Post on 15-Feb-2021
How competition affects evolutionary rescue:
theoretical insight
Matthew OsmondClaire de Mazancourt
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Evolutionary rescue
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Evolutionary rescue
0
Mean fitness
Mean trait value
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Evolutionary rescue
0
Mean fitness
Mean trait value
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Evolutionary rescue
Time
Abundance
00
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Evolutionary rescue
Time
Abundance
00
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Evolutionary rescue
So, which populations are likely to be “rescued” by evolution?
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Evolutionary rescue
Likelihood of rescue increases with:
I genetic variation / mutation rate (faster adaptation)
I initial population size (slower extinction)
Gomulkiewicz & Holt (1995), Orr & Unckless (2008), Chevin & Lande (2010)
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Extending evolutionary rescue
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Extending evolutionary rescue: competition
Competition reduces the abundance of a species ...
Maynard Smith (1989)
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Extending evolutionary rescue: competition
Time (t )
Abundance (
N)
K
dNdt
= rN(
1− NK
)
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Extending evolutionary rescue: competition
Time (t )
Abundance (
N)
K − C
KdNdt
= rN(
1− NK
)dNdt
= rN(
1− N+CK
)
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Extending evolutionary rescue: competition
Competition reduces the abundance of a species ...
... which must make extinction more likely.
Maynard Smith (1989)
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Extending evolutionary rescue: competition
But, competition also influences selective pressures ...
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Competition and selection
Trait value (z)
Fitn
ess (
iso
late
d)
Fitness (in isolation): ——–
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Competition and selection
Trait value (z)
Fitn
ess (
iso
late
d)
Co
mp
etitio
n (
C)
Decreases selectionstrength
Fitness (in isolation): ——–Competition (C): - - - - - - -
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Competition and selection
Trait value (z)
Fitn
ess (
iso
late
d)
Trait value (z)
Co
mp
etitio
n (
C)
Decreases selection Increases selectionstrength strength
Fitness (in isolation): ——–Competition (C): - - - - - - -
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Extending evolutionary rescue: competition
But, competition also influences selective pressures ...
... and selection affects the rate of adaptation.
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Extending evolutionary rescue: competition
But, competition also influences selective pressures ...
... and selection affects the rate of adaptation.
So, competition can sometimes help evolutionary rescue?
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Adaptive dynamics
I Large asexual population
I All individuals have same phenotype
I Beneficial mutations rare and small
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Adaptive dynamics
The rate of evolution:
dz
dt= µ · n(z) · g(z)
z : phenotypeµ: per capita mutational inputn(z): abundanceg(z): selection gradient
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Adaptive dynamics
The rate of evolution:
dz
dt= µ · n(z ,C) · g(z ,C)
z : phenotypeµ: per capita mutational inputn(z ,C): abundanceg(z ,C): selection gradientC: competition
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Competition and adaptation
Trait value (z)
Fitn
ess (
iso
late
d)
Co
mp
etitio
n (
C)
A
B
Decline in abundance: A>BIncreased selection: A
Conclusions
1. Competition lowers abundance, slowing adaptation
2. Competition also impacts selection, potentially speeding adaptation
3. If selection imposed by competition is strong enough toovercome negative effect on abundance, competition can helpevolutionary rescue
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Circumstantial evidence
Extinction rate (Er) of birds on islands increases with sqaure of speciesnumber, suggesting competition increases extinction Moulton & Pimm 1983
Trait value (z)
Fitness (
isola
ted)
Com
petition (
C)
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Evidence
Adaptation of algae to increasedCO2 slowed by competition Collins 2011
Trait value (z)
Fitness (
isola
ted)
Com
petition (
C)
high
2137
high
2699
high
473
high
A
* * * **
* *
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Evidence for ‘helpful’ competition?
Trait value (z)
Fitness (
isola
ted)
Com
petition (
C)
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Adaptive responseTra
it v
alu
e (
ẑ)
z0*
zn*
z0*
zn*
zN c
Time (t )A
bundance (
n̂)
0
K
0
K
00 5000050000
N c
t robst r
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Time at risk influenced by:
Magnitude of change (z0*− z
n
*)
Tim
e a
t ri
sk (
tr)
00
∆z** ∆z*
Critical fraction of max abundance (N c K )
0 10
Tim
e a
t ri
sk (
tr)
Environmental tolerance (σk)σk*
0σk**0
Tim
e a
t ri
sk (
tr)
Ori
gin
al environm
ent
New
environm
ent
z0* zn*zN c Trait value (ẑ)
K
Kn
N c
0
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