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Ch 11: Population Growth + Regulation dN/dt = rN dN/dt = rN(K-N)/K

BRING toLECTURE:1) PRINT ofLecture Outline2) Pg. 75 in Manual

Ch 7: Life Histories and Evolution

Lifetime scheduling of resources and time to maximize fitness…

Objectives• Define life history• Explain how related to evolution• Resource allocation and tradeoffs• Correlated life history traits in contrasting

environments• Explain evolution of life history traits• Age of maturity• Fecundity• Parity (no. times reproduce/lifetime)• Aging and lifespan

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

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

Juvenile survival

Life histories vary along a slow-fast continuum.

Traits are correlated in contrasting environments.

• Slow (often large organisms)• slow development• delayed maturity• low fecundity• high parental investment/offspring• low mortality• long life• Fast: opposite traits

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.

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.

Resource Allocation

• Organisms face a problem of allocation of scarce resources. (compromise? or can organisms increase overall performance without trading off one function against another?)

*** Describe, then explain this tradeoff: reproduction vs. mortality

What is the tradeoff between:parental investment vs. parental survival?

Alternative pathways for resource allocation

growth reproduction maintenance

Energy + matter

reproduction

increased survival

increasedcompetitive ability

immediateprofit

delayed profit

increasednumbers

• 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.

• Tradeoffs: Allocation of time, energy, or materials devoted to one structure or function cannot be allotted to another.

• Costs: Allocation to current reproduction involves tradeoff with survival, growth, and future reproduction.

Explain the law of ‘diminishing returns’: trade-off between fecundity vs. survival

Major life history traits

• 1 Age of Maturity• 2 Fecundity• 3 Parity (# times reproduce)• 4 Aging and lifespan

1) Age of Maturity When should an organism begin to breed?

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

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

Explain: Optimal age at maturity (i.e. maximize lifetime reproduction) varies in direct proportion to life span.

e.g. determinate growth in a lizard: starts to reproduce after reaches maximum size

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

Wide variation among organisms in life history traits.

temperate tropical

***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 selectiondoes this demonstrate?a. Directionalb. Stabilizing

Is genetic variation beingmaintained or reduced?

e.g. indeterminate growth in fish (continue to grow throughout life; fecundity directly related to body size)

Explain: adult longevity determines optimalallocation between growth and reproduction.

Summarize all graphs in one sentence.Explain this evolutionary shift in life histories.(selection by predators on both adults and young occurs)

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-lived emphasize 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

3) Parity• How many times to reproduce per

lifetime?• Semelparous • (monocarpic) once• Iteroparous • (polycarpic) repeated

If semelparous, at what year to undergo ‘big-bang’ reproduction?

• Annual• Biennial• Long-lived

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

Why is cicada:Semelparous?

Synchronous?

• Semelparity: Hypotheses…• When payoff for reproduction is highly

variable but favorable conditions are predictable?

• When pollinators attracted to massive display?

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

Perennials…Repeated breeders…

4) Aging and Lifespan• Senescence is a decline in

physiological function with age. • Causes decline in fecundity and

survival

Strength of selection varies with mortality rate. If high mortality, few reach old agelittle selection for mechanisms to prolong life.

Would greenor orange have strongerselection todelay senescence?

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.

*** Sample exam questionLife 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. ExplanationB. Number of offspring per breeding attempt

vs. adult survival between successive breedings1. Graph 2. Explanation

C. What is the key assumption of tradeoffs?