The population is the unit of evolution. All the genes of its members make up its gene pool, which is characterized by the frequency of alleles. The Hardy-Weinberg theorem states that, if a population meets certain conditions, its allele frequencies will not change. From the Hardy-Weinberg theorem, the forces of evolution can be inferred. The forces are mutation, gene flow, genetic drift, and natural selection.
- CA.9–12.IE.1.l; CA.9–12.LS.6.g; CA.9–12.LS.7.a, b, c, e, f; CA.9–12.LS.8.a, c
- NSES.9–12.A.2.4; NSES.9–12.C.2.3; NSES.9–12.E.2.1
- AAAS.9–12.1.C.4; AAAS.9–12.5.A.1; AAAS.9–12.5.B.1, 5; AAAS.9–12.5.F.3, 5, 6, 7, 9; AAAS.9–12.10.H.5; AAAS.9–12.11.B.1
- Distinguish between microevolution and macroevolution.
- Define gene pool, and explain how to calculate allele frequencies.
- State the Hardy-Weinberg theorem.
- Identify the four forces of evolution.
allele frequency: how often an allele occurs in a gene pool relative to the other alleles for that gene
directional selection: type of natural selection for a polygenic trait in which one of two extreme phenotypes is selected for, resulting in a shift of the phenotypic distribution toward that extreme
disruptive selection: type of natural selection for a polygenic trait in which phenotypes in the middle of the phenotypic distribution are selected against, resulting in two overlapping phenotypes, one at each end of the distribution
gene flow: change in allele frequencies that occurs when individuals move into or out of a population
gene pool: all the genes of all the members of a population
genetic drift: a random change in allele frequencies that occurs in a small population
Hardy-Weinberg theorem: founding principle of population genetics that proves allele and genotype frequencies do not change in a population that meets the conditions of no mutation, no migration, large population size, random mating, and no natural selection
macroevolution: evolutionary change that occurs over geologic time above the level of the species
microevolution: evolutionary change that occurs over a relatively short period of time within a population or species
population genetics: science focusing on evolution within populations that is the area of overlap between evolutionary theory and Mendelian genetics
sexual dimorphism: differences between the phenotypes of males and females of the same species
stabilizing selection: type of natural selection for a polygenic trait in which phenotypes at both extremes of the phenotypic distribution are selected against, resulting in a narrowing of the range of phenotypic variation
Introducing the Lesson
Review how probability can be used to predict the genotypes of offspring of two parents by applying Mendel’s rules of inheritance. Then say that probability can also be used to predict the genotypes in the next generation of a population. Tell students they will learn how when they read this lesson.
Building Science Skills
The Hardy-Weinberg theorem is often difficult for students to understand. Solving the Hardy-Weinberg practice problems at the URL below may improve student understanding.
Create a gallery walk of the four forces of evolution (mutation, gene flow, genetic drift, and natural selection). Write the name of each force on a separate sheet of paper, and post one sheet on each wall of the classroom. Have groups of students circulate around the room, adding what they know to each sheet of paper and reading the comments of the other groups.
Challenge students who excel in math to solve more advanced Hardy-Weinberg problems. Suitable problems can be found at the URL below.
Students can explore natural selection with a virtual simulation at this URL.
A common misconception is that all evolution occurs by chance. Make sure students understand that only mutation and genetic drift are chance processes. Natural selection, the main driver of adaptive evolutionary change, does not occur by chance. It occurs when some individuals have more surviving offspring than others do because of beneficial traits that help them survive and reproduce in their environment.
Reinforce and Review
Copy and distribute the lesson worksheets in the CK-12 Biology Workbook. Ask students to complete the worksheets alone or in pairs as a review of lesson content.
Have students answer the Review Questions listed at the end of the lesson in the FlexBook®.
Points to Consider
Disruptive selection for a polygenic trait results in two overlapping phenotypes. Theoretically, disruptive selection could lead to two new species forming.
- How might this happen? Can you describe how it could occur?
- How else might one species diverge into two?