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Life length and ageing

Explore the concepts of life length, ageing, selection, and reproduction in different organisms. Understand the role of genetic variation and trade-offs in determining life history strategies.

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Life length and ageing

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  1. 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

  2. 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

  3. Prime-age stage Senescent stage Juvenile stage Example: red deer survival rates Age

  4. Caulobacter crescentus Ackermann, Stearns and Jenal 2003.

  5. Asexual reproduction via symmetric division (clones) • ”Mother” and ”daughter” have the same age! • Immortal? • No ageing!

  6. 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

  7. Invertebrates Mammals Variation in life length Effect of phylogeny

  8. 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 history Nt+1 = er Nt = BaNt ; er = Ba r = lnBa Iteroparous life history Nt+1 = er Nt = BpNt + Nt ; er = Bp + 1 r = ln(Bp + 1) Given that the two life histories are equal: Ba = Bp + 1

  9. 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!

  10. Adult survival (S) 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

  11. Roff 2002 Plants Flatworms Snails

  12. A simple graphical method Lines of equal fitness (isoclines) If we assume a trade off curve between adult survival and reproductive effort: Optimal reproductive effort Adult survival

  13. Lines of equal fitness (isoclines) Optimal reproductive effort Adult survival

  14. Lines of equal fitness (isoclines) Optimal reproductive effort Adult survival

  15. Flat late in life Steep early in life The trade off curve between adult survival and reproductive effort ≈ residual reproductive value Lines of equal fitness (isoclines) Prediction: Increasing reproductive investment with age. Optimal reproductive effort Adult survival

  16. Head length/ body mass P (survival to breed again) Reproductive effort Trichoserus vulpecula Brushtail possum Isaac and Johnson 2005. primiparous Middel aged old

  17. 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 Bet-hedging

  18. European perch Heibo, Magnhagen and Vøllestad, 2005

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