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Life-history Characteristics

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Evolutionary Hypothesis for aging If selection can produce longer life spans why does it not do so? Under evolutionary hypothesis for aging, ... – PowerPoint PPT presentation

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Title: Life-history Characteristics


1
Life-history Characteristics
  • All organisms have been selected to maximize
    reproductive success over the course of their
    lifetimes.
  • There is, however, tremendous variation in how
    organisms achieve this.

2
Life-history Characteristics
  • Some organisms produce many offspring at once,
    but live only a short time.
  • Others produce a few offspring over the course of
    a long life.

3
Life-history Characteristics
  • There is also enormous variation in the size of
    offspring. Oysters produce 10-50 million tiny
    eggs whereas whales produce a single large calf.
  • What explains the variation we see?

4
Life-history Characteristics
  • Clearly, there are constraints and trade-offs in
    the strategies that organisms can employ.
  • The best strategies are determined by the
    availability of energy and an organisms
    prospects of survival.

5
Sample life history
  • Consider a hypothetical female opossums life
    history.
  • Born and nursed by mother for about 3 months.
  • Becomes independent and grows to maturity.
  • Age 10 months has first litter of 8 pups. Age 15
    months has second litter of 7 pups
  • Killed by predator at 20 months.

6
Fig 12.2
7
Life-history Characteristics
  • Females energy came from different sources and
    was allocated in different directions over course
    of her life.
  • For first 3 months received energy from her
    mother. After that had to obtain her own.

8
Life-history Characteristics
  • As a juvenile she devoted energy to growth,
    metabolism and repair.
  • After reaching maturity she devoted energy to
    metabolism, repair and reproduction.

9
Life-history Characteristics
  • Fundamentally, differences in how and when energy
    is allocated affect life history strategies.
  • A different opossum might have matured earlier at
    a smaller size, and produced babies earlier, but
    perhaps fewer or smaller ones.
  • Alternatively, more energy might be allocated to
    repair and less to reproduction, perhaps
    resulting in a longer life.

10
Differential energy allocation by sand crickets
  • Sand crickets occur in both long-winged and
    short-winged forms (papers by Zhao and Zera 2002,
    2003).
  • Long-winged forms have well developed flight
    muscles and fuel to power them. This enables
    them to disperse if conditions are poor.
  • Short-winged forms cannot disperse, but can
    develop eggs more quickly.
  • There is a trade-off between dispersal ability
    and early reproduction.

11
Issues in life-history analysis
  • Analyzing life history decisions involves
    cost-benefit analysis and an examination of
    fitness trade-offs as it relates to the following
    questions
  • Why do organisms age and die?
  • How many offspring should an individual produce
    in a given year?
  • How big should each offspring be?

12
Why do organisms age and die?
  • Senesence is a late-life decline in an
    individuals fertility and probability of
    reproducing.
  • Same pattern found in many organisms.

13
Fig 12.4
14
Senesence
  • If senesence reduces reproductive success we
    would expect it to be opposed by selection.

15
Hypotheses explaining senesence
  • Two major hypotheses on why aging persists
  • Rate-of-living theory
  • Evolutionary trade-off theory

16
Rate-of-living Hypothesis
  • This hypothesis suggests that aging is caused by
    accumulation of cellular damage caused by
    accumulation of toxins and accumulation of errors
    during replication, transcription and translation
    of DNA.
  • Hypothesis suggests organisms have reached limit
    of biological repair and no more genetic
    variation exists for improved repair mechanisms.

17
Rate-of-living Hypothesis
  • Hypothesis makes two predictions.
  • 1. Cell and tissue damage are caused by
    metabolism so aging rate should be correlated
    with metabolic rate.
  • 2. Species should not be able to evolve longer
    life spans.

18
Rate-of-living Hypothesis
  • Austad and Fisher (1991) tested prediction 1.
  • Calculated amount of energy expended per gram of
    tissue per lifetime for 164 mammal species.
    Theory predicts rate should be similar across
    groups.
  • Found large range from 39 kcal/g/lifetime in
    elephant shrews to 1,102 kcal/g/lifetime in a
    bat.

19
Fig 12.5
20
Rate-of-living Hypothesis
  • Also found bats have rates similar to those of
    many other mammals but life spans that are 3
    times as long.
  • These patterns dont fit rate-of-living
    predictions.

21
Rate-of-living Hypothesis
  • Luckinbill et al. (1984) tested prediction 2 by
    artificially selecting for longevity in fruit
    flies.
  • Lineages in which they selected for late
    reproduction showed greatly increased longevity
    over the course of 13 generations of selection.
    Average lifespan increased from 35 to 60 days.

22
Fig 12.6
23
Rate-of-living Hypothesis
  • Results of tests thus do not support the
    rate-of-living hypothesis.

24
Evolutionary Hypothesis for aging
  • If selection can produce longer life spans why
    does it not do so?
  • Under evolutionary hypothesis for aging,
    organisms age because the body fails to repair
    cell and tissue damage rather than because it
    cannot do so.

25
Evolutionary Hypothesis for aging
  • Failure to repair may be due to (i) accumulation
    of deleterious mutations or (ii) trade-offs
    between repair and reproduction.

