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?