|new support for an icon of evolution|
There is no shortage of evidence for evolution. Mathematical models all predict it. The fossil record presents "snapshot" like views of the changing population of animals and plants on the planet, with species growing like branches from common ancestors. The analysis of DNA gives us another way to look for common ancestry, and the same "tree" of speciation emerges when looking at it this way as well. Anatomy contains hints of evolution (in the human appendix, the pelvis of some snakes, and the "finger bones" in a whale's fin). And, finally, examples of evolution of antibiotic resistance of infectious agents in the real world or more complex creatures in laboratory settings are common place.
However, there was one key example of evolution in action that used to be mentioned in all of the textbooks but gets much less attention now: the melanic moth of Manchester! (Okay, I've never heard anyone else call it that before, but I thought it had a nice ring to it.)
The story is this: All samples of peppered moths collected in England before the mid 19th century were cream colored. In 1848, a darker "melanic" form was caught and pinned by a moth collector in Manchester. Then, by the turn of the 20th century, nearly all of the peppered moths were of the darker variety, with almost none of the lighter ones around. At the same time, due to the building of factories in the area, the trees which had previously been light colored had turned dark with soot. It was James Tutt in 1896 who suggested the idea that the change in the population could be the result of the fact that their light color -- which had made good camouflage on the lighter trees -- stood out on the newly darkened trees and increased the likelihood that the moths would get eaten by birds.
Actually, this makes a lot of sense as an idea. It is easy to imagine a light colored moth and a (nearly invisible) dark colored moth each sitting on a soot blackened tree in Manchester, England and a bird swooping down to grab the one it can see. This is clearly a disadvantage for the light moths and if this was what happened it would not be long before only dark ones were left.
So, there was a change in the population (light colored moths turned dark over the course of 50 years) and an explanation for how this could have happened through natural selection...but is it actually what DID happen? Even though it made sense, there was some doubt. For example, some people were skeptical about the idea that birds would eat these moths, let alone that their color mattered. If this was some other realm of human activity (like religion or philosophy) there might be nothing left to do other than debate about it endlessly. But, in science, claims like this are tested.
An experiment done by Bernard Kettlewell in the 1950s sought to prove once and for all that the melanic moth of Manchester was indeed selected through predation so as to match the new darker color of the trees. Kettlewell placed light and dark moths on trees in Birmingham (where the trees were soot blackened) and Dorset (where they were not) and then recaptured them with traps. He found (as would be predicted by the evolutionary model) that he captured twice as many dark moths as light moths in Birmingham and twice as many light moths as dark moths in Dorset. Moreover, he observed and filmed birds eating the moths off the trees (more frequently when the colors did not match).
This certainly provides some evidence to support Tutt's suggestion. It shows that birds do eat the moths and that they are more likely to eat the moths which don't match the color of the trees. Actually, it seems like a lot of evidence to me! Combining this with the fact that we know the trees changed color due to the factories, it seems quite reasonable to conclude that once a mutant moth with more melanin was born in Birmingham, natural selection would eventually cause its descendants to replace those of its lighter cousins. And so, lots of biology textbooks listed this as a good example of "evolution in action".
However, in science (unlike in my own field of mathematics), it is only possible to find evidence and not to give a complete proof. So, it is not surprising that Kettlewell's experiment falls somewhat short of "iron clad proof". In the 1990s, Michael Majerus pointed out some ways in which his study falls short of perfection. In particular, he pointed out that Kettlewell placed the moths on tree trunks (which is not the location they pick on their own) and that he placed them out in much higher numbers than one would find naturally.
One thing I like about science is that it has skepticism built right into it. That's a good thing, and I think Majerus had a good point. His question is: "Would Kettlewell's experiment still have worked if the moths were placed in more natural locations and in smaller numbers?"
Of course, one way to respond to such a question is to do the experiment and find out. As it turns out, Majerus himself did exactly that...but since small numbers of moths were used it took a long time.
Unfortunately, in the intervening time, the Discovery Institute and other Creationists leapt on this as an opportunity to remove the moth example from textbooks. This seems like an extreme reaction. If someone had carried out the experiment proposed by Majerus and found that the answer was "no", then one could argue that the evidence did not support it. But, before now, the only experiment done was Kettlewell's and -- even if it was not perfect -- it did provide evidence for the idea that the change in color was the result of predation. Of course, the Creationists presented it as being a more serious problem than just this. Some scientists and textbook authors, being cautious and skeptical as well, stopped using the moth example as a good one for teaching about natural selection.
After seven years, however, Majerus has released preliminary results of his revised experiment in which he did use smaller numbers of moths and placed them in natural locations. As it turns out, the conclusions are the same as in the original Kettlewell experiment: birds do eat the moths and they are more likely to eat moths that do not match the tree color.
Since the adoption of clean air acts in the 1950's, trees are becoming lighter again...and so are the moths. This remains an easy to understand example of evolution in action, and one which is well supported by experimental evidence. Of course, it is possible to point out that all of this evidence still falls short of 100% proof. However, if one asks why the moths are getting lighter and looks to the evidence to provide an answer, all of that evidence points in the same direction: it is an example of natural selection with birds catching the darker moths on the light trees.