Neuropsychology period 2
Ageing
Task 1 - The biological paradox: Time of our lives
Possible exam question:
A) Explain the disposable soma theory of ageing. (6p)
B) Explain what is meant by selection shadow. (4p)
What happens when you age?/why do we age?
Ageing is usually defined as the progressive loss of function accompanied by decreasing fertility and
increasing mortality with advancing age.
- But some species show no age-associated increase in mortality or decline in fertility.
- Thus → ageing cannot be explained simply as the inevitable result of biological wear-and-tear. So, why
does it occur?
An early explanation for evolution of ageing was the idea that senescence (= veroudering) is programmed in
order to limit population size or accelerate the turnover of generations, thereby aiding the adaptation of
organisms to changing environments.
➔ Flaw in this argument → for most species there is not much evidence that senescence contributes
significantly to mortality in the wild.
Natural mortality, as opposed to that seen in protected populations, is mostly due to extrinsic hazards, such as
infection, predation, starvation or cold, and occurs mainly in young individuals.
- Wild animals simply do not live long enough to grow old (figure 1a).
- Therefore, natural selection has limited opportunity to exert a direct influence over the influence of the
process of senescence.
➔ Even in species where senescence does make some contribution to mortality in the wild (for example,
larger mammals and some long-lived birds), any hypothetical ‘accelerated ageing gene’ would be
disadvantageous to the individual.
Three theories of ageing
Mutation accumulation theory
The rarity of aged animals in the wild in fact gives the clue to an important principle underlying all of the
current evolutionary theories of ageing.
- As a result of extrinsic mortality
- (= seen as the result of hazards from the environment; external factors, such as sunlight and
pollutants. As the human body is exposed to these hazards uniformly across ages, extrinsic
mortality would not represent aging and would be constant during life), there is a
progressive weakening in the force of selection with increasing age.
- By an age when wild survivorship has declined to very low levels, the force of selection is too
weak to oppose the accumulation of germ-line mutations with late-acting deleterious
effects (Figure 1b). This ‘selection shadow’ allows a wide range of alleles with late
deleterious effects to accumulate over the generations with little or no check.
➔ This is the ‘mutation accumulation’ theory
o → a build-up across generations of mutations with late-acting harmful effects that cannot
be removed because the power of selection is too weak (= selection shadow)), and because
the deleterious alleles are essentially unselected, we might expect considerable heterogeneity
in the distribution of such alleles among individuals within the population.
Selection shadow = minder effectiviteit van natural selection; natural selection neemt af naarmate je ouder
wordt
, ➔ → no chance of selecting anymore, so bad genes get the chance to come forward.
➔ When someone is reproducing you don’t know if someone is going to develop, for example, dementia.
➔ When someone grows older, selection is not high anymore.
➔ Selection shadow makes the bad genes visual, it is the period where we do not select for bad genes
anymore and don’t reproduce anymore.
➔ We weten dus niet of we slechte genen hebben waardoor we dementie ontwikkelen, wanneer we dus
ouder worden blijkt dat we bijvoorbeeld wel dementie ontwikkelen en deze slechte genen ook hebben
doorgegeven. De selectie schaduw geeft dus weer dat er nu geen natural selection meer is of minder en
je dus uiteindelijk dood gaat.
➔ sexual maturity → animals die soon after they reached sexual maturity
o If they stay alive: mutations become more present → not able to suppress mutations because
the effects of aging
➔ If there would not be a selection shadow
o Perfect knowledge of the genes
o We could eliminate people with diseases
o Perfect genome
➔ Relate it to other theories of aging
Pleiotropy/antagonistic pleiotropy
A second theory is that of pleiotropy also sometimes called antagonistic pleiotropy (→ the further likelihood
that late-acting deleterious effects will accumulate if they are direct byproducts of genes that are retained by
selection because they have early-life fitness benefits).
- Pleiotropic genes with good early effects would be favored by selection even if these genes had bad
effects at later ages (figure 1c).
- Because the contribution to fitness is a composite of both the size of the effect and the probability of
surviving to be affected by it, a small beneficial effect early in life can outweigh a late deleterious effect
even if the latter results in senescence and death.
- Possible addition on selection shadow
Disposable soma theory
The third theory is the disposable soma theory, which is based on optimal allocation of metabolic resources
between somatic maintenance and reproduction.
- Effective somatic maintenance is required only to keep the organism in sound physiological condition
for as long as it has a reasonable chance of survival in the wild (figure 1d).
- Nearly all of the mechanisms required to combat intrinsic deterioration (such as DNA repair or
antioxidant systems) require metabolic resources.
- Resources are scarce, as shown by the fact that the main cause of mortality for wild mice is
cold, owing to failure to maintain thermogenesis.
The disposable-soma theory therefore suggests that the mouse will benefit by investing any spare resource into
thermogenesis or reproduction, rather than into better repair capacity, even though this means that damage will
eventually accumulate to cause ageing.
➔ This theory focusing specifically on mechanisms, particularly the role of somatic maintenance and
repair, whereas the pleiotropy theory is formulated in terms of a general pattern of gene action and may
involve pleiotropic genes of various kinds.
➔ Other article:
o First recognized by Weismann, who argued that in a fundamental sense the germ line must be
regarded as immortal, whereas the soma is often subject to ageing and intrinsic mortality.
o To the extent that continuing survival is secured by physiological investments in processes
of cellular maintenance and repair, the disposable soma concept recognizes that under
pressure of natural selection to make optimal use of metabolic resources, it will only make
evolutionary sense to invest in somatic maintenance to secure functional integrity during the
span of time that the individual has a reasonable chance still to be alive.
