Chapter 10: Life Histories and Evolutionary Fitness

1
Chapter 10 Life Histories and Evolutionary
Fitness

• Robert E. Ricklefs
• The Economy of Nature, Fifth Edition

2
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

3
What is life history?

• The life history is the schedule of an organisms
  life, including
• age at maturity
• number of reproductive events
• allocation of energy to reproduction
• number and size of offspring
• life span

4
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

5
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 find food during the breeding
  season

6
Lacks 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

7
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

8
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?

9
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)

10
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

11
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 others habitat - the reaction norms
  for these genotypes cross

12
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

13
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

14
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?

15
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.

16
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

17
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

18
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 plants)

19
Grimes 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)

20
Stress Tolerators

• Stress tolerators
• grow under extreme environmental conditions
• grow slowly
• conserve resources
• emphasize vegetative spread, rather than
  allocating resources to seeds

21
Ruderals

• Ruderals
• are weedy species that colonize disturbed
  habitats
• typically exhibit
• rapid growth
• early maturation
• high reproductive rates
• easily dispersed seeds

22
Competitors

• Competitors
• grow rapidly to large stature
• emphasize vegetative spread, rather than
  allocating resources to seeds
• have long life spans

23
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

24
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?

25
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

26
Fecundity versus Survival 1

• 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

27
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

28
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

29
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.

30
Semelparity Agaves and Bamboos

• Agaves are the century plants of deserts
• grow vegetatively for several years
• produce a gigantic flowering stalk, draining
  plants 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

31
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

32
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

33
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

34
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.

35
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

36
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.