Unit 7 AP BIO: Natural Selection

KEY IDEAS: the 3 types of selection, hardy-weinberg, high genetic variation makes species more resistant and increases fitness, pre and post zygotic barriers 


  • If chi squared is higher than p value, reject 

  • Directional selection 

  • Stabilizing selection (heterozygote advantage) 

  • Disruptive selection 

  • Convergent evolution is the evolution of similar, or analogous, features in distantly related species (does not provide ancestral data)

    • Analogous structures have functional similarity but structurally differ 

    • Homologous Structures have the same structure but different function, due to common ancestry

  • Microevolution is a change in allele frequencies in a population over generations

  • Bacteria = horizontal gene exchange, including transformation, transduction, and conjugation contribute to genetic variation

  • Sexually reproducing species = crossing over, independent assortment, random fertilization

  • Hardy-Weinberg Equilibrium: p is dominant and q is recessive 

    • Large population

    • No mutation

    • No migration

    • No natural selection

    • No sexual selection (must have random mating)

  • The biological species concept: a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations

  • Allopatric speciation = speciation occurs with geographic isolation

  • Sympatric speciation = speciation occurs without geographic isolation, often due to sexual selection

  • punctuated equilibrium: sudden change

  • Adaptive radiation = new habitat with little competition, many niches available, resulting in many new, related species

  • high genetic variation are more likely to persist through environmental disturbances (more likely to possess adaptive advantages, mutations) 


  • Prezygotic Barriers 

    • Temporal - timing 

    • Habitat 

    • Behavioral - different mating dances, etc. 

    • Mechanical - physical structure

    • Gametic - can’t fertilize 

  • Postzygotic Barriers 

    • Reduced hybrid viability - die before maturing 

    • Reduced hybrid fertility - survive but sterile 

    • Hybrid breakdown - first generation is fine but future generations have reduced fitness - either they die or can not reproduce