Notes for disposable soma theory
➔ We try to make an optimal division of resources, so that our body is most optimal when we reproduce
→ thus, body tries to be in the best shape to reproduce → we don't reproduce when metabolic state is
not optimal
, ➔ Aging = evolutionary trade off
o DNA repair on one side
o Growth and reproduction on other side
➔ Because of reproduction → not enough resources to repair
➔ More investment in one side leads to loss on the other side
o Accumulation → loss of repair → damage
o Telomere loss
o Mitochondrial damage
Figure 1 Evolutionary theories of ageing.
a, Extrinsic mortality in wild environments
occurs to an extent that senescence-associated
mortality is rare, undermining any idea that
genes specifically for ageing have evolved.
b, The ‘selection shadow’ at older ages may
permit an accumulation of late-acting
deleterious mutations (mutation-accumulation
theory).
c, Pleiotropic genes that benefit organisms early
in life will be favored by selection even if they
have bad effects at later ages (pleiotropy theory).
d, Selection pressure to invest metabolic
resources in somatic maintenance and repair is
limited; all that is required is to keep the
organism in sound condition for as long as it
might survive in the wild (disposable-soma
theory).
Try to explain the figures!
These three theories provide complementary explanations for why ageing occurs. Each also addresses the
question: why do species have the life spans they do?
- The principal determinant in the evolution of longevity is predicted to be the level of extrinsic
mortality → If this level is high, life expectancy in the wild is short, the force of selection attenuates
fast, deleterious gene effects accumulate at earlier ages, and there is little selection for a high level of
somatic maintenance.
Evolutionary theories tested
The ageing theories assume an age-structured population, that is, a population in which individuals can be
segregated by age, and therefore predict that ageing should not arise in populations where age classes cannot be
assigned.
- At a unicellular level, it is therefore unsurprising that ageing is generally not seen in bacterial
populations.
- In multicellular organisms, the need for age structure also applies and ageing is generally predicted to
require a clear separation between germline and
soma.
An important prediction of the evolutionary theories is that
altering the rate of decline in the force of natural selection will
lead to the evolution of a concomitantly altered rate of ageing.
Most selection experiments have used the fruitfly Drosophila
melanogaster.
➔ By restricting reproduction to later ages, the intensity
of selection on the later portions of the life span was
increased. This consistently extended the longevity of
the selected populations.
➔ Furthermore, a general correlate of delayed
senescence has been reduced fecundity (=
, vruchtbaarheid) in the long-lived flies, which supports the idea of a trade-off between fertility and
survival, as suggested by the disposable-soma and pleiotropy theories.
➔ The general finding from selection experiments in Drosophila is therefore that retarded ageing is
associated with depression of fitness components in early life, although there is variation in which
fitness components are affected.
Trade-offs have also been reported in the nematode Caenorhabditis elegans where a host of long-lived mutants
has been identified.
➔ Mixed results of there are the same trade-offs between longevity and early-life fitness components
as found in Drosophila.
➔ A recent study of the long-lived age-1mutant in C. elegans showed that the relative fitness effects of
mutations can be strongly affected by environment.
Adaptations that reduce extrinsic mortality (for example, wings, protective shells or large brain) are generally
linked with increased longevity (in bats, birds, turtles and humans).
Addition Kirkwood 2017
Why does ageing occur?
That ageing has been influenced by selection is indicated inter alia by the fact that each species shows a
characteristic pattern of survival and longevity.
Ageing is present when there is a progressive, generalized impairment of physiological functions, resulting in
an increasing age-specific death rate.
- Humans exhibit an approximately exponential increase in age-specific death rates; so-called
Gompertzian mortality.
There are variations in the patterns of species’ life histories that make the study of ageing a particularly rich
field of investigation.
- In some instances death comes abruptly after reproduction → In these so-called semelparous life
histories, where the organism has only one opportunity to reproduce before dying and thus mobilizes
all its effort towards maximizing its single chance of success once reproductively mature
- The signicance of post-reproductive death is different from the age-related decline in ’iteroparous’
organisms that can reproduce repeatedly.
Most adaptations are beneficial for the organism. Ageing is unusual in that its effects on fertility and survival
(→ two principal determinants of Darwinian fitness) are deleterious. It has been suggested that ageing evolved
1. To control population size
2. To accelerate the turnover of generations and thereby facilitate adaptation to changing environments
3. To free up space and other resources for the benefit of the group.
These suggestions are contradicted by the facts that most organisms die from extrinsic causes at ages well
below those when senescence becomes apparent and that it is extremely hard to devise plausible biological
scenarios where the disadvantage to the individual is outweighed by any benefit accruing to the group.
The evolution of ageing is now generally understood to have occurred not through programming of ageing as an
adaptive benefit in its own right, but because the force of natural selection falls off strongly across the course of
the lifespan.
- Even if organisms were physiologically capable of preserving their integrity so that no decline occurs
in the chance of surviving from one time unit to the next, the fraction of survivors must inevitably
shrink because of deaths from other causes.
The force of selection at any age is proportional to the fraction of the organism’s expected lifetime reproductive
output that is yet to come.
➔ It was the recognition of this fundamental principle that led to the classic ideas of mutation
accumulation,
o i.e., a build-up across generations of mutations with late-acting harmful effects that cannot be
removed because the power of selection is too weak (= selection shadow),
