BIO326 : Population : Life Histories and Evolution : Lesson

Glossary terms that are important in this lesson: Annual, bet hedging, genetic load, genotype, indeterminate growth, industrial melanism, iteroparity, perennial, phenotype, reaction norm, ruderal, semelparity, senescence, trade-off Lady beetle life history stages.

Organisms are faced with environmental challenges and have limited resources. The allocation of these resources to growth and reproduction results in a life history which tends to maximize fitness under prevailing conditions.

I. Life Histories

Introduction Pattern of maturity, parity, fecundity and length of life Reproduction takes resources and involves risks Optimal life history is a set of compromises Genotype: environmental interactions can lead to specialization Reciprocal transplant experiments reveal genetic flexibility Swallowtail butterfly, Papilio canadensis Fence lizard, Sceloporus undulatus Japanese quail vs starlings Tadpoles Humans

2. Variation in Life Histories
   • Birds lay fewer eggs in tropics than at higher latitudes
   • Life histories are molded by natural selection: hypothesis
   • In persistent populations births and deaths balance on average: there are various ways this balance can be achieved
     • Elephant, albatross, giant tortoise, oak: long life, slow development, delayed maturity, high parental investment, low reproductive rate
     • Mice, fruit fly, weeds

3. Allocation of Limited Time and Resources
   • Trade-off expected
     • Most offspring are produced from a clutch of intermediate size
     • Sometimes most productive clutch size is larger than the commonest size
     • Magpie, blue tit, great tit, European kestrel, guppy

4. Evolution of Life Histories
   • Three questions
     • Costs and benefits to age of first reproduction
     • Balancing clutch size and numbers of reproductions
     • Balancing reproduction and survival
   • Annual versus perennial depends on ratio of survival of adults and immatures
   • Reproductive effort varies with fecundity and survival

5. Life Histories in Variable Environments
   • Allocation shifts away from the life history stage with greatest uncertainty: bet hedging
   • Mosquito fish (Gambusia affinis)

6. Semelparity
   • Rare in perennial organisms
   • Occurs when the preparation for reproduction is costly
   • Occurs when payoff for reproduction is highly variable and predictable
   • Agave, bamboo, Lobelia

7. Senescence
   • Rates influenced by natural selection: increasing mortality and decreasing fecundity
   • Rate of senescence is inversely correlated with the adult survival rate

II. Population Genetics and Evolution

1. Introduction
   • Pests evolve resistance to chemical treatment: cyanide and scale insects
     • Human caused environmental change: relatively rare alleles for resistance to cyanide which were already in the population increased survival and spread through the population
   • Evolution by natural selection
     • Variation among individuals in a population
     • Inheritance of that variation has a genetic basis
     • Differences in reproductive success (fitness) are related to that variation: interaction between organisms and environment
   • Natural selection is an ecological process

Fitness and Evolution in Natural Populations Genetic load increases under strong selective pressure Primary effect of selection is to change allele frequencies, not population size Peppered moth (Biston betularia): when its environment was changed, predation altered its allele frequencies (there are two moths in the picture) Variation in Quantitative Traits Heritability of body form is high, but heritability of fecundity and life history characteristics is low Simulated predation in experiments (Tribolium, Japanese quail) shows that populations respond (changes in fecundity and growth) to "natural" selection

4. Conclusions for Ecologists
   • Genetic variation is the basis of adaptation to the environment
   • Rapid environmental change may exceed ability of populations to adapt
   • Populations can respond both ecologically (by current population members) or evolutionarily (genetically)

When you have completed this lesson, go on to Review Questions