life length and ageing selection for increased life length? –selection is strong prior to...
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Life length and ageing
• Selection for increased life length?– Selection is strong prior to reproduction– Selection is relaxed thereafter
• What is ageing (senescence)?– Accumulation of mutations– Reduced selection after reproduction
• Reproduce once / several times?– Semelparity/monocarpy– Iteroparity/polycarpy
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Sexual reproduction
– Germ cells differentiated from soma
– Germ cells are younger than the body that
produces them!
– Selection on germ cells:
• indirect via the body that produces them
Asexual reproduction with asymmetric
division
– Differential ages
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Age
Senescent stage
Juvenile stage
Prime-age stage
Example: red deer survival rates
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Caulobacter crescentus
Ackermann, Stearns and Jenal 2003.
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Asexual reproduction via symmetric division (clones)
”Mother” and ”daughter” have the same age!
Immortal?
No ageing!
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Concepts • Antagonistic pleiotropy
– Genes with a positive effect early in life may have a negative effect late in life (effect of selection reduced with age)
• Accumulation of mutations– Effect of selection reduce with age
• Intrinsic vs. extrinsic mortality factors– Intrinsic factors are sensitive to allocation rules– Allocation to the repair of mutations
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Variation in life length
Invertebrates Mammals
Effect of phylogeny
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Cole’s paradox”for an annual species, the absolute gain in intrinsic population
growth which could be achieved by changing to a perennial reproductive habit would be exactly equivalent to adding one
individual to the average litter size”.
Semelparous life historyNt+1 = er Nt = BaNt ; er = Ba
r = lnBa
Iteroparous life historyNt+1 = er Nt = BpNt + Nt ; er = Bp + 1
r = ln(Bp + 1)
Given that the two life histories are equal:Ba = Bp + 1
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Given juvenile (Pj) and adult mortality (Pa)
Semelparous life history Nt+1 = Pj BaNt
Iteroparous life history
Nt+1 = Pj BpNt + Pa Nt = Nt (Pj Bp + Pa)
Given that the two life histories are equal:
Ba = Bp + Pa/Pj
Two important points: Increased Pa and reduced Pj favours semelparity because that values of juveniles increased relative to adults.
Assumption: age at maturity is the same!
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Lets introduce variation in age at maturity (Charlesworth 1980):
Fitness of an iteroparous and semelparous life history is equal when:
Bp / Ba = 1 - Sa/ = Nt+1 / Nt
Sa = adult survival
Adult survival (S)
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Roff 2002
Plants
Snails Flatworms
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A simple graphical method
Adult survival
Opt
imal
rep
rodu
ctiv
e ef
fort Lines of equal fitness (isoclines)
If we assume a trade off curve between adult survival and reproductive effort:
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Adult survival
Opt
imal
rep
rodu
ctiv
e ef
fort Lines of equal fitness (isoclines)
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Adult survival
Opt
imal
rep
rodu
ctiv
e ef
fort Lines of equal fitness (isoclines)
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Adult survival
Opt
imal
rep
rodu
ctiv
e ef
fort Lines of equal fitness (isoclines)
The trade off curve between adult survival and reproductive effort ≈ residual reproductive value
Steep early in life
Flat late in life
Prediction: Increasing reproductive investment with age.
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Trichoserus vulpeculaBrushtail possum
primiparous Middel aged old
Reproductive effort
P (survival to breed again)
Head length/ body mass
Isaac and Johnson 2005.
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What selects for a long reproductive life
• Large variation in progeny survival– Mean and variance of progeny variance:
– Large variance: geometric mean << arithmetic mean
– Small variance: geometric mean ≈ arithmetic mean
€
Xa =1n
xii=1
∑
€
Xg =ix∏n
Bet-hedging
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0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Reproductive lifelength
14 16 18 20 22 24 26 28 30Reproductive investment
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
Reproductive lifelength
1 2 3 4 5 6Age at maturity
European perch
Heibo, Magnhagen and Vøllestad, 2005