life history evolution: the evolution of senescencegow/biol418/pdf/5c_life_history.pdfantagonistic...

Life History Evolution:The evolution of senescence

Why?

1. Constraint

2. Mutation accumulation (Medawar, 1946)

3. Antagonistic pleiotropy (Medawar, 1946; Williams, 1957)

Biol 418: Evolutionary EcologyLife History Evolution

Jennifer Gow

Senescence

The evolution of senescence

Theory:ConstraintMutation accumulation Antagonistic pleiotropy

Empirical evidence:Comparative testsSelection experimentsAging genes

Charlesworth (2000) Genetics 156: 927-931.


Jennifer Gow

SenescenceConstraint: ‘wear and tear’





Jennifer Gow

Senescence

Birds in zoo

Ricklefs (1998) Am. Nat. 152: 24-44.

Constraint





Jennifer Gow

SenescenceMutation accumulation

So after 1 month 10 break

10% breakage/month

100 tubes





Jennifer Gow

Senescence

10% breakage/month

+ 1/9 fecundity per unbroken individual

… so population number is stable

So after 1 month 10 break

100 tubes

100 tubes

Mutation accumulation





Jennifer Gow

SenescenceMutation accumulation





Jennifer Gow

Senescence

= ‘brittle’ mutation has 20% breakage rate

So after 1 month:9/90 ‘regular’ break2/10 ‘brittle’ break

Differential breakage rate

100 tubes






Jennifer Gow

Senescence

= ‘brittle’ mutation has 20% breakage rate

So after 1 month:9/90 ‘regular’ break2/10 ‘brittle’ break

Differential breakage rate

fecundity replaces broken individuals100 tubes

100 tubes






Jennifer Gow

Senescence

Early expression of ‘brittle’

Late expression of ‘brittle’



Theory:ConstraintMutation accumulationAntagonistic pleiotropy



Jennifer Gow

SenescenceEarly expression of ‘brittle’

Late expression of ‘brittle’



Theory:ConstraintMutation accumulationAntagonistic pleiotropy



Jennifer Gow

SenescenceAntagonistic pleiotropy

Pemberton et al. (1991) Evolution 45: 93-103.Hedrick (1999) Heredity 82: 126-133.

A gene influences two traits simultaneously: selection for one trait is counteracted by selection against another:

genotypes that were associated with enhanced juvenile survival tended to reduce adult reproductive success.

4

4.2

4.4

4.6

ss fs/ff

Genotype

Age

at f

irst c

alvi

ng

00.20.40.60.8

ss fs ff

Juve

nile s

urviv

al

0.4

0.5

0.6

0.7

ss fs/ffMpi Genotype

Fecu

ndity





Jennifer Gow

Senescence

Positively effects fitnessearly in life

Antagonistic pleiotropy

Negatively effects fitnesslate in life

Gene ‘A’





Selection is ineffective at removing mutations with effects in old age.

Antagonistic pleiotropy

Early reproduction is favoured by natural selection.

Mutation with positive effect on fitness when young can increase even if it

has negative effects late in life:

“necessary cost of processes beneficial to youth”


Jennifer Gow

Senescence




Partridge & Barton (1993) Nature 362: 305-311.

Mutation accummulation & antagonistic pleiotropy:


Jennifer Gow

Senescence

•Senescence greatest when extrinsic

mortality is high because: few individuals

survive that long, therefore, there will be

little selection against deleterious mutations

in old age.

•Why invest in repair if you’re going to die

for other reasons first?

Extrinsic mortality rate

Rat

e of

sen

esce

nce






Jennifer Gow

SenescenceEmpirical evidence: comparative tests

Birds in zoo

Ricklefs (1998) Am. Nat. 152: 24-44.




Mutation accummulation & Antagonistic pleiotropy:


Jennifer Gow

Senescence

•Senescence greatest when extrinsic

mortality is high because: few individuals

survive that long, therefore, there will be

little selection against deleterious mutations

in old age.

•Why invest in repair if you’re going to die

for other reasons first?

Extrinsic mortality rate

Rat

e of

sen

esce

nce






Jennifer Gow

SenescenceEmpirical evidence: comparative tests

Ricklefs (1998) Am. Nat. 152: 24-44.





Jennifer Gow

SenescenceEmpirical evidence: selection experiments

Service et al. (1988) Evolution 42: 708-716.




1. Select for high late-life fitness2. Select for high early-life fecundity


Jennifer Gow

SenescenceEmpirical evidence: selection experiments


No change





Jennifer Gow

SenescenceEmpirical evidence: aging genes

Hekimi & Guarente (2003) Science 299: 1351-1354.

Nematode Caenorhabditis elegans

Reduces ROS production outside the mitochondria

Detoxifies ROS in the mitochondria

Reduces ROS production in the mitochondria




The evolution of senescenceTheory:

ConstraintMutation accumulation Antagonistic pleiotropy



Jennifer Gow

Overview


Jennifer Gow

SenescenceReferences

Charlesworth (2000) Genetics 156: 927-931.

Hedrick (1999) Heredity 82: 126-133.

Hekimi & Guarente (2003) Science 299: 1351-1354.


Pemberton et al. (1991) Evolution 45: 93-103.

Ricklefs (1998) Am. Nat. 152: 24-44.


life history evolution: the evolution of senescencegow/biol418/pdf/5c_life_history.pdfantagonistic...

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