Transcript Chapter 10: Life Histories and Evolutionary Fitness

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CHAPTER 7: LIFE HISTORIES
AND EVOLUTIONARY FITNESS
Life Histories
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Consider the following remarkable differences in life
history between two birds of similar size:
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thrushes
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reproduce when 1 year old
produce several broods of 3-4 young per year
rarely live beyond 3 or 4 years
storm petrels
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do not reproduce until they are 4 to 5 years old
produce at most a single young per year
may live to be 30 to 40 years old
Parental
investment
What is life history?
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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
What influences life histories?
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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
A Classic Study
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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 to find
food during the breeding season
Lack’s Proposal
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Lack made 3 key points, suggesting that life histories
are shaped by natural selection:
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2.
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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
An Experimental Test
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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
Life Histories: A Case of Trade-Offs
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Organisms face a problem of allocation of scarce
resources (time, energy, materials):
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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
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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)
Life history: set of rules and choices
influencing survival and reproduction
The Slow-Fast Continuum 1
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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
The Slow-Fast Continuum 2
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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 plants)
Grime’s Scheme for Plants
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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)
Grime’s Scheme for Plants
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Stress Tolerators
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Stress tolerators:
 grow
under extreme environmental conditions
 grow slowly
 conserve resources
 emphasize vegetative spread, rather than allocating
resources to seeds
Ruderals
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Ruderals:
 are
weedy species that colonize disturbed habitats
 typically exhibit
 rapid
growth
 early maturation
 high reproductive rates
 easily dispersed seeds
Competitors
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Competitors:
 grow
rapidly to large stature
 emphasize vegetative spread, rather than allocating
resources to seeds
 have long life spans
Life histories resolve
conflicting demands.
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Life histories represent tradeoffs among competing
functions:
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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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Parental
investment
affects
parental
survival
Life histories balance tradeoffs.
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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?
Age at First Reproduction
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At each age, the organism chooses between
breeding and not breeding.
The choice to breed carries benefits:
 increase
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in fecundity at that age
The choice to breed carries costs:
 reduced
survival
 reduced fecundity at later ages
Long-lived organisms mature later than short-lived ones
Fecundity versus Survival 1
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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
Fecundity versus Survival 2
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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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In other words…
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The number of offspring produced today can
reduce the number produced tomorrow
Natural selection should optimize the trade-off
between present and future reproduction
What factors influence the resolution of this conflict?
 High
mortality rates for adults… ?
 Long adult life span… ?
Fecundity and mortality rates for 33 species of birds:
vary together
Growth versus Fecundity
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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
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Semelparous organisms breed only once during
their lifetimes, allocating their stored resources to
reproduction, then dying in a pattern of
programmed death:
 sometimes
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called “big-bang” reproduction
Iteroparous organisms breed multiple times during
the life span.
Semelparity: Agaves and Bamboos
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Agaves are the century plants
of deserts:
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grow vegetatively for several
years
produce a gigantic flowering
stalk, draining plant’s stored
reserves
Bamboos are woody tropical
to warm-temperate grasses:
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grow vegetatively for many years
until the habitat is saturated
exhibit synchronous seed
production followed by death of
adults
Agaves: semelparous
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Why semelparity versus
iteroparity?
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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
More on Semelparity in Plants
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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
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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
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Other processes lead to greater mortality:
 incidence
of tumors and cardiovascular disease
Senescence… (males in English population in 1980s)
Why does senescence occur?
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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
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In this sense the body is like an automobile, which
eventually wears out and has to be junked.
Why does aging vary?
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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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Life histories respond to variation in
the environment
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Storage of food and buildup of reserves
Dormancy  physiologically inactive states
Hibernation  spending winter in a dormant state
Diapause  (insects) – water is chemically bound or
reduced in quantity to prevent freezing and
metabolism drops so low to become barely
detectable
What are the stimuli for change
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Proximate factors (day length, for example) – an
organism can assess the state of the environment but
these factors do not directly affect its fitness
Ultimate factors (food supplies, for example) –
environmental features that have direct
consequences on the fitness of the organism
Photoperiod: the length of daylight: proximate
factor to virtually all organisms
Relationships between age and size at maturation
may differ when growth rates differ
Food Supply and Timing of Metamorphosis
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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?
Metamorphosis Under Varied Environments
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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?
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Solution is typically a compromise between mass and
age, depending on risks and rewards associated with
each possible combination.
An Experiment with Tadpoles
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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 size
Size…
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Risks of all sorts depend on size and those risks
influence the allocation of resources between
functions that support growth and those that support
maintenance and survival
In the Kalahari sand vegetation of Zimbabwe…
Animals and feeding
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Optimal feeding: what do you think that means?
Central place foraging – offspring in one location
and parents search for food at some distance
Risk sensitive foraging: every activity carries a risk
of mortality