Chapter 11: The

Evolution of Populations

Warm-Up: PG. 339The graph at the right shows sexual selection patterns in guppies. Three experiments were run to determine if female guppies prefer males with specific tail sizes. In each experiment, female guppies were given the choice of two males to mate with, each having a different tail size: large vs. small, large vs. medium, and medium vs. small.

Warm-Up Questions 1. Analyze what tail sizes were compared in

each experiment. 2. Analyze the relationship between tail size in

male guppies and female preference for mates.

3. Infer why the difference in preference is larger in experiment 1 than in experiment 2.

Objectives: 1. Students will be able to describe how natural

selection acts on the distribution of traits in a population.

2. Students will be able to explain the three way natural selection can change the distribution in a population

3. Students will be able to identify the five factors that influence evolution.

Review of Concepts: In order to have different phenotypes a population must have ____________________.A gene pool is the ____________________________of all the individuals in a population.The two types of sexual selection are ____________________ and _____________________.

Review of Concepts Allele frequency is a measure of how common a certain allele is in a population. It can be expressed as the ratio of one allele to the total number of alleles for that gene in a gene pool. In a population an allele is present that codes for the presence of freckles. Freckles are a dominant trait represented by F. Calculate the allele frequency for F if 5 people are homozygous dominant, 4 people are heterozygous, and 7 people are homozygous recessive. What is the allele frequency for f?

Review of Concepts:Changes in allele frequencies due to chance are known as ______________________.The three sources of genetic variation are:1.2.3.

Key Concept Populations, not individuals, evolve.

Stop and Think…How would you describe a person’s appearance?

Natural Selection acts on distributions of traits

When a frequency is highest towards the mean value and decreases towards each extreme, it is called a normal distribution.

A normal distribution will produce a bell shaped curve when graphed. In a normal distribution, all of the possible phenotypes have an equal chance of survival.

Natural Selection acts on distributions of traits

The most common phenotypes are generally found in the center of the distribution.

Less common phenotypes are found near the outer extremes.

Natural selection can change the distribution of a trait in one of three ways

Microevolution: the observable change in allele frequencies of a population over time.

One process that can lead to microevolution is natural selection What is natural selection?

Natural selection can change the distribution of a trait in one of three ways

1. Directional Selection • This is a type of selection that favors phenotypes at

one extreme of a traits range. • Causes a shift in the phenotypic distribution of a

population. • A phenotype that once was rare, becomes the most

present. • The mean will shift in the direction of the more

favorable phenotype.

Natural selection can change the distribution of a trait in one of three ways

1. Directional Selection

Natural selection can change the distribution of a trait in one of three ways

2. Stabilizing Selection• The intermediate phenotype is favored and

becomes more common in a population. • Increases the number of individuals with

intermediate phenotypes. • The genetic diversity of a population declines with

stabilizing selection.

Natural selection can change the distribution of a trait in one of three ways

2. Stabilizing Selection

Natural selection can change the distribution of a trait in one of three ways

3. Disruptive Selection• Disruptive selection occurs when both of the

extreme phenotypes are favored. • Individuals with the intermediate phenotype are

selected against by nature.

Natural selection can change the distribution of a trait in one of three ways

3. Disruptive Selection

Posted Note Activity Each student will be given a sticky note. On this note, the student will right down his/her approximate height. The data collected will be graphed and we will be able to determine the distribution that is present within the class.

There are five factors that can lead to evolution

Populations evolve in response to their environments. There are five different factors that can lead to evolution

1. Genetic Drift 2. Gene Flow 3. Mutation

4. Sexual Selection 5. Natural Selection

There are five factors that can lead to evolution1. Genetic Drift

There are five factors that can lead to evolution2. Gene Flow

There are five factors that can lead to evolution3. Mutation

There are five factors that can lead to evolution4. Sexual Selection

There are five factors that can lead to evolution5. Natural Selection

Evolution is continuous In nature populations evolve:

Expected in all populations most of the time Respond to changing environments

Reflection1. Describe how natural selection acts on the

distribution of traits in a population.2. Explain three ways that natural selection can

change the distribution of a trait in a population.