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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Life length and ageing Selection for increased life length? –Selection is strong prior to reproduction –Selection is relaxed thereafter What is ageing.
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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
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
Age
Senescent stage
Juvenile stage
Prime-age stage
Example: red deer survival rates
Caulobacter crescentus
Ackermann, Stearns and Jenal 2003.
Asexual reproduction via symmetric division (clones)
”Mother” and ”daughter” have the same age!
Immortal?
No ageing!
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
Variation in life length
Invertebrates Mammals
Effect of phylogeny
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
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!
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)
Roff 2002
Plants
Snails Flatworms
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:
Adult survival
Opt
imal
rep
rodu
ctiv
e ef
fort Lines of equal fitness (isoclines)
Adult survival
Opt
imal
rep
rodu
ctiv
e ef
fort Lines of equal fitness (isoclines)
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.
Trichoserus vulpeculaBrushtail possum
primiparous Middel aged old
Reproductive effort
P (survival to breed again)
Head length/ body mass
Isaac and Johnson 2005.
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