Natural Selection

Life History Strategies

K-selected Species

  • Traits and Habitat

    • Stable habitats with increased competition.

    • Offspring need advantages to survive in competitive environments.

  • Reproductive Characteristics

    • Produce fewer, large offspring.

    • Invest significantly in parental care.

  • Additional Traits

    • Often exhibit large body size.

    • Have longer lifespans.

    • Reproduction is delayed, typically associated with Type I survivorship curve.

r-selected Species

  • Traits and Habitat

    • Found in disturbed or unpredictable habitats.

    • Need to produce as many offspring as possible to ensure survival.

  • Reproductive Characteristics

    • Produce many, small offspring.

    • Provide little to no parental care.

  • Additional Traits

    • Often small in size.

    • Early reproduction with shorter lifespans.

    • Often associated with Type II survivorship curve.

Questions on Natural Selection

  • Differences between r-selected and K-selected life history strategies.

  • Conditions that favor each type of strategy.

  • Mechanisms of natural selection.

  • Sources of variation that drive selection.

  • Factors determining phenotype expression in a genotype.

Reproduction Strategies

  • Reproductive Frequency

    • Semelparous: reproduce once or twice at maximum.

    • Iteroparous: reproduce multiple times across their lifespan.

  • Life History Strategies

    • Trade-offs between growth and reproduction.

  • Timing of Reproduction

    • Early vs. late investment in offspring impacts growth, lifespan, and offspring size.

Stable vs. Unstable Environments

K-selected Life History (Stable Environment)

  • Offspring: more likely to survive to adulthood.

  • Resulting Competition

    • Increased competition among adults.

  • Energy Investment

    • Invest energy in fewer, larger offspring for better adult competitiveness.

    • Delay breeding to focus on personal growth leading to larger body size, longer lifespan.

r-selected Life History (Unstable Environment)

  • Offspring: unlikely to survive.

  • Strategy

    • Produce numerous offspring to enhance the chance of survival.

  • Size and Lifespan

    • Smaller offspring, earlier reproduction, shorter lifespan.

Iteroparous vs. Semelparous

  • K-selected species are typically iteroparous.

  • r-selected species are usually semelparous.

  • Mixed reproductive strategies are most common.

  • Example: Oak trees exhibit many K-selected traits but also produce many small offspring during reproductive events.

Natural Selection: Mechanisms and Variation

Heritable Variation

  1. Individuals vary in their characteristics.

  2. Variability must be heritable to influence future generations.

    • DNA structure supports common ancestry as a shared genetic code.

    • Each cell inherits a complete genetic blueprint (genotype).

Sources of Genetic Variation

  • Recombination during sexual reproduction.

  • Genetic diversity due to crossing over and independent assortment in meiosis.

  • Mutation is a vital source of new alleles.

    • Responsible for introducing novel genetic variants.

Differential Fitness

  • Individuals better suited to leave more offspring demonstrate higher fitness in their environment.

  • Relative fitness varies among individuals within a specific context.

  • Resulting in positive selection for traits that enhance reproductive success and survival.