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Ch 10: Life Histories and Evolution
Lifetime scheduling of resources and time to
maximize fitness…
Objectives
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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
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
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:
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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.
(or can organisms increase
overall performance without
trading off one function against
another?)
Alternative pathways for resource
allocation
Energy + matter
growth
increased
competitive
ability
reproduction
maintenance
immediate
profit
increased
numbers
delayed
profit
reproduction
increased
survival
• 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 affects survival,
growth, and future reproduction.
*** Explain this tradeoff: reproduction
vs. mortality
Wide variation among organisms in life
history traits.
temperate
tropical
Clutch size of birds increases with latitude.
Clutch
size
***Possible hypothesis?
Latitude
***Experimental test of hypothesis:
Number of eggs per clutch is limited by
food supply. Normal clutch size = 7.
Do the data support the hypothesis?
What type of selection
does this demonstrate?
a. Directional
b. Stabilizing
Is genetic variation being
maintained or reduced?
Major life history traits
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Age of Maturity
Fecundity
Parity (# times reproduce)
Aging and lifespan
1) Age of Maturity
When should an organism begin to breed?
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:
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May have risk of mortality with time
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May have reduced fecundity at later
ages
*** Summarize the major result.
What explains the pattern?
2) Fecundity: How many offspring per
reproductive bout?
• Fecundity vs. parental
investment/offspring
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seed size vs. seed number
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egg size vs. egg number
• Great variation in seed and egg size
among species
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: 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?
Semelparity: Hypothesis: When preparation
for reproduction is extremely costly?
Iteroparity: When low current reproduction
results in maintaining high future
reproduction.
4) Aging and Lifespan
• Senescence is a decline in
physiological function with age.
• Causes decline in fecundity and
survival
Hypotheses
• Antagonistic pleiotrophy
good early tied to bad late
• Accumulation of harmful mutations
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.
Strength of selection varies with mortality
rate. If high mortality, few reach old
agelittle selection for mechanisms to
prolong life.
Would green
or orange
have stronger
selection to
prolong life?
Individuals in populations with high
extrinsic mortality rates age faster.
A search for a set of rules when particular
traits affecting reproduction and survival
may be favored by natural selection.
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
*** 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?