Peppered Moths

Kettlewell wanted to see if wild birds would preferentially eat live peppered moths depending on the amount of pollution in the area. He knew that it was very hard to observe wild peppered moths in the countryside. He also needed to work with a few hundred moths so his results would be significant.

He decided to catch moths with a light trap. He put a paint mark on the underside of each moth's wing. Moths land on trees and lie flat all day so birds wouldn't be able to see the paint mark. He counted the number of light and dark moths that he had marked. At dawn he released the moths in a polluted forest. The moths flew up into the trees and waited for the next night.

For the next few nights Kettlewell set up his light trap and recaptured the moths. He recorded the number of marked moths that he recaptured.

Polluted Forest
	white moths	dark moths
number released	201	601
number recaptured	34	205
percent recaptured	16%	34%

As scientists we need to analyze this data. In this polluted forest, which did Kettlewell recapture more, white or dark moths?

Was it a fair trial? Did Kettlewell release the same number of dark and white moths?

How can the percent recaptured be used to overcome the unfairness of the experiment?

Kettlewell wanted to show that in a polluted forest more white moths would be eaten. Are there other explanations that could explain why more dark moths were recaptured?

The next year he repeated the experiment but this time he released the moths in a forest that wasn't polluted.

Unpolluted Forest
	white moths	dark moths
number released	496	473
number recaptured	62	30
percent recaptured	12%	6%

Was this a fair trial? Did Kettlewell start with the same number of dark and white moths?

Which kind of moth was more likely to be recaptured in the non-polluted forest?

In the polluted forest, how much more likely is it that the dark moths will survive and be recaptured?

In the non-polluted forest, how much more likely is it that the white moths will survive and be recaptured?

When you compare the two experiments do you see evidence that one kind of moth is better able to avoid recapture because they are stronger, smarter, weaker, etc?

Kettlewell acknowledged that some of the moths might have flown out of the area and weren't recaptured. He also realized that some of the moths may have died of natural causes. Peppered moths only live as moths for a few days. Some of the released moths may have been 4 or 5 days old and died that day of old age.

Kettlewell assumed that these other reasons for not recapturing some of the moths would have affected both dark and white moths the same. The same percent (5%, 25% or 50%) of both dark and white moths would be lost. Kettlewell didn't know the percent lost but if he used the percent recaptured that would make the experiment fair.

Kettlewell analyzed the percent recaptured and concluded that twice as many dark moths were recaptured in a polluted forest so twice as many white moths were eaten by birds. In an unpolluted forest he found the opposite results. Twice as many white moths were recaptured. Kettlewell concluded that in an unpolluted forest the birds ate twice as may dark moths as white moths.

As scientists you need to analyze both Kettlewell's experiment and his conclusions. You can check the accuracy of his conclusion by building a spreadsheet. Set up the first 5 rows of your spreadsheet by entering the text as seen in the image below.

In cells A6 through A15 enter 0, 0.1, 0.2, 0.3... 0.9.

In cell B6 you will calculate the rate at which the moths were eaten if none were lost. B3/B2 gives you the rate that the moths were recaptured. If none of the moths were lost every moth that wasn't recaptured was eaten. The number of moths released minus the number recaptured equals the number eaten. In rates that is 1-B3/B2. Put this in cell B6. Remember to start the equation with "=" so the spreadsheet will know it is a formula not text.

In cell B7 you will calculate the rate at which the moths were eaten if 10% were lost. Again it will be 1 minus the number of moths released divided by the number eaten. Start with =1-B$3/. The $ in front of the 3 tells Excel not to change the 3 if the cell is copied. You will want Excel to do an absolute reference to B2 and B3 when you copy the cell for all of the other emigration rates.

B$3 is divided by the number of moths eaten. The number eaten is the number released minus the number that were lost. The number lost is the emigration rate times the number released or B$2-A7 * B$2. Notice that A7 doesn't have a $. That is because as you copy down, you want to also use the rate as you go down.

Copy cell B7 down to cell B15. This will give you the predation rates for the white moths.

Repeat these procedures for the dark moths using the appropriate relative and absolute references.

Column E calculates the ratio of white predation rate divided by dark predation rate. For cell E6 that is =B6/D6. Again these are relative references. Copy this down for all of the emigration rates.

Column F is the additional predation. Think about the ratio of white predation to dark predation. For no emigration the ratio or dark to dark is 1 and the ratio of white to dark is 1.26. The additional predation on the white is the difference between 1 and 1.26 or 1.26-1. In cell F6 enter =E6-1. again relative reference. Copy this down for all emigration rates. Set the format to percent.

Repeat these steps to set up the calculations for the Non-Polluted forest. Depending on where you set up the cells you will have to be sure to reference the correct cells.

What does it mean when the predation rate is negative? Could the emigration rate be 90%?

Describe how much additional predation was there on white moths in the polluted forest?

Describe how much additional predation was there on dark moths in the non-polluted forest?

Kettlewell said that there was twice as much predation on the white moths in a polluted forest and twice as much predation on dark moths in a non-polluted forest. Was he: 100% right, 100% wrong, part right and part wrong?

If he was part right and part wrong, explain where you agree and disagree with his conclusion.

Kettlewell's overall position was that natural selection by bird predation on the white colored moths caused the change in moth ratios from 98% white to 98% dark. Kettlewell based this on his experiments and his conclusion that the wrong colored moths were eaten at twice the rate of the camouflaged moths. If Kettelwell was wrong and it wasn't twice as much predation, should you conclude that his overall position was wrong?

4. Staged
3. Mutation
2. Background
1. Introduction

6. Modeling