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Ch 10: Life Histories and Evolution

Ch 10: Life Histories and Evolution

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Ch 10: Life Histories and Evolution

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  1. Ch 10: Life Histories and Evolution Lifetime scheduling of resources and time to maximize fitness…

  2. Objectives • Define life history • Explain how related to evolution • Resource allocation and tradeoffs • Age of maturity • Fecundity • Parity (no. times reproduce/lifetime) • Aging and lifespan • Correlated life history traits in contrasting environments

  3. A search for a set of rules when particular traits affecting reproduction and survival may be favored by natural selection.

  4. Life history results from rules and choices influencing survival and reproduction. Juvenile survival

  5. Lack: life history in an evolutionary context. • As life history traits contribute to reproductive success, they influence evolutionary fitness. • Life histories vary consistently with environmental factors; hence may be molded by natural selection.

  6. Life history: schedule of organism’s life, including: • age at first reproduction (maturity) • number and size of offspring (fecundity) • number of reproductive events (parity) • aging (life span) • The values of these traits are solutions to the problem of allocating limited time and resources among various structures, physiological functions, and behaviors.

  7. Resource Allocation • Organisms face a problem of allocation of scarce resources. (or can organisms increase overall performance without tradingoff one function against another?)

  8. Energy + matter Alternative pathways for resource allocation growth reproduction maintenance increased survival immediate profit increased competitive ability increased numbers delayed profit reproduction

  9. Life histories balance trade-offs between current reproduction and future reproduction. • Great variation among organisms in resolving the fundamental tradeoff between fecundity and adult growth and survival. • Principle: limited time and resources are allocated among competing functions so as to maximize lifetime reproductive success.

  10. Tradeoffs: Allocation of time, energy, or materials devoted to one structure or function cannot be allotted to another. • Costs: Allocation to current reproduction affects survival, growth, and future reproduction.

  11. ***Explain this tradeoff: reproduction vs. mortality

  12. Wide variation among organisms in life history traits. temperate tropical

  13. Clutch size of birds increases with latitude. Clutch size Latitude ***Possible hypothesis?

  14. ***Experimental test of hypothesis:Number of eggs per clutch is limited byfood supply. Normal clutch size = 7.Do the data support the hypothesis? What type of selection does this demonstrate? Directional Stabilizing Is genetic variation being maintained or reduced?

  15. Major life history traits • 1 Age of Maturity • 2 Fecundity • 3 Parity (# times reproduce) • 4 Aging and lifespan

  16. 1) Age of MaturityWhen should an organism begin to breed?

  17. What determines age of maturity? • Affects generation time and rate of entry of genes into gene pool • Benefit to not delay: immediate fecundity • Benefit to delay: (if have relatively long lifespan)  may have age-related gains in fecundity from growth or experience • BUT cost to delay: • May have risk of mortality with time • May have reduced fecundity at later ages

  18. *** Summarize the major result.What explains the pattern?

  19. 2) Fecundity: How many offspring per reproductive bout? • Fecundity vs. parental investment/offspring • seed size vs. seed number • egg size vs. egg number • Great variation in seed and egg size among species

  20. Growth vs. Fecundity • If indeterminate growth, • Fecundity is related to body size; • Increased fecundity in one year reduces growth, and thus fecundity, in future. • Short-livedemphasize fecundity over growth • High extrinsic adult mortality rates favor increased reproductive effort, or investment in offspring, at expense of adult survival and future reproduction. • Long-lived emphasize growth over fecundity

  21. 3) Parity • How many times to reproduce per lifetime? • Semelparous • (monocarpic) once • Iteroparous • (polycarpic) repeated

  22. If semelparous, at what year to undergo ‘big-bang’ reproduction? • Annual • Biennial • Long-lived

  23. Semelparity: Hypotheses… • When payoff for reproduction is highly variable but favorable conditions are predictable from environmental cues? • When pollinators attracted to massive display? • When seed predators become satiated?

  24. Semelparity: Hypothesis: When preparation for reproduction is extremely costly?

  25. Iteroparity: When low current reproduction results in maintaining high future reproduction.

  26. 4)Aging and Lifespan • Senescence is a decline in physiological function with age. • Causes decline in fecundity and survival

  27. Hypotheses • Antagonistic pleiotrophy good early tied to bad late • Accumulation of harmful mutations

  28. Why does aging vary? • Not all organisms senesce at same rate, suggesting that aging may be subject to natural selection and evolutionary modification. • Strength of selection diminishes on traits expressed at progressively later ages.

  29. Strength of selection varies with mortality rate. If high mortality, few reach old agelittle selection for mechanisms to prolong life.Would greenor orange have strongerselection toprolong life?

  30. Individuals in populations with high extrinsic mortality rates age faster.

  31. A search for a set of rules when particular traits affecting reproduction and survival may be favored by natural selection.

  32. Life histories vary along a slow-fast continuum. Traits are correlated in contrasting environments. • Slow (K-species) = in persistent habitats population near carrying capacity • slow development • delayed maturity • large adult size • low reproductive rate • high parental investment/offspring • low mortality • long life • low dispersal • Fast (r-species) = in disturbed habitats population can grow rapidly • opposite traits

  33. *** Sample exam question Life history traits often represent ‘tradeoffs’. Graph the expected relationship for each pair of traits. Label axes. Then explain the nature of each tradeoff. A. Seed size vs. seed number 1. Graph 2. Explanation B. Number of offspring per breeding attempt vs. adult survival between successive breedings 1. Graph 2. Explanation C. What is the key assumption of tradeoffs?