MadSci Network: Evolution

Re: Is the Peppered Moth Theory False?

Date: Fri Mar 9 19:31:19 2001
Posted By: Mark Sullivan, Medical Student
Area of science: Evolution
ID: 983934622.Ev

This is something I recently heard about lately and I had to do a little investigating to see what is going on with this classic example of evolution. So, I will assume you know about the idea behind the peppered moths in that the industrial revolution caused a shift in the light color of trees to dark thereby protecting dark colored moths from being eaten by birds. As hypothesized in 1986 by JW Tutt the moth population had changed from predominantly light colored to mostly dark after the industrial revolution. This was supposedly due to soot deposit on trees which killed a light colored lichen species which offered camoflage protection for the white moths. This was hailed as the needed proof for natural selection which states that species that have a selective advantage in their environment will survive to a greater degree. So, the idea is this, dark trees....dark moth will live, white moth will get eaten and the total population will show a larger number of dark moths to white. So this was reported and scientists all cheered. But it wasn't really proof, it just sounded really good.

Now, cut to the 1950's when it was noticed that the white moths were making a comeback in the population. It was hypothesized that the clean air acts caused another shift in tree color that would affect the black and white moth numbers in an opposite manner from above. So a scientist named Kettlewell devised an experiment to test this hypothesis. First, he tacked dead moths to low branches so he could witness birds taking them. He compared how often white and dark moths were eaten and concluded that the hypothesis was true as now birds ate the dark moths more frequently than the white ones. However, there are some flaws in his experiment that prove it to be invalid. First is that peppered moths, white or dark, don't land on the types of places he put them. They like to lie out of sight on the underside of the branches in the shade. Also, these moths are nocturnal and would only be out in the open at night. So basically his experimental deisgn placed the moths in such a fasion so that they were un-naturally available to their predators. You cannot make any worthy claims about natural selection if your experimental design has a direct effect on the outcome. This is called confounding bias.

Experiments should be devised so that there is something called a "null hypothesis" and the "alternative hypothesis". The null hypothesis for this experiment would be something like, "there is no difference in evolutionary fitness of the dark and white peppered moths based on their color." The alternative hypothesis is just the opposite, that there is a difference. So, what you need to do is create an experiment that attempts to reject the null hypothesis, and accept the alternative. So, with Kettlewell's experiment, his results showed that black moths were eaten more often that white, so there must be a difference. Therefore, he rejected the "null hypothesis" and accepted the "alternative hypothesis" However, he didn't realize his results were biased due to his experimental design. So he rejected the null hypothesis when he shouldn't have, and nobody knew until more information about the habits and environment about the moth was understood. It is hard to make sure your experimental design doesn't have any bias towards a certain result, and that is why science uses peer review to check the work of others so that mistakes like this aren't made. What was supposed to be a lesson in evolution has become a lesson in study design.

What does this mean for the theory of natural selection? Not much one way or the other. What we found out is that Kettlewell failed to reject the null hypothesis, which means that his experiment could not show a real difference between the moths, it doesn't mean that a difference doesn't exist. The original hypothesis regarding the dark moths and the industrial revolution was never proved, only a thought. Better experiments would help determine this, but the problem is coming up with a way to test this without directly affecting the outcome. Maybe you can think of a way. Good luck to you.

Mark Sullivan

Current Queue | Current Queue for Evolution | Evolution archives

Try the links in the MadSci Library for more information on Evolution.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-2001. All rights reserved.