unit 7

7.1 Introduction to Natural Selection

  1. What is natural selection? 

    1. Organisms with adaptations suited for an environment survive and reproduce more.

  2. What factors influence it? 

    1. Natural selection is influenced by the environment, genetic variation, adaptations, and fitness

  3. How does it affect populations? It can increase or decrease phenotypes.


  • Natural selection is a major mechanism of evolution

    • Evolution: change in the genetic makeup of a population over time

    • Natural selection: organisms with adaptations survive/reproduce more → passing adaptation to subsequent generation

      • Charles Darwin credited with theory of natural selection

      • Conditions must be met for evolution by natural selection

  • competition for limited resources

    • Not all organisms have the same survival chance

    • Competition: struggle for limited resources → space, food, materials, mates, light, nutrients

      • Different phenotypes determine competition → differential survival

  • Favorable phenotypes improve chances of survival

    • Variation: genetic diff among organisms in a pop.

      • Mutation + sexual reproduction increase variation

    • Adaptation: traits proving advantages

      • Greater chance of survival and reproduction

      • # of individuals with adaptation inc over time

  • Evolutionary fitness measured by reproduction

    • Fitness: ability to survive and produce fertile offspring

    • Reproductive success: production of offspring

    • Heritability: ability to pass on adaptation to generations

      • Selection → more reproductive success traits become more common

    • Reproductive success + Heritability = evolutionary fitness

  • Ecosystem stability = rate + direction of evolution

    • biotic/abiotic env. Remain more or less stable

      • Major disruptions and change quickly or slowly over time

      • Stability = less likely evolution over time

      • More unstable → faster rate of evolution

7.2 Natural Selection

  1. Why is phenotypic variation important in a population?

    1. It increases the probability a population will continue in unstable environments.

  2. How does natural selection act on phenotypic variation?

    1. It can increase or decrease variation by selecting individuals with the most advantageous traits based on the environment.

  3. How do changing environments apply selective pressures to populations?

    1. It can affect which individuals are selected for and against.

  4. What variables increase or decrease the fitness of an organism?

    1. Phenotypes, adaptations, and mutations can increase or decrease fitness.


  • Natural selection acts on phenotypic variations

    • Genetic variation: genotypic and phenotypic differences b/w individuals

      • Increase probability of survival under changing environment

        • One phenotype might be more suited

    • Favorable traits increase survival

  • Env. change and apply selective pressures

    • Selective pressure: biotic or abiotic factors influencing survival

      • Diseases, predation, climate, food availability, changing env.

      • Individual fitness is relative to env. conditions

      • Phenotypes selected for can be selected against with env. Changes

  • Phenotypic variations can inc. or dec. fitness of an organism

7.3 Artificial Selection

  1. How can humans affect genetic diversity within a population?

    1. They can affect through artificial selection, by selecting desirable traits. 

  2. What is convergent evolution?

    1. Convergent evolution is when unrelated species develop similar traits

  3. How does convergent evolution occur?

    1. Environmental changes cause similar evolution over populations due to similar selective pressures.


  • Through artificial selection, humans affect variation in other species

    • Artificial selection: the process by which humans select desirable traits in other species and selectively breed individuals with desired traits

      • Can result in phenotypes not otherwise in nature

      • Lead to more OR less genetic diversity

    • Humans can select any trait/combo as desirable → breed individuals w/in the population to get the desired outcome

      • Depending on which traits are selected/how often genetic diversity can change over time

  • Similar selective pressures result in similar phenotypic adaptations

    • Convergent evolution: similar env. conditions select similar traits in different populations over time

      • distant/unrelated species

      • Analogous structures



7.4 Population Genetics

  1. What factors drive evolution?

    1. Evolution is driven by random occurrences.

  2. How do mutations contribute to natural selection?

    1. Mutations increase genetic variation which provides new phenotypes for natural selection and evolution.

  3. How does genetic drift impact population size?

    1. It occurs in small populations through bottleneck and founder effects.

  4. How does reduction of genetic variation impact populations of the same species?

    1. It can increase differences between populations of the same species and make them more likely to be negatively affected by environmental changes.


  • Evolution is driven by random occurrences

    • Mutation: random change in genome, alteration in DNA

      • Changes to genetic makeup of pop. = genetic variation

      • New phenotypes that contribute to evolution by natural selection

    • Founder effect → random rprocess reducing genetic variation in small pop due to sepertation from larger population

      • Migration, geological events

      • Founder population genetic makeup diff to original

    • migration/gene flow: movement of inviducaual bw pop. = exchange of allele

      • New genes = more genetic variation

      • Too much continued migration = uniform gene pool → less diversity

  • Reduction in genetic variation can increase differences b/w pop of same species

    • Genetic drift: random change in frency of particular elle

      • Non selective, small population → inc death, less repdocution

      • Natural catastrophes

      • Bottle neck events → large diverse to sudden small pop.

  • Random process 

    • Genetic variation is raw material of evolution

    • Fitness is relative to environment

    • Diff phenotypes selected for or against based on env changes

Evolution can’t occur without genetic variation