Life history 2: ageing and senescence

Question 1

Senescence vs ageing

Answer 1

Senescence: Growing older AND losing vitality
- Components of fitness (mortality rate and rate of reproduction) impact senescence.

Ageing: Growing older
- There are visible signs of ageing, and natural selection has selected organisms that are able to recognise these signs.

Question 2

Longevity

Answer 2

This is the lifespan of and organisms

There is huge diversity

Question 3

Evolutionary theory of senesence

Answer 3

August Wiesmann

He did not think senescence was a purely physiological process believing it was a ‘make room strategy’.

There was groups selection due to the ‘need of the species’ for organisms to die.

This explains the maintanance but not the evolution of senesence.

He also made the correct observation that organisms that separate germ line and soma to reduce mutations in the soma.
- Live longer, invest more into survival, germline generates mutations, cannot reproduce during alter stages of life -> may still be chosen to mate with if experience outweighs genetic degredation
- Live less time, invest less into survival and less time for germline mutations, can reproduce up until death.

Question 4

Classical theory of senesence

Answer 4

Mutation theory: Mutations are accumulate faster than they can be repaired.

Mathematical theory

Disposable soma: There is a trade-off between resource allocation to reproduction and maintenance (soma or germline) that cannot be maintained forever. -> they are both accumulating errors

Antagonistic pleiotropy: Genes may be beneficial early in life so selected for but detrimental later in life leading to senescence.

Question 5

Combined conclusion about theory of senesence

Answer 5

• Increase in mortality and decrease in fertility is inevitable due to senesence
• ‘Protected’ species evolve longer life spans
• Lifespan correlates with time of reproduction
• Senescence begins immediately after sexual reproduction.

Question 6

Hamilton’s view

Answer 6

He believed that all organisms underwent senescence, which is not the case (e.g. hydra and flatworms).

Question 7

What factors effect life span (other than senesence)

Answer 7

Size: large individuals can collect more resources and escape predators.

Flight: flight allows individuals to access the ground and sky resources and escape predators

Protection: Protected species have a reduced mortality

Active period: Crepuscular species are exploded to diurnal and nocturnal species.

Sociality: Social groups can lead to division of labout and greater protection.

cliff nesting, ocean-going, tree burrowing: Protection from predators.

Question 8

How to measure senesence

Answer 8

Observational: observe specific factors at different ages

Experimental: Grow individuals and test their strength

Question 9

What is demographic senesence

Answer 9

This involves the shift in demographic processes during senescence (e.g. mortality rate and fertility)

Question 10

Longevity and senesence are distinct

Answer 10

Longevity (lifespan) and senesence are distinct.

e.g. a species with a high mortality rate throughout life will be affected by the same rate of senescence differently compared to a species with a low mortality rate throughout life.

Question 11

Life history and senensence correlation

Answer 11

Within clades there are strong life history correlations with lifespan/ senescence.

e.g. The higher the generation time the lower the rate of senesence.

Question 12

Can you study senescence in the field

Answer 12

No- organisms will die from other causes other than senescence. Must be studied in a lab.

Yes- studying lifespan and mortality rate tells us about senescence.

Question 13

Species that don’t senesce

Answer 13

Under controlled conditions with no resource limitation and predation:

Hydra and flatworms

This is due to undifferentiated cells available to replace damaged cells

Question 14

what factors determine the level of senescence

Answer 14

1) The rejuvenation/ regeneration ability: allows replacement of damaged cells.

2) Separation of germ line from soma: reduces resource trade-off

3) Life history traits (e.g. regeneration time)

4) Development/ size

5) Architecture

Question 15

Example of different longevities

Answer 15

Humans: 122

Rockey mountain Bristlecone: 5,060 (potential for longer)

Bowhead whale: >210 (harpoon)

Laysan albatross: 72

Question 16

Overview

Answer 16

Senescence is the process of ageing and losing vitality. As a result, senescence affects the lifespan of organisms, but they are not always directly coupled (due to differences in underlying mortality rates).

Life history traits correlate with senescence (e.g. generation time), and senescence affects demographic processes like fertility and mortality rate.

There are both evolutionary and mechanistic theories for why organisms senesce

There are some organisms that don’t senesce (Hydra and flatworms) which goes against Hamilton’s view.

There are many factors that effect the life span of organisms other than senescence (e.g. flight and protection).

Question 17

Seperation of germline and some

Answer 17

Seperation leads to trade off between germline and soma (reproduction and survival)

Trade-off between the repair and maintenance of the germ cells and growth and maintenance of the soma

In Male zebrafish exposed to stressful conditions, the experimental removal of the germ line improves somatic recovery

In stochastic environments invest more into the germline (e.g. produce lots of sperm) for explosive reproduction and less into reproduction and tend to die from extrinsic factors rather than senesence

In constant environments organisms invest more into survival to maximise life long fitness (e.g. produce less sperm). They esperience slow senesence that limits there propensity to reproduce later in life.