Evolution of Life Histories

Life Histories

• Concerned with– 1. Size at reproductive maturity– 2. Age at reproductive maturity – 3. Number of offspring produced – 4. Size of offspring produced

• Variation from two sources:– 1. Epigenetic factors

• Phenotypic plasticity– 2. Adaptations

• Set by genotypes

Conceptual Problems

• 1. Life history characteristics have low heritability.– Relationship between life history characteristics and

fitness– But, fitness usually has a large, variable, environmental

component.– Genetic variation for life history characteristics is

maintained by shifting selection pressures.

• 2. Principles summarizing the diversity of life history strategies are rare.

Life histories and reproductive mode

• Two principles:• 1. Small genetic component in life history

variation.– Flexibility is important

• 2. Life history characteristics have not evolved in order to perpetuate a species.– Shaped by natural selection increasing fitness of

individuals.

Energy allocation principle

• Energy available to an individual is finite• A constraint on r (per capita rate of increase) • Energy shunts:

– 1. maintenance– 2. growth– 3. reproduction

• Energy constraints on reproduction results in two fundamental strategies:– 1. Large number of small young – 2. Small number of large young

• LH strategy example: KiwisNew ZealandLS: 20 yearsHuge reproductiveinvestment in a few individuals

Chicken-size6 lbsProportionatelylargest eggs ofany bird (1 lb)

Incubation by male (c. 11 wks.)Loses 20% BW

Chicks not fed byadultSelf reliant

• LH example: Thrip egg mites

LS: 4 days

Life History Principles

• Generally begin with birds• Reproductive output is accessible.• Reproductive output can be easily manipulated

and adjusted.• Individuals can be marked for identification.

The evolution of clutch size

• Optimal clutch size• ? How much energy should an individual allocate

to an episode of reproduction; e.g., how many eggs?

• Trade-off: The more offspring produced, the fewer resources available for each individual.

• Lack’s prediction: Selection should favor a clutch size that maximizes the number of surviving offspring.

• Clutch size should be a reproductive strategy.

Tradeoff:Probability of individual survival< with > clutch size

Prediction: Number of survivingoffspring = clutch sizex probability of individual survival

Optimal clutchsize = 5

Assumptions: 1. eggs are all thesame size2. current reproductive effort does notaffect subsequentperformance

Starting hypotheses

A test of the prediction: 1960-1982

Number ofClutchesN = 4489Mean clutch size = 8.5

Numbersurvivingas a functionof clutch size

Parental lifetime fitness can decrease from care necessitated by large broods.

Future effects of clutch size on daughters’ performance

Collared Flycatchers

Effect of age at first reproduction on size of subsequent clutches

• e.g. Collared Flycatchers

Begin atdifferent ages

Begin with extra eggs

How large should offspring be?

• Trade-off between number and size of offspring.• Produce many small OR few large? • The determining factor can be based on the size of

the individuals produced; a size with adaptive value.

A fitness enigma

Aspidoscelis tesselata (2n)SVL = 92.1 mmClutch size = 3.9 eggs A.neotesselata (3n)SVL = 84.8 mmClutch size = 2.6 eggs

A.sexlineata (2n)SVL = 67.6 mmkClutch size = 2.8 eggs

Taylor, H. L., B. A. Droll, and J. M. Walker. 2006. Proximate causes of a phylogenetic constraint on clutch size in parthenogenetic Aspidoscelis neotesselata (Squamata: Teiidae) and range expansionopportunities provided by hybridity. Journal of Herpetology 40:294-304.

• Intraspecific divergence in life histories in A. tesselata (a parthenogenetic species)

Fort Sumner

Aspidoscelis tesselataPattern class E Pattern class C

Taylor, H. L., J. M. Walker, J. E. Cordes, and G. J. Manning. 2005.Application of the evolutionary species concept to parthenogeneticentities: comparison of postformational divergence in two clones ofAspidoscelis tesselata and between Aspidoscelis cozumela and Aspidoscelis maslini (Squamata: Teiidae). Journal of Herpetology39:266-277.

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53 59 65 71 77 83 89 95 101SVL (mm)

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