1. End Show Slide 1 of 40 Copyright Pearson Prentice Hall 16-2 Evolution as Genetic Change

2. End Show 16-2 Evolution as Genetic Change Slide 2 of 40 Recall that polygenic traits are controlled by two or more genes. One polygenic trait can have many possible genotypes and phenotypes. Height in humans is a polygenic trait. Copyright Pearson Prentice Hall

3. End Show 16-2 Evolution as Genetic Change Slide 3 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits How does natural selection affect the phenotypes of polygenic traits?

4. End Show 16-2 Evolution as Genetic Change Slide 4 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Natural selection can affect the distributions of phenotypes in any of three ways: 1. directional selection 2. stabilizing selection 3. disruptive selection

5. End Show 16-2 Evolution as Genetic Change Slide 5 of 40 Turn your book to pg. 398 and look at Figure 16-6 Copyright Pearson Prentice Hall

6. End Show 16-2 Evolution as Genetic Change Slide 6 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end, directional selection takes place.

7. End Show 16-2 Evolution as Genetic Change Slide 7 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve, stabilizing selection takes place.

8. End Show 16-2 Evolution as Genetic Change Slide 8 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits Disruptive Selection When individuals at the upper and lower ends of the curve have higher fitness than individuals near the middle, disruptive selection takes place.

9. End Show 16-2 Evolution as Genetic Change Slide 9 of 40 Some biologists ask: Are there any conditions under which evolution will not occur? The answer to these questions is provided by the Hardy-Weinberg Principle. Copyright Pearson Prentice Hall

10. End Show 16-2 Evolution as Genetic Change Slide 10 of 40 Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium The Hardy-Weinberg principle states that allele frequencies in a population will remain constant unless one or more factors cause those frequencies to change. When allele frequencies remain constant it is called genetic equilibrium. If the allele frequencies do not change, the population will not evolve.

11. End Show 16-2 Evolution as Genetic Change Slide 11 of 40 Copyright Pearson Prentice Hall Evolution Versus Genetic Equilibrium Five conditions are required to maintain genetic equilibrium from generation to generation: 1. there must be random mating 2. the population must be very large 3. there can be no movement into or out of the population 4. there can be no mutations 5. there can be no natural selection