Transcript Chapter 10: Life Histories and Evolutionary Fitness Robert E. Ricklefs

Chapter 10: Life Histories
and Evolutionary Fitness
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 W.H. Freeman and
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Life Histories
Consider the following remarkable differences
in life history between two birds of similar
size:
thrushes
reproduce when 1 year old
produce several broods of 3-4 young per year
rarely live beyond 3 or 4 years
storm petrels
do not reproduce until they are 4 to 5 years old
produce at most a single young per year
may live to be(c)30
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What is life history?
The life history is the schedule of an
organism’s life, including:
age at maturity
number of reproductive events
allocation of energy to reproduction
number and size of offspring
life span
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What influences life histories?
Life histories are influenced by:
body plan and life style of the organism
evolutionary responses to many factors,
including:
physical conditions
food supply
predators
other biotic factors, such as competition
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A Classic Study
David Lack of Oxford University first
placed life histories in an evolutionary
context:
tropical songbirds lay fewer eggs per
clutch than their temperate counterparts
Lack speculated that this difference was
based on different abilities to find food for
the chicks:
birds nesting in temperate regions have longer
days in which
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during the breeding
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season
Lack’s Proposal
Lack made 3 key points, suggesting that
life histories are shaped by natural
selection:
because life history traits (such as number of eggs per
clutch) contribute to reproductive success they also
influence evolutionary fitness
life histories vary in a consistent way with respect to
factors in the environment
hypotheses about life histories are subject to
experimental tests
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An Experimental Test
Lack suggested that one could artificially
increase the number of eggs per clutch to
show that the number of offspring is limited
by food supply.
This proposal has been tested repeatedly:
Gören Hogstedt manipulated clutch size of
European magpies:
maximum number of chicks fledged
corresponded to normal clutch size of seven
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Life Histories: A Case of
Trade-Offs
Organisms face a problem of allocation of
scarce resources (time, energy, materials):
the trade-off: resources used for one function
cannot be used for another function
Altering resource allocation affects fitness.
Consider the possibility that an oak tree
might somehow produce more seed:
how does this change affect survival of seedlings?
how does this change affect survival of the adult?
how does this change affect future reproduction?
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Components of Fitness
Fitness is ultimately dependent on
producing successful offspring, so many
life history attributes relate to
reproduction:
maturity (age at first reproduction)
parity (number of reproductive episodes)
fecundity (number of offspring per
reproductive episode)
aging (total length
of life)
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Phenotypic plasticity allows
an individual to adapt.
A reaction norm is the observed relationship
between the phenotype and environment:
a given genotype gives rise to different phenotypes
under different environments
responsiveness of the phenotype to its surroundings
is called phenotypic plasticity
example: the increased rate of larval development of
swallowtail butterfly larvae at higher temperatures
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Genotype-Environment
Interaction
When populations have differing reaction
norms across a range of environmental
conditions, this is evidence of a
genotype-environment interaction.
Such an interaction is evident in
development of swallowtail larvae:
genotypes from Alaska and Michigan: each
performs worse in the other’s habitat - the
reaction norms for these genotypes cross
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What is specialization?
Genotype-environment interactions are the basis
for specialization.
Consider two populations exposed to different
conditions over time:
different genotypes will predominate in each
population
populations are thus differentiated with different
reaction norms
each population performs best in its own
environment
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Reciprocal Transplant
Experiments
Reciprocal transplant experiments involve
switching of individuals between two
localities:
in such experiments, we compare the
observed phenotypes among individuals:
kept in their own environments
transplanted to a different environment
such experiments permit separating
differences caused by genetic differences
versus phenotypic plasticity
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Food Supply and Timing of
Metamorphosis
Many organisms undergo
metamorphosis from larval to adult
forms.
A typical growth curve relates mass to
age for a well-nourished individual, with
metamorphosis occurring at a certain
point on the mass-age curve.
How does the same genotype respond
when nutrition varies?
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Metamorphosis Under
Varied Environments
Poorly-nourished organisms grow more slowly
and cannot reach the same mass at a given
age.
When does metamorphosis occur?
fixed mass, different age?
fixed age, different mass?
different mass and different age?
Solution is typically a compromise between
mass and age, depending on risks and rewards
associated with each
possible
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An Experiment with
Tadpoles
Tadpoles fed different diets illustrate
the complex relationship between size
and age at metamorphosis:
individuals with limited food tend to
metamorphose at a smaller size and later
age than those with adequate food
(compromise solution)
the relationship between age and size at
metamorphosis is the reaction norm of
metamorphosis with respect to age and
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The Slow-Fast Continuum 1
Life histories vary widely among different
species and among populations of the
same species.
