1. LIFE HISTORY EVOLUTION: Why do we get old and die?

2. Can life histories evolve through natural selection?
An organism’s life history is the stages it goes through in its lifetime:
birth--> growth --> reproduction --> death
Life history traits: # and size of offspring, age at first reproduction, reproductive life span, etc.
Hence, life histories include many components that contribute to an individual’s fitness

3. What life history traits are favored by natural selection?
Selection favors genotypes that have higher fitness: individuals that pass on more of their genes to future generations
Natural selection should favor individuals that mature at birth, produce lots of high quality offspring and live forever

4. So why don’t we live forever and have millions of offspring?
Energy/resources are limiting!!!
This sets up TRADE-OFFS between different life history traits
Energy/resources devoted to one function can’t be used for others

5. The cost of reproduction
The trade-offs between current reproduction and other components of fitness is referred to as the cost of reproduction
Fundamentally, differences in life histories concern differences in the allocation of resources to different life history traits

6. Evidence for trade-offs
Relationship between clutch size and offspring size
Age at first reproduction
Reproductive events per lifetime

7. Natural selection will act on life histories to adjust energy/resource allocation to maximize TOTAL lifetime fitness
Evolutionary biologists studying life histories are interested in understanding factors that favor different LIFE HISTORY STRATEGIES
The following are some examples of different life history strategies

8. Parthenogentic aphids may
carry embryos even before it
is born
A blue whale gives birth
To a single offspring the
Weight of an adult
elephant

9. Bristlecone pine trees can live to be 4600 years old
Brown kiwi birds lay a single egg that can be up to 20% of the female’s body weight

10. Let’s look at 2 specific life history traits:
Age Schedule of Reproduction
When and how often should an organism reproduce?
Life Span and Senescence
How long does an organism live?

11. Age schedule of Reproduction
Semelparity: individuals reproduce once and then die (annual plants, salmon, century plants)
Iteroparity : individuals reproduce more than once over their lifetimes (humans, elephants, perennial plants, most animals)

12. When is iteroparity selected for? When is semelparity selected for?
If juvenile mortality is high compared to adult mortality --> i.e., once you make it to maturity, you have a good chance of living longer
Keep on reproducing: iteroparity
If adult mortality is high compared to juvenile mortality --> i.e., once you reach maturity, your time is near
Go for it why you can: semelparity

13. Overall: a simplification
"K"-Selected populations
- If juvenile mortality is high compared to adult mortality:
Iteroparous reproduction Reproductive delay
Small brood sizes Large eggs
Characters that result in a low intrinsic rate of increase
Such populations tend to find an equilibrium near K (carrying capacity)
“r”-Selected populations
If populations tend to experience many periods of exponential growth or high adult mortality selection may favor:
Semelparous (or at least early) reproduction
Fast development
Large brood sizes
Such populations tend to maximize their intrinsic rate of increase, r

14. Trinidad Guppies (David Reznick)
Reznick transplanted guppies from a low predation stream into a high predation stream (w/cichlids) in Trinadad
High juvenile mortality
High adult mortality

15. Probability of surviving to next year is high-->K-selected
Surviorship Curves
Probability of surviving longer is low--> r-selected

16. Evolution of Life Span and senescence
We need to distinguish between:
Senescence/aging: physiological degeneration and death over time
Extrinsic mortality: death due to predation, disease, etc.
All else being equal, aging should be opposed by natural selection

17. A Non-Evolutionary Explanation for Aging
Hypothesis I: aging is a byproduct of accumulation of damage to cells and tissues- “RATE OF LIVING” or “PARTS WEAR OUT”
Selection for longer life in flies yields response (2x in 13 generations)- contradicts rate of living hypothesis
Predicts:
 Damage is a byproduct of metabolism - aging and metabolic rates should be positively correlated
Ability to replace or repair has been maximized by selection - species are constrained from evolving longer life spans
No evidence at broad taxonomic levels- marsupials have lower metabolic rates than comparably-sized placental mammals. but shorter life spans

18. Evolutionary Explanations for Aging
If selection can produce longer life spans, why don’t organisms evolve them?
Hypothesis 2: Accumulation of deleterious mutations
Medawar (1946) - selection on genes that have negative effects late in life (“aging genes”) is low because many individuals are already dead due to environmental causes by the time they show their effects
Selection is weak on old individuals, so mutations with deleterious effects late in life are not removed by selection

19. Evidence for Mutation-Accumulation Hypothesis
Inbreeding depression exposes recessive deleterious alleles
If mutation-accumulation hypothesis is true, inbreeding depression (reduction in fitness due to inbreeding) should increase with age (Hughes et al, 2002)
I.e., there are more mutations that affect individuals late in life

20. Evolutionary Explanations for Aging
Hypothesis 3: Antagonistic Pleiotropy
Williams (1957) - genes with two effects (pleiotropy)
“Aging” genes may be those that are advantageous effect in youth but disadvantageous in old age:
Such genes would be selected for as many individuals will benefit from its advantages in youth
but fewer will suffer its disadvantages in old age

21. Evolutionary Explanations for Aging
Hypothesis 3: Antagonistic Pleiotropy
Genes that exhibit antagonistic pleiotropy:
Age-1 in C. elegans
Worms with hx546 mutation live longer, wildtype age-1 allele increases early reproduction at expense of longevity
Indy gene in Drosophila
Indy loss of function mutants have 2x the lifespan as wildtypes
Under restricted diets, wildtypes have higher fecundity

22. Evolutionary Explanations for Aging
An organism’s lifespan is determined by balancing the trade-off between allocation to repair and allocation to reproduction
A decrease in extrinsic mortality may favor an increase in allocation to repair --> delayed senescence (and vice versa)
Austad (1993) followed opossums on mainland (South Carolina) and island (Sapelo Island) populations that have been isolated about 4500 yrs

23. Life History Evolution- a natural experiment Virginia opossums
Mainland Population
High extrinsic mortality- dogs,bobcats, etc.
Island Population
Low extrinsic mortality- no mammalian predators
Performance of mainland mothers decreases in second year (reproductive senescence)

24. Life History Evolution- a natural experiment Virginia opossums
Mainland Population
High extrinsic mortality- dogs,bobcats, etc.
Island Population
Low extrinsic mortality- no mammalian predators
Island females have slower rate of physiological aging (collagen crosslinks reduce flexibility and increase with age)
Island possums have delayed senescence and longer lifespans

25. Don’t try this at home It may reduce your life expectancy
Studies have shown that mating can reduce the life span of female Drosophila
Male sperm induces female to invest more in her offspring at cost to her longevity