Chapter 13

Aging and Other Life History Characteristics

Life History Analysis: Study of Reproductive Strategies

• Reproduce early & fast or later & slow?– Deer mice mature at 7 weeks, have litters 3-4

times/year– Black bears mature at 4-5 years and have cubs every 2

years• Range of lifespans: single season to years– California poppy flowers & dies yearly (annual)– Black cherry flowers yearly for decades (perennial)

• Reproduce lots of little eggs or a few large ones?– Oysters (10-50 million eggs @ 55 µm)– Clams (<100 eggs @ 300 µm)

Sockeye Salmon

• Will defend egg nests as long as they have enough energy to stay alive

• Laying eggs earlier in life requires less energy, fish live longer

• Also better able to defend nest

Thrips Eat Their Way Out (of Mom)

Brown Kiwis Lay Extreme Eggs

BASIC ISSUES IN LIFE HISTORY ANALYSIS AND WHY DO ORGANISMS AGE AND DIE?

Sections 13.1-13.2

An Example of Life History: Opossum

Life History TradeoffsShort-winged female• Hatch with poorly-

developed flight muscles & lower triglyceride levels

• Use that energy to make phospholipids for eggs

• Reproduce earlier

Long-winged female• Hatch with well-developed

flight muscles & loads of triglycerides (flight fuel)

• Some can fly: may enable her to seek better environment

• Ovaries grow more slowly

Why do Organisms Age and Die?

• Senescence refers to a late-life decline in fertility and survival

• Should be opposed by natural selection• Two theories:

1. Rate-of-living theory: populations lack sufficient genetic variation to keep evolving longer lifespans: irreparable damage to cells/tissues

2. Evolutionary theory: tradeoff between directing energy to reproduction versus repair/survival

Costs of Aging

Rate-of-Living Theory

• Damage to cells/tissues occurs during replication, transcription, translation, accumulation of toxic metabolic by-products

• Two predictions:1. Aging rate should be correlated to metabolic

rate2. Species should not be able to evolve longer

lifespans due to selection (natural or artificial)

Does Aging Rate=Metabolic Rate?

Aging & Mutation

Can Lifespan Change or Evolve?

Researchers artificially selected for longevityUnclear if metabolic rate also contributed

Hayflick Limit & Telomeres• Caps on the ends of

chromosomes– Kinda like aglets– Sequence TTAGGG

• Shorten with each cell division– When too short, cells stop

dividing– Eventually die: part of

aging• Only stem cells, cancer

cells, germ cells exempt

Telomere Length & Lifespan

Altering Lifespan

The Evolutionary Theory

• Recap: Tradeoff between self-maintenance and reproduction

• Aging caused by failure to FULLY repair damage: slower path to death

• 2 reasons:1. Deleterious mutations2. Trade-offs between repair and reproduction

Basic Model for Evolution of Aging

• Critter lives 16 years• 80% chance of survival

year-to-year• Sexual maturity at age 3• One offspring/year• Overall expected

reproductive success is 2.419

Effect of Deleterious Mutation

• Mutation causes early death at 14 years

• NO other alterations• Overall expected

reproductive success is 2.340

• Slightly reduced relative to WT population (not a big change in survival): WEAK SELECTION

Tradeoff: Reproduction vs. Death• 2 mutations

1. Lifespan decreased to 10 years (should lower overall success)

2. Maturation reduced to 2 years rather than 3 (should increase overall success)

• Overall expected reproductive success is 2.663

• Moderate increase relative to WT: OFFSETS SOMEWHAT STRONGER SELECTION

Mutation Accumulation

Reduced fitness in small populations (inbreeding depression) may be due to accumulation of deleterious mutations

Longevity in small populations declines faster for late-acting deleterious genes (neutral)

Tradeoffs: Reproduction vs. Stress Resistance

• Normal flies die younger but reproduce better; methuselah flies live longer but reproduce less: TEMPERATURE DEPENDENT EFFECT

Antagonistic Pleiotropy

• One gene affects 2 traits: longevity & survival• If beneficial, should increase; if deleterious, should decrease• Hx546 increases longevity: deleterious when food is scarce (right)

HOW MANY OFFSPRING SHOULD AN INDIVIDUAL PRODUCE IN A GIVEN YEAR?

Section 13.3

Optimal Clutch Size

Lack’s hypothesis: Average clutch size should equal optimal size

Optimum Clutch Size for Great Tits

Optimal=12, Average=8. Violates Lack’s hypothesis.

Family Size Affects Next Generation

• Researchers added or removed eggs from nest

• The daughters “compensated” in the next generation

• Suggests a quality vs. quantity tradeoff– Daughters produce more

eggs if they got more care

Lack’s Hypothesis & Parasitoid Wasps

Species Optimal ActualAnagasta 4 1-2Ellopia 7 5-8Bupalus 9 5-8

HOW BIG SHOULD EACH OFFSPRING BE?

Section 13.4

Size vs. Quantity Tradeoff

The Optimal Compromise… for Everyone

SMITH-FRETWELL hypothesis:•More offspring decreases fitness of offspring•Bigger size increases fitness of offspring•PARENTAL fitness is better for smaller clutches

Testing Smith-Fretwell

• Larger clutches of small eggs: CONFIRMED

• Larger eggs have better survival: CONFIRMED

• Parental fitness: intermediate clutches of intermediate eggs: CONFIRMED– Drops in hatcheries due to

safer environment

Phenotypic Plasticity• What role does environment play in egg size?– Good host/environment, smaller eggs & more of them– Poor host/environment, larger eggs & fewer of them

Phenotypic Plasticity• Acacia tree is a good host• More eggs laid

• Cercidium tree is a poor host• Fewer eggs laid

• If you change hosts:• GoodPoor: egg size

increases• PoorGood: egg size

decreases