W5 L1 Human Life History I - Longevity

Question 1

Stages of human life

Answer 1

Gestation/Childhood: GROWTH and maintenance
Early adulthood :REPRODUCTION and maintenance
Post-reproductive age:MAINTENANCE

Question 2

what is life history

Answer 2

the timing of and relative investment of energy in survival, growth, and reproduction

Question 3

The ‘ideal’organism (Darwinian Demon)

Answer 3

• reproduces during its entire, long life
• produces large number of large offspring
• lavishes parental care on all offspring
• outcompetes its competitors
• always escapes predation
  Even the most successful organism can’t be good at everything

Question 4

Allocation of finite energy and resources

Answer 4

Fundamental concept for understanding life histories: Trade-offs among resources to maximise fitness

Question 5

What is trade-off

Answer 5

Negative correlation between traits
Increased allocation of energy/resources to one trait usually results in reduced allocation to another.
Genetic correlations can cause changes in one trait (such as growth rate) due to selection on another trait (such as fecundity).

Question 6

Common life-history trade-offs

Answer 6

• Survival vs reproduction
• Current vs future reproduction
• Reproduction vs growth
• Offspring quality vs quantity

Question 7

Approaches to study life-history evolution

Answer 7

Trade-offs are studied using optimality theory - which combination of trait values maximises fitness, or population growth rate r
Assumes organisms are optimally adapted to their environment and any phenotype is possible.
Genetic models are used to estimate:
* Heritability
* Response to selection (evolvability)
* Genetic correlations between traits
* Interaction between genes and environment

Question 8

Studying human life history

Answer 8

Rarely or never opportunities to conduct experimental studies
Heavy reliance on correlational or comparative data
Remarkable records of reproduction and life span for many societies; exceptional demographic and genomic datasets

Question 9

Multiple reasons for observed correlations

Answer 9

1. A causes B (direct causation)
2. B causes A (reverse causation)
3. A and B are consequences of a common cause, but do not cause each other
4. A causes C, which causes B (indirect causation)
5. Coincidence (there is no connection between A and B)

Question 10

Mutation accumulation

Answer 10

Animals tend to die of extrinsic causes (predation, disease, accidents) well before they reach old age
Weak selection for weeding out random, detrimental mutations that don’t show up until late in life (not disadvantageous to majority)
The older we become, the more we accumulate these mutations – these eventually kill us

Question 11

Antagonistic pleiotropy

Answer 11

Many genes are pleiotropic (affect two or more traits)
 The same gene can have positive as well as negative effects
 For instance, a gene that promotes calcium deposits in bones but also in arteries has early fitness benefits, but exacts a cost later on
Traits that are beneficial early in life will be selected for (even if they come with a price tag later in life)
So, negative effects in old age may result from selection for pleiotropic genes with beneficial effects early in life

Question 12

Why do we age? Disposable soma theory

Answer 12

Reproduction is costly; investment in reproduction diverts resources from maintenance and repair of cells, causing ageing
V
Prediction: shortage of food should exacerbate this trade-off
But …
 Caloric restriction experiments find animals live longer when fed less
 Males invest much less into reproduction than females, yet females live longer

Question 13

Longevity and reproduction tradeoff in humans

Answer 13

-Fundamental life-history trade-off between survival and reproduction because reproduction is costly
-Are longevity and reproduction negatively correlated in humans?
Anecdotal accounts of non- reproducing individuals living for
longer (celibate nuns and priests; neutered pets)

Question 14

study of longevity and reproduction in humans

Answer 14

• Study of 18th & 19th century British aristocracy
• Minimised effects of socio-economic conditions on reproduction and lifespan by restricting study to individuals that were relatively well-off
• Studied the relationship between reproductive success and longevity in married women in cohorts up to 1876
• 33,497 individuals
  Prediction: delayed age at first reproduction and fewer offspring should increase female longevity
  -Early child bearing is associated with a shorter life span
  -For women ages 60+ number of offspring is negatively correlated with longevity

Question 15

Longevity and ‘children ever born’

Answer 15

Framingham Heart Study Dataset’ – 3 generations
Phenotypic and genetic correlations between Lifespan and ‘Children Ever Born’
Genome Wide Association Study (GWAS) to identify genes responsible
-Trade-off between longevity and reproduction in women, but not men
-found 5 Single Nucleotide Polymorphisms (SNPs) associated with this relationship

Question 16

Change in sex-specific longevity supports cost of reproduction utah study

Answer 16

Demographic transition from high to low birth and death rates 4 birth cohorts that span the transition in Utah
Change in selection on life history traits: predict greater increase in lifespan for women relative to men