(c) 2001 W.H. Freeman and Company Chapter 10: Life Histories and Evolutionary Fitness Robert E....

(c) 2001 W.H. Freeman and Company

Chapter 10: Life Histories and Evolutionary Fitness

Robert E. RicklefsThe Economy of Nature, Fifth Edition

Journals related to ecology and evolution

Journal of EcologyJournal of Molecular EcologyEcologyOikosEcology LettersTrends in Ecology and EvolutionScience/Nature


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 oldproduce several broods of 3-4 young per yearrarely live beyond 3 or 4 years

storm petrels ( 海燕）do not reproduce until they are 4 to 5 years oldproduce at most a single young per yearmay live to be 30 to 40 years old


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）


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 conditionsfood supplypredatorsother biotic factors, such as competition


A Classic StudyDavid 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 season

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


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

Figure 10.3


Life Histories: A Case of Trade-OffsOrganisms 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?

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) aging (total length of life)


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

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

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

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 environmentstransplanted to a different environment

such experiments permit separating differences caused by genetic differences versus phenotypic plasticity

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?


Metamorphosis Under Varied EnvironmentsPoorly-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.

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 size

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


The Slow-Fast Continuum 2Life 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 lifeslow developmentdelayed maturityhigh parental investmentlow reproductive rates

at the “fast” end of the continuum are organisms with the opposite traits (mice, fruit flies, weedy plants)


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)


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


Ruderals

Ruderals杂草 : are weedy species that colonize

disturbed habitats typically exhibit

rapid growthearly maturationhigh reproductive rateseasily dispersed seeds


Competitors

Competitors: grow rapidly to large stature （体型）

emphasize vegetative spread, rather than allocating [ˈæləˌkeɪt] resources to seeds.

have long life spans


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


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?


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


Fecundity versus Survival 1

How do organisms optimize the trade-off between current fecundity and future growth?

Critical relationship is:

= S0B （幼体生育增加） + SSR （成体存活率提高）

where: is the change in population growthS0 is the survival of offspring to one year

B is the change in fecundityS is annual adult survival independent of reproductionSR is the change in adult survival related to reproduction


Fecundity versus Survival 1


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


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.


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 reservesBamboos 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


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


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


Senescence 衰老 is a decline in function with ageSenescence 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 mechanismsOther processes lead to greater mortality 死

亡率 : incidence of tumors 肿瘤 and cardiovascular 心血

管 disease


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.


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

Global warming and flowering time



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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(c) 2001 W.H. Freeman and Company Chapter 10: Life Histories and Evolutionary Fitness Robert E....

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