evolution of life histories. life histories concerned with –1. size at reproductive maturity –2....
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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
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.
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.
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.