Unit 4 - Mathematics of Natural Selection

​Unit 4
3 Types of Natural Selection

8-LS4-6. Use mathematical representations to support explanations of how natural selection may lead to increases and decreases of specific traits in populations over time.

What is Natural Selection?

Today's Driving Question:

​How can we use mathematical representations (graphs) to show that natural selection may lead to increases or decreases of specific traits in a population over time?

We will learn about 3 different types of natural selection.

Ideally, the normal distribution of individuals with a trait would look like this...

3 Different Types of Natural Selection Include:

1. Directional Selection

2. Stabilizing Selection

3. Disruptive Selection

Knowing this is NORMAL SELECTION...

Draw

What do you think a graph of DIRECTIONAL SELECTION would look like?

DIRECTIONAL SELECTION

Draw

Draw a DIRECTIONAL SELECTION graph again:

Knowing this is NORMAL SELECTION...

Draw

What do you think a graph of STABILIZING SELECTION would look like?

STABILIZING SELECTION

Draw

Draw a STABILIZING SELECTION graph again:

Knowing this is NORMAL SELECTION...

Draw

What do you think a graph of DISRUPTIVE SELECTION would look like?

DISRUPTIVE SELECTION

Draw

Draw a DISRUPTIVE SELECTION graph again:

If I give you an example, can you tell which type of natural selection it is?

Multiple Choice

In a population of mice, individuals with medium brown fur are less likely to be seen by predators than light brown and dark brown mice. What type of selection is this, and what would the distribution curve for the mouse population look like?

• disruptive; two peaks at either end of the graph with a dip in the middle

• disruptive; a tall, narrow peak in the center of the graph

• stabilizing; two peaks at either end of the graph with a dip in the middle

• stabilizing; a tall, narrow peak in the center of the graph

Multiple Choice

Paleontologists observed changes in the sizes of black bears as their environment changed. During warmer periods between ice ages, most black bears were small. During ice ages, most black bears were large. Which type of selection is this, and how would a graph of the distribution curve of black bear size change as Earth moved out of an ice age?

• directional; the curve would shift to the left

• directional; the curve would shift to the right

• disruptive; the curve would break into two peaks with a dip in the middle

• stabilizing; the curve would become taller in the middle and shorter on each side

Multiple Choice

Over generations, female reindeer in a Finnish population give birth to larger offspring. Which type of selection is occurring, and how would a distribution curve for this trait change over time?

• stabilizing selection; the curve would become shorter in the middle

• directional selection; the curve would shift to the left

• stabilizing selection; the curve would become taller in the middle

• directional selection; the curve would shift to the right

Multiple Choice

In peppered moth populations, light and dark moths are selected for in different environments. Which type of selection is this, and what would a distribution curve for all peppered moths look like?

• disruptive selection; a tall, narrow peak in the center of the graph

• disruptive selection; two peaks at the extremes of the graph with a dip in the middle

• stabilizing selection; two peaks at the extremes of the graph with a dip in the middle

• stabilizing selection; a tall, narrow peak in the center of the graph

Poll

Reflect on today's lesson.

• I understand the 3 different types of natural selection and can identify each in an example.

  10

• I understand the 3 different types of natural selection, but I have a difficult time identifying each in an example.

  5

• I do not understand the difference between the 3 types of natural selection and cannot identify any of them.

  1