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: Kiwis New Zealand LS: 20 years Huge reproductive investment in a few individuals Chicken-size 6 lbs Proportionately largest eggs of any bird (1 lb) Incubation by male (c. 11 wks.) Loses 20% BW Chicks not fed by adult Self 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 clutch size Prediction: Number of surviving offspring = clutch size x probability of individual survival Optimal clutch size = 5 Assumptions: 1. eggs are all the same size 2. current reproductive effort does not affect subsequent performance Starting hypotheses

A test of the prediction: Number of Clutches N = 4489 Mean clutch size = 8.5 Number surviving as a function of 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 at different 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 mm Clutch size = 3.9 eggs A.neotesselata (3n) SVL = 84.8 mm Clutch size = 2.6 eggs A.sexlineata (2n) SVL = 67.6 mmk Clutch size = 2.8 eggs

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

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

Fort Sumner

Aspidoscelis tesselata Pattern class EPattern class C

Taylor, H. L., J. M. Walker, J. E. Cordes, and G. J. Manning Application of the evolutionary species concept to parthenogenetic entities: comparison of postformational divergence in two clones of Aspidoscelis tesselata and between Aspidoscelis cozumela and Aspidoscelis maslini (Squamata: Teiidae). Journal of Herpetology 39: