Aging and Other Life-History Characters

Basic issues in life-historyo Reproductive success is the trait on which natural selection actso Life history– attempts to make sense of the diversity in reproductive

strategieso Organisms face fundamental trade-offs in their use of energy and time

Amount of energy an organism can harvest is limited Energy and time tradeoff

Size tradeoff– energy cost Time required to grow is time during which predators,

diseases, or accidents may strike An individual that takes the time to grow large thus incurs a

greater risk of dying without reproducing Whenever there is a trade-off between different components of

fitness, we expect natural selection to favor individuals that allocate energy and time with an optimal balance between benefits and costs, thereby maximizing lifetime reproductive success

o Changes in life history are caused by changes in allocation of energy Allocating less energy to reproductions means having smaller litters Natural selection on life histories leads to adjustments in energy

allocation that maximize the total lifetime production of offspring Why do organisms age and die?

o Aging (senescence) is a late-life decline in an individual’s fertility and prob- ability of survival

o Aging reduces an individual’s fitness Aging should be opposed by natural selection

o Rate of living theory Populations lack the genetic variation to respond any further to

selection against aging Aging caused by accumulation of irreparable damage to cells and

tissue Damage caused by errors during replication Accumulation of metabolic products

All organisms have been selected to resist and repair cell and tissue damage to the maximum extent physiologically possible

Have reached the limit of biologically possible repair Populations lack the genetic variation that would enable them

to evolve more effective repair mechanisms that they already have

Aging rate should be correlated with metabolic rate Species should not be able to evolve longer life spans whether

subjected to natural or artificial selection

According to the theory, all should expend about the same amount of energy per gram per lifetime, whether they burn it slowly over a long life or rapidly over a short one

BUT data on variation in metabolic rate and aging among mammals refute this theory

Organisms age and die due to intrinsic physiological limits on cells and tissues (little flawed)

o Evolutionary theory Trade-off between allocation of energy to reproduction versus repair Aging is caused not so much by cell and tissue damage itself as by the

failure of organisms to completely repair such damage This failure leads to gradual decay and ultimate collapse

Failure to completely repair damage is ultimately caused by either deleterious mutations or trade-offs between repair and reproduction

Natural selection is weak late in life so alleles that cause aging are only mildly deleterious

They may persist in mutation-selection balance or rise to high frequency by drift

In populations where mortality rates are high, individuals tend to invest more heavily in early reproduction

The evolutionary theory of senescence hinges on the observation that the power of natural selection declines late in life

This is because most individuals die—due to predators, diseases, or accidents—before reaching late life.

Two mechanisms can lead to the evolution of senescence: o (1) Deleterious mutations whose effects occur late in

life can accumulate in populations; o and (2) when there are trade-offs between

reproduction and maintenance, selection may favor investing in early reproduction even at the expense of maintaining cells and tissues in good repair

o Evolutionary explanation for menopause In humans, reproductive capacity declines earlier and more rapidly in

women than men Nonadaptive artifact of our modern lifestyle

Reconstructing the demography of ancient peoples have often concluded that in premodern cultures, virtually all adults died by age 50 or 55

Not an adaption because hunter-gatherer ancestors never lived long enough to experience it

Life History adaptation Menopause- life history adaption associated with the

contribution grandma makes to feeding her grandchildren Human children depend on mom for food several years after

weaning

1. Probability that she will live long enough to be able to nurture another baby from birth to independence declines

2. risks associated with pregnancy and childbirth rise 3. her own daughters will themselves start to have children

suggests that older women may reach a point at which they can get more additional copies of their genes into future generations by ceasing to reproduce themselves and instead helping to provision their weaned grandchildren so their daughters can have more babies.

grand- mothers face a trade-off between investment in children versus grandchildren

age at menopause is heritable and later menopause confers higher lifetime reproductive success

How many offspring should an individual produce in a given year?o Trade offs: more offspring a parent (or pair of parents) attempts to raise at

once, the less time and energy the parent can devote to caring for each oneo Clutch size in birds

Selection will favor clutch size that produces the most surviving offspring

Probability that any individual offspring will survive decreases with increasing clutch size

Adding eggs reduces survival rate for individual chickso Ability of parents to feed any individual offspring

declines as the number of offspring increases Number of surviving offspring reaches a maximum at an intermediate

clutch size. It is this most-productive clutch size that Lack’s hypothesis predicts will evolve by natural selection

Lack’s hypothesis predicts that parents will attempt to rear that number of young that maximizes the number of surviving offspring.

Data indicate that parents often rear fewer offspring. Efforts to identify which of Lack’s assumptions are violated have led to the discovery of additional trade-offs and improved estimates of lifetime fitness

o Criticisms of Lack’s hypothesis 1. Lack’s hypothesis assumes there is no trade-off between a parent’s

reproductive effort in one year and its survival or reproductive performance in future years

When reproduction is costly and selection favors withholding some reproductive effort for the future, the optimal clutch size may be less than the most productive clutch size

2. Lack’s hypothesis assumes that the only effect of clutch size on off- spring is in determining whether the offspring survive. Being part of a large clutch may, however, impose other costs on individual offspring than just reducing their probability of survival

clutch size affects not only offspring survival but also offspring reproductive performance

When larger clutches entail lower offspring reproductive success, the optimal clutch size will be smaller than the most numerically productive clutch size

3. discrepancy between Lack’s hypothesis and the behavior of individual birds may sometimes be more apparent than real

Note: assumed that clutch size is fixed for any given genotype. In fact, clutch size is often phenotypically plastic

o If clutch size is plastic, and if birds can predict whether they are going to have a good year or a bad year, we would predict that individuals will adjust their clutch size to the optimum value for each kind of year

o Larger clutch sizes may reduce offspring fitness Trade-offs between a female’s investment in a particular clutch and

her own future survival or reproductive performance How big should each offspring be?

o Individual offspring will have a better chance of surviving if they are larger Minimum size below which offspring have no chance of survival. As

offspring get larger, their probability of surviving rises Expected fitness of a parent producing offspring of a particular size is

the number of such offspring the parent can make multiplied by the probability that any individual offspring will survive

Key point: selection on parents often favors offspring smaller than the size favored by selection on offspring themselves

Survival probability increases with offspring size, but the rate of increase decline

Increasingly large offspring gain a progressively smaller survival benefit

Intermediate offspring size give the highest parental fitnesso 1. Trade off between size and number of offspringo 2. Above minimum size, probability that any individual offspring will survive

is an increasing function of its sizeo Problem: organisms face a trade-off between making many low- quality

offspring or a few high-quality offspring. Selection on parents favors a compromise between the quality and quan- tity of offspring, but selection on individual offspring favors high quality.