26
Evolution of senesence in a hypothetical
population.
  • Population has annual probability of survival
    each year of 0.8 (death by accident, predation,
    etc.). Population declines exponentially over
    time.
  • Individuals with wild-type genotype mature at age
    3 and die at age 16 (if not killed). Have one
    offspring a year.
  • Population expected lifetime reproductive success
    of 2.419.

27
12.9a
28
Evolution of senesence in a hypothetical
population.
  • New mutation occurs which causes death at age 14.
    Rest of life history unchanged.
  • Expected lifetime RS reduced to 2.34 offspring, a
    small reduction and 96 of the lifetime RS of the
    wildtype.
  • Few individuals live beyond 14 in wildtype
    population so effect is small.

29
12.9b
30
Evolution of senesence in a hypothetical
population.
  • In general, mutations that cause death late in
    natural life will be only weakly selected
    against.
  • Mutation that causes death at a young age, of
    course, will be strongly selected against.
  • Such mutations may be maintained in population by
    mutation-selection balance.

31
Evolution of senesence in a hypothetical
population.
  • An example of the kind of mutation that could
    cause death only late in life might be one that
    causes cells to not repair themselves as well as
    is possible.
  • For example, in humans, a DNA mismatch repair
    mutation causes a form of colon cancer. Median
    age of diagnosis is 48 (range 17 to 92) well
    after reproduction has begun.

32
Evolution of senesence in a hypothetical
population.
  • In our hypothetical population a second mutation
    occurs that causes reproduction to begin at age 2
    and death at age 10.
  • There is thus a trade-off between age of first
    reproduction and longevity.
  • Expected lifetime RS of individuals with mutation
    is 2.66, which is 1.1 times the wildtypes RS.

33
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34
Evolution of senesence in a hypothetical
population.
  • Most individuals reap benefit of early
    reproduction, bur few pay cost of earlier death.
  • This mutant allele should spread rapidly.
  • A gene that causes less energy to be devoted to
    cellular damage repair and more to be devoted to
    reproduction would fit profile of such a mutant.
    Several have been identified in fruit flies and
    nematodes.

35
Evidence of a trade-off caused by early
reproduction.
  • In a study of Collared Flycatchers individuals
    that bred at age 1 had smaller clutches at ages
    2-4 than individuals who dont first breed until
    age 2.

36
12.13
37
Evidence of a trade-off caused by early
reproduction.
  • Also, females whose clutches were artificially
    enlarged in year 1 had progressively smaller
    clutches in years 2-4.

38
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39
Evidence of a trade-off caused by early
reproduction.
  • Conclusion is that there is a trade-off between
    early and late reproduction in Collared
    Flycatchers.
  • First year breeders do have higher life time RS
    than second year breeders.

40
Evolution of ageing in Opossums
  • We expect populations with low rates of mortality
    due to factors such as predation to evolve
    delayed senesence.
  • Under these circumstances mutations that cause
    senesence are more likely to make themselves felt
    because animals live to be older and so will be
    selected against.

41
Evolution of ageing in Opossums
  • Austad (1993) studied two populations of Opossums
    one on Georgia mainland, the other on Sapelo
    Island off the coast.
  • Opposums on mainland have high mortality rates
    from predators (gt50 of all deaths).
  • No mammalian predators on Sapelo Island.

42
Evolution of ageing in Opossums
  • Austad followed life histories of radio-collared
    opossums on both sites.
  • Island populations aged more slowly than mainland
    populations on several measures including rate of
    survival, reproductive performance, and
    connective tissue physiology.

43
Fig 12.14
44
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45
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46
How many offspring should an individual produce
in a year?
  • In life history decisions a fundamental choice is
    how many offspring to produce in a year.
  • The more offspring produced in a year, the less
    each can be cared for and additional offspring
    affect the parents prospects for survival.

47
Clutch size in birds
  • The question of how many young is optimal has
    been extensively studied in birds.
  • David Lack (1947) suggested that selection would
    favor the clutch size that produced the most
    surviving offspring.

48
Clutch size in birds
  • If probability of average offspring surviving
    falls with increasing clutch size then we can
    calculate optimal clutch size by multiplying
    clutch size by probability of survival.
  • An intermediate clutch size is thus optimal.

49
Fig 12.16
50
Clutch size in birds
  • There have been numerous field studies that have
    tested Lacks hypothesis.
  • Many studies (including ones in which additional
    eggs are added to the brood) have found that the
    most productive clutch is often several eggs
    larger than that laid by the birds.

51
Fig 12.17
52
Clutch size in birds
  • How do we explain the observation that many birds
    appear to lay clutches that are smaller than the
    apparent optimum?
  • Several plausible hypotheses have been put
    forward.

53
Clutch size in birds
  • (i) Lacks hypothesis assumes that effort in one
    breeding season has no effect on effort in future
    years.
  • Many studies have shown that birds forced to
    raise larger broods in one year, lay smaller
    clutches the next year. Also, birds that raise
    larger clutches have lower survival to the next
    year.

