EEB 321 Community Ecology: phylogenetics lecture
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Transcript of EEB 321 Community Ecology: phylogenetics lecture
EEB321: Community Phylogenetics
The study of how evolutionary relationships among species affect the structure of communities
Lecture outline
The effect of evolutionary relatedness on the structure of communities
1. Classic theory – limiting similarity and why relatedness might matter
2. Contemporary community phylogenetics – inferring pattern from process
3. The future of phylogeny-coexistence relationships: stabilizing differences, fitness differences, and beyond
The community assembly process
Regional species pool
Habitat filter
Biotic filter
Local community
Size of available seeds (cm)
Freq
uenc
y
2.0 2.51.0 1.5 3.0
Community assembly and limiting similarity
Size of available seeds (cm)
Freq
uenc
y
2.0 2.51.0 1.5 3.0
Community assembly and limiting similarity
Many cases of and , but no
Community assembly and limiting similarity
But, how do we measure similarity?
Assumptions:• we know which traits
are important
• the important traits can be measured
• trait combinations are either known or not important
Hutchinson: the niche is an n-dimensional hypervolume
Evolutionary relationships as a proxy for species similarity
Genus 1 Genus 2
Early evidence: species to genus ratios
Elton 1946: Fewer species per genus are present locally than are regionally available
Elton et al. 1946 J of Animal EcologySimberloff 1970 Evolution
Genus 1 Genus 2 Genus 3
But, taxonomic groupings provide coarse estimates of evolutionary time
More time = more opportunity for ecological specialization
Fact: we now know that many initial classifications were actually wrong
1 mya 30 mya
• 1970s: molecular + statistical techniques improved our ability to estimate evolutionary time– expensive and time consuming
• 1990s: new computational tools to expedite the process
• Now: $5 per sequence, compared to $1000s
New phylogenetic tools
Environmental filteringCompetitive interactions
Over-dispersion Under-dispersion
Webb et al. 2002 Annu Rev Ecol Syst
Phylogenetic distance to estimate community relatedness
Because branch lengths are proportional to evolutionary time, we can sum them for the community
2 2
Phylogenetic distance = 4 units
Phylogenetic distance to estimate community relatedness
Because branch lengths are proportional to evolutionary time, we can sum them for the community
6 6
Phylogenetic distance = 12 units
Phylogenetic distance to estimate community relatedness
Because branch lengths are proportional to evolutionary time, we can sum them for the community
46
Phylogenetic distance = 14 units = 4.6 units per species
2 2
Testing phylogenetic patterns
• we need to formally test if communities are phylogenetically over- or under-dispersed
• we can use a NULL model approach, where we compare our observed data to an expected pattern if assembly was RANDOM– observed PD > expected = overdispersed– observed PD < expected = underdispersed
Null models in phylogenetic community analyses
RANDOM
Freq
uenc
yobserved value
Distribution of expectedvalues under a random pattern
The observed value is not significantly different from the null expectation
Environmental filteringCompetitive interactions
Over-dispersion Under-dispersion
Webb et al. 2002 Annu Rev Ecol Syst
Null models in phylogenetic community analyses
RANDOM
observed value
The observed value is significantly greater than the null expectation under competition
Freq
uenc
y
Environmental filteringCompetitive interactions
Over-dispersion Under-dispersion
Webb et al. 2002 Annu Rev Ecol Syst
Null models in phylogenetic community analyses
RANDOM
observed value
The observed value is significantly less than the null expectation under environmental filtering
Freq
uenc
y
Environmental filteringCompetitive interactions
Over-dispersion Under-dispersion
Phylogenetic patterns actually tend to be weak, or are inconsistent with Webb et al.’s predictions when the ecological mechanisms are known. WHY?
Problem: differences can promote or preclude coexistence via competition alone
Stabilizing differencespromote coexistence
Fitness differenceslimit coexistence
Chesson 2000; Adler et al. 2006 Ecol Lett
per c
apita
gro
wth
rate
per c
apita
gro
wth
rate
rare common rare common
Evolutionary trajectories of stabilizing (ρ) to fitness (κ) differences
Mayfield & Levine 2010 Eco Lett
Coex
isten
ce m
etric
(Δ
ρ /Δ
κ)
Phylogenetic distance
stab. diffsevolve faster
COEXISTENCE ZONE
EXCLUSIONZONE
Estimating stabilizing (ρ) to fitness (κ) differences in a competition experiment
Experimentally estimate competition coefficients and finite rates of increase, and subthose into equations that calculate stabilizing and fitness differences
20 40 60 80
0.0
0.2
0.4
0.6
0.8
1.0
Phylogenetic distance (mya)
Stab
ilizi
ng d
iffer
ence
20 40 60 80
0
10
20
30
40
Phylogenetic distance (mya)
Fitn
ess
diffe
renc
e
Our results: stabilizing and fitness differences evolve at similar rates
Close relatives: stabilizing and fitness differences minimalDistant relatives: stabilizing and fitness differences large
Coexistence is equally likely between close and distant relatives – phylogenetic overdispersion via competition
Coexistence is equally likely between close and distant relatives
Our results: stabilizing (ρ) and fitness (κ) differences evolve at similar rates
Coex
isten
ce m
etric
(Δ
ρ /Δ
κ)
Phylogenetic distance
stab. diffsevolve faster(Webb et al.)COEXISTENCE
ZONE
EXCLUSIONZONE
stab. and fit. diffsevolve at similar rates (our results)
The dynamic interplay between ecology and evolution
Evolution Ecology
We have really only scratched the surface of this question!