MODULE 16: LIFE HISTORY EVOLUTION

What is the main focus of life history analysis?

Understanding how organisms allocate limited time, energy, and resources toward growth, survival, and reproduction under evolutionary trade-offs.

What major questions does life history theory try to answer?

Why organisms age, when they should reproduce, how many offspring to produce, how large offspring should be, and how organisms balance current vs. future reproduction.

What is senescence?

The age-related decline in physiological function, fertility, and survival probability as organisms grow older.

Why is senescence an evolutionary problem?

Natural selection should favor traits that promote long life and maintenance, yet aging is nearly universal, indicating evolutionary forces shape it.

What does the Rate of Living Theory propose?

That aging results from accumulated metabolic damage and that species have fixed limits on energy expenditure and repair capacity.

Why is the Rate of Living Theory rejected?

Species differ widely in lifetime energy use, and lifespan can evolve under artificial selection, contradicting the theory's predictions.

What physiological factors contribute to aging but do not explain its evolution?

Telomere shortening, oxidative stress, and p53-mediated tumor suppression that trades off with tissue maintenance.

Why does natural selection weaken with age?

Because traits expressed after reproduction contribute little to fitness, reducing selective pressure against late-acting harmful effects.

What is the Mutation Accumulation Hypothesis?

The idea that late-acting harmful mutations persist because natural selection is too weak to remove them.

What evidence supports mutation accumulation?

Human diseases with late onset and fly populations selected for early reproduction that accumulate late-life harmful mutations.

What is the Antagonistic Pleiotropy Hypothesis?

The idea that alleles increasing early-life reproduction or survival are favored even if they cause negative effects later in life.

Examples of antagonistic pleiotropy

C. elegans Age-1 mutants with longer lifespan but reduced reproduction; Drosophila Methuselah allele improving stress resistance but reducing reproductive output in some conditions.

How does extrinsic mortality influence aging rates?

High extrinsic mortality favors early reproduction and faster aging; low extrinsic mortality favors slower aging and delayed senescence.

Example of extrinsic mortality affecting senescence

Island opossums evolve slower aging and longer reproductive spans due to reduced predation compared to mainland populations.

What is the trade-off between number and size of offspring?

Producing many small offspring increases quantity but reduces investment per offspring, while producing few large offspring increases survival but reduces number.

Why do larger offspring generally have higher survival?

They have more energy reserves, better competitive ability, and greater resilience to environmental stress.

What does Lack's Optimal Clutch Size Model predict?

That parents should produce the number of offspring that maximizes the number of surviving young.

Why do real organisms produce smaller clutches than Lack's prediction?

Because parents must balance future reproduction and survival, and larger clutches reduce offspring quality.

What trade-offs reduce optimal clutch size in birds?

Producing more young reduces parental survival and reduces the long-term performance and survival of offspring.

What determines the optimal offspring size?

The balance between increased survival of larger offspring and the reduced number a parent can produce as size increases.

Example of offspring size evolution

Chinook salmon evolve different egg sizes depending on whether they reproduce in wild rivers or hatcheries, due to differing survival pressures.

What are the major takeaways of life history evolution?

Aging evolves because selection weakens with age; trade-offs shape reproduction and survival; and offspring number and size evolve through optimization rather than maximization.