
residents : body size ↓
frequency
body size
X
body size
Later:
-invader evolves to
optimum body size
- eventually, resident
driven to extinction
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
Erlich & Raven (1964):
2° metabolites → new adaptive zones
MFOs → new adaptive zones
• leads to cycle of adaptive radiations
& ↑ diversity
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 interactions:host evolves defenses
should parasite ↑ or ↓ virulence?
depends!
Virulence
1) 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
3) Type of Transmission:
• Horizontal: ↑ virulence
• Vertical: ↓ virulence
“Arms Race” : adaptive advances must be
countered or face extinction!
e.g. “Brain Size Race” b/w
Ungulates & Carnivores:
archaic
a) Ungulate
b) Carnivore
paleogene
neogene
Brain:Body size ratio
recent
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
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
Taxonomic Survival Curves
• Does mortality (extinction) depend on age ?
age
1
2
3
4
5
6
7
8
9
species 1
1000
900
810
729
656
590
531
478
430
species 2
1000
Sp. 1: 10% die yearly,
740
regardless of age
600
580
570
Sp. 2: mortality high for
young & old; mortality low
560
in middle age
550
540
460
log (number of survivors)
Log - linear analysis :
Age - independent mortality is linear
3.0
species 1
2.9
2.8
species 2
2.7
2.6
0
1
2
3
4
5 6
age
7
8
9 10
Taxonomic Survival Curves
• log (# of taxa surviving) vs. age of taxon
• for most taxa: linear → age - independent
• 2 interpretations:
time
a) constant rate of extinction
time
b) variable rate of extinction
independent of age
Extinction
Probability of Extinction: New Taxa = Old Taxa
• What causes extinctions?
• Biotic factors: antagonistic interactions
(pred’n, parasitism, compet’n)
lag load: L = opt - 
opt
Diff’n b/w mean & optimum genotype
L ↑ : rate of evolution ↑
Why? selection coefficient ↑
L ↑ : probability of extinction ↑
Why? falling behind in the “arms race”
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
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 deteriorating
due to arms race
• “running as fast as they can
to stay in the same place!”
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
2. Weak
•Abiotic factors negligible •Abiotic & Biotic factors
imp.
•Extinctions due to spp.
inter’ns
•likely true, but untestable
•improbable, but testable
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
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.
Background Rate
• marine families: → relatively constant
• ~ 5 - 10 families / my
mass
extinctions
e.g. Sepkoski &
Raup (1982)
Ecological Significance of Mass
Extinctions
1. Open up vast niche spaces
2. 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
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?
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
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?
4.) Permian (286 - 245 mya)
• formation of Pangea: continental area > 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?