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

€

Xa =1n

xii=1

∑

€

Xg =ix∏n

Bet-hedging

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