Evolution and Life Histories II

• 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 = 4489

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?

• Can be dictated by environmental context; i.e., the reproductive strategy may include phenotypic plasticity

• e.g. Stator limbatus (a seed beetle)

Seed beetle: Stator limbatus

Blue palo verde seedPoor host: < 1/2 larvae survive

Cat-claw acacia seedGood host: most larvae survive

Optimal offspringsize for parent islarger on poor host

Minimum size atwhich offspringsurvive is smalleron good host

The model: Size decreases with anincreased number ofsibs

Offspringsurvival

Parentalfitness

Prediction: largereggs on poorer host

Stator limbatus

1st egg onon host -then switched

Restricted to one host

• A phylogenetic constraint on clutch size.

• A fitness enigma?

A. tesselata (24.2 mL)

A. neotesselata (17.0 mL)

A. sexlineata (7.1 mL)

Karyotype of Aspidoscelis neotesselata

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

Fort Sumner

Aspidoscelis tesselataPattern class E Pattern class C

53 59 65 71 77 83 89 95 101

0

2

4

6

8

Num

be

r

GravidNongravid

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

0

2

4

6

8

Num

be

r

GravidNongravid

Sumner C

Sumner E

• Assignment: Sunday March 21• Watch “Life”: Cable TV, Discovery Channel • Making the series, 6 P.M.• Reptiles and Amphibians, 7 P.M.