Transcript Chapter 10: Life Histories and Evolutionary Fitness

Chapter 10: Life Histories
and Evolutionary Fitness
Robert E. Ricklefs
The Economy of Nature, Fifth Edition
(c) 2001 W.H. Freeman and
Company
Journals related to ecology
and evolution
Journal of Ecology
Journal of Molecular Ecology
Ecology
Oikos
Ecology Letters
Trends in Ecology and Evolution
Science/Nature
(c) 2001 W.H. Freeman and
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Chapter Opener
A female sockeye salmon红鲑鱼, after swimming up to 5,000 km from
her Pacific Ocean feeding ground to the mouth of a coastal river in
British Columbia, faces another 1,000 km upriver journey to her
spawning ground产卵地. There she lays thousands of eggs, and then
promptly dies, her body wasted from the exertion（用尽）.
Figure 10.1
Life Histories
Consider the following remarkable differences
in life history between two birds of similar
size:
Thrushes（鸫鸟）
reproduce when 1 year old
produce several broods of 3-4 young per year
rarely live beyond 3 or 4 years
storm petrels (海燕）
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
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What is life history?
The life history is the schedule日程表 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（寿命）
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What influences life histories?
Life histories are influenced by:
body plan体型 and life style生活型态 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（鸣禽） 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 find food during the breeding
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Figure 10.2
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（喜鹊）:
maximum number of chicks fledged
corresponded to normal clutch size of seven
(c) 2001 W.H. Freeman and
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Figure 10.3
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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Figure 10.4
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事
件)
Fecundity结实力 (number of offspring per
reproductive episode)
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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（敏感度） of the phenotype to its
surroundings is called phenotypic plasticity
example: the increased rate of larval development of
swallowtail butterfly（燕尾蝴蝶） larvae at higher
temperatures
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Figure 10.5
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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Figure 10.6
What is specialization?
Genotype-environment interactions are the basis
for specialization（特化）.
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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Figure 10.7
Reciprocal Transplant交互迁移
Experiments
Reciprocal transplant experiments
involve switching（转换） 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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Figure 10.8
Figure 10.9
Food Supply and Timing of
Metamorphosis（变态，形变）
Many organisms undergo
metamorphosis from larval to adult
forms.
A typical growth curve relates mass 重量
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（重量） 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 combination.
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Figure 10.10
An Experiment with
Tadpoles[ˈtædˌpoʊl]（蝌蚪）
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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Figure 10.11
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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(c) 2001 W.H. Freeman and
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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[ˈtɔrtəs], 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体系 for Plants
English ecologist J.P. Grime envisioned相像
life history traits of plants as lying between
three extremes:
stress tolerators (tend to grow under most
stressful conditions)
Ruderals杂草 (occupy habitats that are
disturbed)
competitors[kəmˈpetɪtər] (favored by
increasing resources and stability)
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Stress Tolerators
Stress tolerators（压力耐受者）:
grow under extreme environmental
conditions
grow slowly
conserve resources
emphasize vegetative spread, rather than
allocating resources to seeds
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Ruderals
Ruderals杂草:
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（体型）
emphasize vegetative spread, rather than
allocating [ˈæləˌkeɪt] resources to seeds.
have long life spans
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Life histories resolve conflicting
demands (冲突的需求).
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（红隼） 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表述 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（幼体生育增加）+ SSR（成体
存活率提高）
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 (c)
in 2001
adult
survival
related to reproduction
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Fecundity versus Survival 1
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Growth versus Fecundity
Some species grow throughout their lives,
exhibiting indeterminate growth无限生长:
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一次繁殖 and
Iteroparity多次繁殖
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龙舌兰
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（饱和）
exhibit synchronous（同时产生的） seed production
followed by death of adults
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Bet Hedging赌注保护 versus
Timing定时
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悖论 may be resolved by
considering the advantages of timing定时
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衰老 is a decline
in function with age
Senescence is an inevitable [ɪnˈevɪtəb(ə)l]
decline in physiological function with age.
Many functions deteriorate:
most physiological indicators (e.g., nerve
conduction神经传导, kidney function肾功能)
immune system and other repair mechanisms
Other processes lead to greater mortality死
亡率:
incidence of tumors肿瘤 and cardiovascular 心
血管disease
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Why does senescence
occur?
Senescence may be the inevitable wearing out
of the organism, the accumulation of molecular
defects（缺陷）:
ionizing radiation（电离辐射） 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内在的 shorter life
spans may experience weaker selection for
mechanisms that prolong life延长寿命
repair and maintenance are costly;
investment in these processes reduces
investment in current fecundity
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Global
warming and
flowering
time
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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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http://en.wikipedia.org/wiki/Walden
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