Natural Selection

Paraphrase:

Take a close look at your classmates. Does everyone look exactly the same? Though we’re all part of the same species, our differences are pretty obvious. Some of us are tall and some are short. Some of us have darker skin or eyes, while others of us have lighter complexions. Some of us are able to jump really high. Others are super flexible.

Show image of moth and ask how they would describe its basic traits.

This section of the lesson is available as an interactive activity.

The panels and animations convey the basic story of how the population of peppered moths in England changed from mostly light-colored to mostly dark-colored in a relatively brief period of time.

Each panel has one or two prompts for students:

Panel 1: Students slide the indicator left and right to simulate the passage of time and notice the changes to the environment.

Panel 2: Students see the same image as Panel 1, but with a group of moths on the tree. Initially we are displaying an equal number of light and dark color moths to help the students theorize later.

Panel 3: Students advance time to 1825 and answer the prompt. Student are likely to say that the darker color moths seem easier to see.

Panel 4: Students advance time to 1855 and answer the prompt. Student are likely to say that the darker and lighter moths seem equally easy to see.

Panel 5: Students advance time to 1895 and answer the prompt. Student are likely to say that the lighter color moths seem easier to see.

Panel 6: Student examine a model of the actual population change of the peppered moths in England.

Important points to encourage students to see:

Before the 1800s, nearly all of the peppered moths in England were light-colored and very hard for birds to see against light-colored tree bark and lichen. Dark-colored pepper moths weren’t common at all. The dark-colored moths that did exist were easy targets for birds to see and eat.

Throughout the 1800s, industry was on the rise. Steam-powered factories and commercial railroads burned coal to make energy. Burning coal produced dark soot that coated the tree bark and killed the lichen. The light-colored moths were now easier for birds to see against the darkened tree trunks, while the dark-colored moths were harder for birds to see. More dark-colored moths survived and were able to reproduce, passing their genetic traits on to the next generation. Over time, the population of light-colored moths decreased. The population of dark-colored moths increased.

By the end of the 1800s, nearly all of the peppered moths in the industrialized areas of England were dark-colored.

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Note: Many students may not know what the “British Parliament” or “Clean Air Act” mean. Encourage students to work in pairs to try to figure out the meanings. You may want to use the illustration of the London Fog of 1952 to show them what the British House of Parliament looks like. (Some students may have heard of Big Ben, the bell in the building’s famous clock tower.) Ask students what they think might happen in a big, famous building like this in a nation’s capitol.

Students can work in pairs or independently to complete the storyboard in the worksheet.

Intended answers:

Efforts to clean up the polluted air have just begun. The tree trunks are still dark-colored. There are more dark moths than light moths. The birds have an easier time seeing (and eating!) the light moths. More dark moths are able to live and reproduce and pass their traits on to the next generation.

(Most of the moths should be shaded in dark; the moth in the bird’s beak should be left light-colored.)

The air is getting cleaner. There is less dark soot on the tree trunks. Light-colored lichen is beginning to grow back. There is a mix of both light and dark moths on the tree trunks.

(There should be a roughly even number of light and dark moths; one bird should have a light moth and the other a dark moth to show the two colors’ equal advantage in this intermediate environment.)

The air is much cleaner! The tree trunks are now light-colored and the light-colored lichen has grown back. The light moths are well-protected from birds since they’re hard for the birds to see. The occasional dark moth is usually snatched away and eaten by a bird. More light moths have been able to live and reproduce. There are mostly light moths on the tree trunks once again.

(None or very few moths are shaded. The moth in the bird's beak is shaded.)

Debrief by paraphrasing:

Frequently, evolution occurs at a very slow pace. Peppered moths are an example of natural selection working pretty quickly. One reason a change like this one can occur quickly in moths is that they reproduce more frequently than, say, humans or tortoises. The sooner a living thing can reproduce, the faster evolution can happen, although it isn’t the only factor influencing the pace of evolution.

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