Biology—Semester B (2014-2015)

Sample Lesson - Introduction

Lesson 7.02: Natural Selection

"Survival of the fittest." Surely you've heard the term before. And no, we're not talking about physically fit people fighting each other to be the last one standing, Hunger Games-style. We're talking about a different meaning of "fittest" that doesn't necessarily involve washboard abs.

Survival of the fittest describes a process a little like Dancing with the Stars where the dancers with the lowest scores get the boot from the competition. Only the best dancers survive and remain on the show. Well, usually. Unfortunately, there are no glittery leotards or samba music involved in the biological version.

You don't have to be this chiseled to be considered the "fittest" in "survival of the fittest." Slackers, rejoice!
(Source)

We know evolution happens based on the finches from the last lesson. One species of finch evolved to give rise to different new species. But, how does it happen?

Natural selection describes how evolution occurs. In natural selection, only those organisms with traits most suitable for the environment will survive to reproduce and pass those traits on. The poor unfortunate organisms that don't have these traits die off.

Let's think about it this way: let's say you've decided to go on a health kick and get rid of all of the candy in your house. After scouring the pantry, you collect some loose black jellybeans, individually wrapped lifesavers, mini-Snickers bars, and Starburst. You put all of the candy in a bowl and set it somewhere for all to eat.

After a couple of days, you notice that all of the candy has been taken except for the black jellybeans. Something about those jellybeans prevented them from being eaten by wily human predators. Maybe it is because they weren't individually wrapped like the other candy which means they must be covered in germs…gross. Maybe it's because they're probably the least liked type of jellybean. Who likes black jellybeans anyway? Whatever the reason, the black jellybeans had traits that made them suitable to stay in the candy bowl. The candy that was taken had traits less suitable for the candy bowl and which made them good candidates for predation.

Natural selection works somewhat like the candy bowl model. Some organisms survive because they have traits that make them more likely to pass on their genes. A trait leading them to not be eaten by predators is one example of increased fitness.

In this lesson, you'll learn more about how natural selection works and understand why it's nicknamed "survival of the fittest." We're also guessing you will probably forever link Dancing with the Stars with natural selection. Sorry about that...Just imagine Darwin participating in Dancing with the Stars, grimace and all.

Sample Lesson - Reading

Reading 7.7.02a: Natural Selection

When Darwin studied finches, he identified that beak size was a trait associated with fitness. Let's say that a plant with a large type of seed was prevalent one year because it was the only one that survived a drought. The birds with a large beak size would have increased fitness (because they could eat the large seeds) and would have more offspring. As a result, the next generation would have a higher percentage of birds with large beaks. Large beaks were naturally selected for.

After several generations, the population would have a larger average beak size than before. The prevalence of a specific type of large seed was a selective pressure. It caused a change in the population by giving some individuals a higher fitness and advantage over others.

A large beak size did not occur because of the large seeds; there were already birds with large beaks. The selective pressure merely turned a large beak into the new hottest thing (or rather, made the big-beaked birds better nourished and attractive to hungry females). This means that genetic diversity is the key to natural selection. There must be differences within a population in order for some individuals to have more or less fitness. Otherwise, all birds would have the same advantages and disadvantages.

Remember, that evolutionary fitness just means that an individual is more likely to successfully reproduce and pass on their genes. Traits can cause fitness in many different ways such as: helping evade predators, helping to find food, helping to attract a mate, helping to protect offspring until they can reproduce, etc.

What makes you different from other people? If someone made an announcement tomorrow that made a particular human trait the key to survival, what would you want them to announce?

In this reading, learn about natural selection changing a population of snails. Trust us, it is more interesting than it sounds.

Natural Selection at Work: A Case Study

In the reading, the southern predators caused a selective pressure. When predators are around, being fast is the golden ticket, the new hottest thing, and the key to fitness. Initially, there were likely fast snails in both environments, but after several generations, the southern population has a higher percentage of fast snails because they have survived and passed on their genes at a higher frequency.

Sample Lesson - Reading

Reading 7.7.02b: Different Types of Natural Selection

We hope you're feeling tough, because things are about to get squirmy and slimy. This lesson will teach you about three different types of natural selection. Make sure you grasp all the concepts before the slip away from you.

Natural Selection
Sexual Selection

The different types of natural selection explain how different members of a population can be favored. Sometimes, the "average" traits are favored, sometimes it is one extreme, sometimes it is both. Which individuals are favored will determine how the future populations will look. Where do you think the human race is going? What will the human population look like in a thousand years?

Sample Lesson - Activity

Activity 7.02a: Natural Selection Simulation

This virtual lab will take you through the process of natural selection.

Instructions:

1. Download the program by clicking the blue button that says download. You may have to adjust your security settings in order to download it.
2. When you open the program you will see an interface with a lot of buttons. There will be a black line that moves across the screen and single white rabbit hopping around. The black line represents the passing of time. We recommend pressing the pause button and then advancing the generations with the little "step" button to the right of it.
3. The first thing you must do is click the "Add Friend" button in the lower left of the picture screen. This will add another bunny to mate with bunny #1. Go ahead and see what happens if you never give your bunny a mate.
4. The next things you can do (on the right side) are:
   • add mutations to give variety to your population of bunnies. You can change the fur color, tail length, and teeth length. If you select one of these mutations, you can also select which trait is dominant and which is recessive.
   • add in selective pressures of wolves or food.
   • change the environment location.
   • check the genotype of a particular bunny.

This simulation will show you what happens to a population of bunnies when different selection pressures are added. We want you to play out these different scenarios and then play them out again while answering the questions for each one.

Sample Lesson - Activity