Chapter 10: Life Histories and Evolutionary Fitness Robert E. Ricklefs
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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
Company
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
toW.H.
40 Freeman
years old
2001
and
Company
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
to W.H.
findFreeman
foodand
during the breeding
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Company
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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Freeman andcombination.
Company
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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size
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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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plants)
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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:
= S0B + SSR
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.
(c) 2001 W.H. Freeman and
Company