Several generalizations emerge:
life history traits often vary consistently with
respect to habitat or environmental conditions
variation in one life history trait is often
correlated with variation in another
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The Slow-Fast Continuum 2
Life history traits are generally organized
along a continuum of values:
at the “slow” end of the continuum are organisms
(such as elephants, giant tortoises, and oak trees)
with:
long life
slow development
delayed maturity
high parental investment
low reproductive rates
at the “fast” end of the continuum are organisms
with the opposite traits (mice, fruit flies, weedy
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Grime’s Scheme for Plants
English ecologist J.P. Grime envisioned life
history traits of plants as lying between
three extremes:
stress tolerators (tend to grow under most
stressful conditions)
ruderals (occupy habitats that are
disturbed)
competitors (favored by increasing
resources and stability)
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Stress Tolerators
Stress tolerators:
grow under extreme environmental
conditions
grow slowly
conserve resources
emphasize vegetative spread, rather than
allocating resources to seeds
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Ruderals
Ruderals:
are weedy species that colonize disturbed
habitats
typically exhibit
rapid growth
early maturation
high reproductive rates
easily dispersed seeds
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Competitors
Competitors:
grow rapidly to large stature
emphasize vegetative spread, rather than
allocating resources to seeds
have long life spans
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Life histories resolve
conflicting demands.
Life histories represent trade-offs among
competing functions:
a typical trade-off involves the competing
demands of adult survival and allocation of
resources to reproduction:
kestrels with artificially reduced or enlarged
broods exhibited enhanced or diminished adult
survival, respectively
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Life histories balance
tradeoffs.
Issues concerning life histories may be
phrased in terms of three questions:
when should an individual begin to produce
offspring?
how often should an individual breed?
how many offspring should an individual
produce in each breeding episode?
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Age at First Reproduction
At each age, the organism chooses
between breeding and not breeding.
The choice to breed carries benefits:
increase in fecundity at that age
The choice to breed carries costs:
reduced survival
reduced fecundity at later ages
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Fecundity versus Survival 1
 How do organisms optimize the trade-off between
current fecundity and future growth?
 Critical relationship is:
 = S0B + SSR
where:  is the change in population growth
S0 is the survival of offspring to one year
B is the change in fecundity
S is annual adult survival independent of reproduction
SR is the change in adult survival related to reproduction
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Fecundity versus Survival 2
When the previous relationship is rearranged,
the following points emerge:
changes in fecundity (positive) and adult survival
(negative) are favored when net effects on
population growth are positive
effects of enhanced fecundity and reduced survival
depend on the relationship between S and S0
one thus expects to find high parental involvement
associated with low adult survival and vice versa
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Growth versus Fecundity
Some species grow throughout their lives,
exhibiting indeterminate growth:
fecundity is related to body size
increased fecundity in one year reduces growth, thus
reducing fecundity in a later year
for shorter-lived organisms, optimal strategy
emphasizes fecundity over growth
for longer-lived organisms, optimal strategy
emphasizes growth over fecundity
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Semelparity and
Iteroparity
Semelparous organisms breed only once
during their lifetimes, allocating their
stored resources to reproduction, then
dying in a pattern of programmed
death:
sometimes called “big-bang” reproduction
Iteroparous organisms breed multiple
times during the life span.
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Semelparity: Agaves and
Bamboos
Agaves are the century plants of deserts:
grow vegetatively for several years
produce a gigantic flowering stalk, draining plant’s
stored reserves
Bamboos are woody tropical to warm-temperate
grasses:
grow vegetatively for many years until the habitat is
saturated
exhibit synchronous seed production followed by
death of adults
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Bet Hedging versus Timing
Why semelparity versus iteroparity?
iteroparity might offer the advantage of bet
hedging in variable environments
but semelparous organisms often exist in
highly variable environments
this paradox may be resolved by considering
the advantages of timing reproduction to
match occasionally good years
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More on Semelparity in
Plants
Semelparity seems favored when adult survival
is good and interval between favorable years is
long.
Advantages of semelparity:
timing reproductive effort to match favorable years
attraction of pollinators to massive floral display
saturation of seed predators
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Senescence is a decline in
function with age
Senescence is an inevitable decline in
physiological function with age.
Many functions deteriorate:
most physiological indicators (e.g., nerve
conduction, kidney function)
immune system and other repair
mechanisms
Other processes lead to greater mortality:
incidence of tumors and cardiovascular
disease
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Why does senescence
occur?
Senescence may be the inevitable wearing out
of the organism, the accumulation of molecular
defects:
ionizing radiation and reactive forms of oxygen break
chemical bonds
macromolecules become cross-linked
DNA accumulates mutations
In this sense the body is like an automobile,
which eventually wears out and has to be
junked.
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Why does aging vary?
Not all organisms senescence at the same
rate, suggesting that aging may be
subject to natural selection:
organisms with inherently shorter life spans
may experience weaker selection for
mechanisms that prolong life
repair and maintenance are costly;
investment in these processes reduces
investment in current fecundity
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Summary
Life history traits are solutions to the
problem of allocating limited resources to
various essential functions.
Variation in life history traits is influenced
by body plan, life style of the organism,
and evolutionary responses to many
factors, including biotic and abiotic
environmental factors.
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