54
Clutch size in birds
  • (ii) Increasing clutch size may reduce the
    quality of the offspring.
  • Schluter and Gustafsson (1993) added or removed
    eggs from nests of Collared Flycatchers.
  • Monitored chicks subsequent life histories.

55
Clutch size in birds
  • Found young from nests with enlarged clutches
    laid smaller clutches than did birds from nests
    with reduced clutches.
  • There appears to be a trade-off between number
    and quality of offspring so that most productive
    clutch size is smaller than that which produces
    the most surviving offspring.

56
Fig 12.18
57
How big should each offspring be?
  • Logically there must be a trade-off between
    number and size of offspring.
  • A cake can be cut into a few large pieces or many
    small pieces, but not many large pieces.

58
How big should each offspring be?
  • Elgar (1990) documented a clear negative
    correlation between clutch size and egg size in
    26 families of fish.
  • Fish that produce larger eggs produce fewer eggs
    per clutch.
  • A similar correlation between egg size and clutch
    size has also been documented for 3 orders of
    insects Berrigan (1991).

59
Fig 12.21
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61
Selection on offspring size
  • Smith and Fretwell (1974) analyzed the problem of
    how parents could strike a balance between size
    and number of offspring.
  • Their analysis was based on two assumptions (i)
    there is a trade-off between size and number of
    offspring

62
Fig 12.22a
63
Selection on offspring size
  • (ii) Larger offspring have a better chance of
    surviving.
  • There must be a minimum size below which
    offspring have no chance of surviving, but above
    this survival probability increases sharply with
    size before leveling off (as it cannot exceed a
    probability of 1).

64
Fig 12.22b
65
Selection on offspring size
  • Given the two assumptions it is easy to determine
    an optimal balance for a pair of curves.
  • Parental fitness for an offspring size is given
    by multiplying number of offspring by survival
    probability.
  • Plotting fitness against offspring size allows
    optimum to be identified.

66
Fig. 12.22c
67
Selection on offspring size
  • Optimal offspring size will differ depending on
    the shapes of the curves used in the analysis.
  • However, an intermediate offspring size will be
    favored. If relationship between survival and
    size was linear rather than curvilinear extreme
    offspring size might be favored instead.

68
Selection on offspring size
  • Note that parental and individual offspring
    optima differ.
  • Producing more, but smaller offspring enhances
    parental fitness, but smaller offspring have
    reduced survival probabilities.

69
Selection on offspring size in salmon
  • Smith and Fretwells model has been tested in
    salmon.
  • Heath et al. (2003) studied Chinook salmon at a
    commercial hatchery.
  • They confirmed Smith and Fretwells first
    assumption that there is a trade-off between egg
    size and number of eggs laid.

70
12.23 A
Mean egg mass
71
Selection on offspring size in salmon
  • They also examined the relationship between egg
    size and survival of young fish (fry).

72
Fig 12.23b
73
Selection on offspring size in salmon
  • Using the two curves, Heath et al. calculated an
    optimal egg mass of 0.15g for hatchery salmon.
  • Optimal egg size for hatchery salmon is smaller
    than it is for wild salmon because smaller fry
    survive better in the hatchery than in the wild.

74
Fig 12.23c
75
Selection on offspring size in salmon
  • The hatchery population was founded from wild
    stock in the late 1980s and given the
    reproductive advantage females with smaller eggs
    have, the population has been evolving towards
    smaller egg sizes since then.

76
12.23d
77
Conflicts of interest between life histories
  • Mammals nourish their offspring using a placenta.
  • This system of nourishing the offspring allows an
    opportunity for conflict between paternal and
    maternal genes.

78
Conflicts of interest between life histories
  • The conflict stems from the fact that males would
    prefer the female to invest heavily in current
    offspring, whereas the female also wishes to
    invest in future offspring (likely fathered by
    other males).

79
Conflicts of interest between life histories
  • Selection should favor males that can coerce the
    female to invest more heavily in the current
    offspring and mechanisms to do this have been
    found.
  • Certain genes are biochemically imprinted during
    gamete production, which allows male and female
    alleles to be distinguished.

80
Conflicts of interest between life histories
  • Imprinting affects transcription of genes within
    the embryo.
  • For example, in mice the paternal allele of a
    hormone called Insulin-like Growth Factor II (IGF
    II) is widely expressed, but the maternal copy is
    hardly transcribed.

81
Conflicts of interest between life histories
  • This pattern of imprinting is puzzling because
    equal expression of alleles is the norm.
  • Females turning off their allele runs the risk
    of the fetus not producing an essential enzyme if
    the males version is non-functional.

82
Conflicts of interest between life histories
  • Haig et al. have explained the observed pattern
    of imprinting as the result of a tug-of-war
    between male and female alleles within the fetus.
  • The paternally transcribed IGF-II is selected to
    maximize rates of cell division (and hence growth
    and monopolization of female resources). The
    female allele is turned off to preserve resources
    for future reproduction.

83
Conflicts of interest between life histories
  • Consistent with the expectation that males will
    attempt to influence resource distribution when
    they can, genomic imprinting does not occur in
    birds and frogs where all resources are
    distributed before fertilization.
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