Unit 7: Natural Selection and Artificial Selection

Natural Selection

• A mechanism of evolution (a way evolution occurs)

• by which organism w/ traits most suited to their environments survive and leave offspring, increasing those traits in the next generation

• Based on 4 key ideas:

  • 1. Variation

    • differences exist b/t individuals in a population due to genetics

    • natural selection only acts upon variation, does not create it

    • no variation? no natural selection

    • new mutations? new variations

      • might be good

      • might be bad

  • 2. Overproduction and Competition

    • usually there are more individuals than resources available

    • leading to competition

    • some individuals are better suited than others

      • so they are more successful

  • 3. Adaptation and Fitness

    • adaptation: physical or behavioral traits that increase fitness

    • fitness: how well an organism can survive and reproduce in its environment

    • better adaptations = greater fitness

    • relative to the environment

    • high fitness in one place does not equal high fitness in another

  • 4. Selection

    • individuals with greater fitness are more likely to survive and pass on their genes

    • over many generations these traits become more common within the species

• for “discrete” traits:

  • ones with only two options (dominant v. recessive)

  • only 1 becomes more common

  • other = less common (may not “die out”)

• for “continuous” traits

  • those on a phenotype spectrum

  • results will vary depending on which portion(s) of that spectrum have an average

• types of selection

  • stabilizing selection

    • favors the “average” phenotype

    • “average” becomes more common over time

    • tends to occur in environments that are relatively stable

  • directional selection

    • favors one “extreme” end of the spectrum

    • curve shifts toward that end

    • usually in response to an environmental change

  • disruptive selection

    • favors both “extreme” phenotypes

    • seen when there is intense competition & individuals w/ different phenotypes can access different resources

    • can potentially lead to the evolution of distinct species

• another type = sexual selection

  • promotes traits that increase an individual’s ability to find and attract mates

    • intrasexual: between individuals of the same sex

    • intersexual: between individuals of the opposite sex

Artificial Selection

• Darwin was partially influenced by artificial selection

  • aka “selective breeding”

  • when humans breed organisms that have useful or desirable traits