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

2. End Show 16-2 Evolution as Genetic Change Slide 2 of 40 Copyright Pearson Prentice Hall 16-2 Evolution as Genetic Change I. Variation & Gene Pools A. A gene pool consists of all the genes, of all possible alleles, that are present in a population of a species. 1. The relative frequency of an allele is the relative proportion of an allele compared to the total number of all alleles. a. It is often expressed as a percentage. B. In genetic terms, evolution is any change in the relative frequency of alleles in a population. 16-2 Evolution as Genetic Change

3. End Show 16-2 Evolution as Genetic Change Slide 3 of 40 Copyright Pearson Prentice Hall 16-2 Evolution as Genetic Change II. Natural Selection & Gene Pools A. Evolution never acts directly on genes. Why? 1. It is entire organisms that either live or die a. If an individual dies without reproducing, it does not contribute its alleles to the population’s gene pool. b. If an individual produces many offspring, its alleles stay in the gene pool and may increase in frequency. 2. Thus, only populations can evolve, not individuals

4. End Show 16-2 Evolution as Genetic Change Slide 4 of 40 Copyright Pearson Prentice Hall Natural Selection on Single-Gene Traits III. Examples of Natural Selection at Work A. A lizard population: 1. They are normally brown but have mutations that produce red and black forms. 2. Suppose red lizards are more visible to predators. a. Less likely to survive & reproduce so allele for red color will decrease in population 3. Black lizards may warm up faster on cold days. This may give them energy to avoid predators. a. More likely to survive & reproduce so allele for black color will increase in population

5. End Show 16-2 Evolution as Genetic Change Slide 5 of 40 Copyright Pearson Prentice Hall Natural Selection on Single-Gene Traits

6. End Show 16-2 Evolution as Genetic Change Slide 6 of 40 Copyright Pearson Prentice Hall Natural Selection on Single-Gene Traits B. Results of Natural Selection over Time 1. Natural selection can affect the distribution of phenotypes in one of three ways: a. Directional Selection When individuals at one end of the curve have higher fitness than individuals in the middle or at the other end. Birds with larger beaks survive better & beak size increases over time

7. End Show 16-2 Evolution as Genetic Change Slide 7 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits b. Stabilizing Selection When individuals near the center of the curve have higher fitness than individuals at either end of the curve. Human babies born of an average mass are more likely to survive than either smaller or larger babies

8. End Show 16-2 Evolution as Genetic Change Slide 8 of 40 Copyright Pearson Prentice Hall Natural Selection on Polygenic Traits c. Disruptive Selection When individuals at both ends of the curve have higher fitness than individuals near the middle. If the pressure of natural selection is strong enough and long enough, the curve will split, creating two distinct phenotypes. If average seeds become scarce, the population might split into two groups: one of small beaks & one of large beaks