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Henry Bernard Davis Kettlewell was an English entomologist, one who studies insects, who, in the 1950's, graduated from Oxford and ran experiments on idustrial-melanism-affected moths, like the peppered moth. Like any scientist, Kettlewell tested many predictions for a cause: he wanted to find out if natural selection really was the force behind the industrial melanism of peppered moths. To perform experiments, scientists make scientific predictions, or guesses, based on theories, explenations for certain events, to test those theories. In this case, Kettlewell made four predictions based on natural selection to test:
Heavily polluted forests will have mostly dark peppered moths.
Clean forests will have mostly light peppered moths.
Dark moths resting on light trees are more likely than light moths to be eaten by birds. The reverse should be true on dark trees.
Dark moths released into polluted forests would live longer than light moths, but die sooner in clean forests.
There are three different types of scientific investigations: Descriptive investigations are used to find information on an existing thing and its status. Comparative investigations are used to compare two things to reach a new idea or conclusion. Experimental investigations are studies where a scientist attempts to find support or opposition for an explenation for a certain event.
First, Kettlewell worked with other amateur entomologists to map the populations of peppered moths in England. Many dark moths were found in polluted forests, while they were not as common as light moths in clean forests. Comparing his data to other studies, he found that almost no peppered moths were dark until the Industrial revolution. Now, dark moth populations were restricted to polluted, dark forests. This study would be comparative.
Kettlewell then directly observed bird predation by placing light and dark moths on tree trunks and recorded how many times a bird found a moth. He found that birds were twice as likely to eat a dark moth on a light tree than a light moth on the same color tree, and vice versa. He ended up with the conclusion that dark moths do have an advantage in dark forests. This study would be an experimental investigation.
After that, Kettlewell decided to directly observe life span of different color moths. He marked two groups of moths, one dark and one light, and released them into the wild. When he set out traps to capture them two days later, he found that twice as many light moths in a clean forest live to be recaptured than dark moths, and half as many light moths were recaptured in dark forests as dark moths. He had scientifically supported the fact that if the moth's color is the same as the forest's, it has a better chance to survive; All of his predictions were right. This study would be a descriptive investigation.
The scientific observation, or outcome of the experiment, was as expected. However, no information recieved can be proven, it can only be a scientific inference, or supported thought, because not all data on one subject can possibly be collected. If this was possible, anything could be proven.
In 1959, Kettlewell had his results published in Scientific American. His work provided great evidence for natural selection.