CHAPTER 7: LIFE HISTORIES AND EVOLUTIONARY FITNESS

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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 30 to 40 years old

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Parental investment

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 to find food during the breeding season

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Lack’s Proposal

Lack made 3 key points, suggesting that life histories are shaped by natural selection:1. because life history traits (such as number of

eggs per clutch) contribute to reproductive success they also influence evolutionary fitness

2. life histories vary in a consistent way with respect to factors in the environment

3. 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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Life history: set of rules and choices influencing survival and reproduction

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 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)

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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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Grime’s Scheme for Plants16

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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Parental investment affects parental survival

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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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Long-lived organisms mature later than short-lived ones

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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In other words…

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… ?

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Fecundity and mortality rates for 33 species of birds: vary together

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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Agaves: semelparous

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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Senescence… (males in English population in 1980s)

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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Life histories respond to variation in the environment 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

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What are the stimuli for change 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

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Relationships between age and size at maturation may differ when growth rates differ

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 combination.

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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 size

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Size…

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…

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Animals and feeding

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

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