lecture 22: coevolution reciprocally induced evolutionary Δ’s in 2 + spp. or pop’ns mutualistic...
TRANSCRIPT
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 + +
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
Acacias Ants• swollen thorns
(nest sites)
• petioles (nectaries)
• Beltian bodies (protein)
• attack herbivores
• remove fungal spores
• attack shading plants
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
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
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 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!
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
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:
a) Ungulateb) Carnivore
archaic
paleogene
neogenerecent
Pop
ulat
ion
dist
’n
Brain:Body size ratio
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 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
Log - linear analysis : Age - independent mortality is linear
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
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
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 deterioratingdue 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 •Abiotic factors negligible•Extinctions due to spp. inter’ns•improbable, but testable
2. Weak•Abiotic & Biotic factors imp.•likely true, but untestable
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
massextinctions
e.g. Sepkoski & Raup (1982)
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
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?
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 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?