Examples of Natural Selection

Here I want to show you some examples of natural selection to make it clearer how it works in nature. We call natural selection a theory, but it is a testable theory and has been tested many times. Darwin, when he proposed it, could only turn to examples of artificial selection, the breeding of domesticated plants and animals, where the selective agent was the breeder, not the environment. Since Darwin’s time, however, natural selection has been demonstrated to occur in many cases. There are well documented examples both in wild populations and in laboratory populations.

The Peppered Moths

This is a famous, or infamous, example of natural selection. It involves a small moth living in England, the English peppered moth. This example is notable not only because it offers definitive proof for natural selection occurring, but because it was the first widely publicized example of natural selection occurring in a wild population. It also provides a good example for illustrating some of the key tenets of natural selection.

Butterfly collectors are numerous among natural historians. For literally hundreds of years, professional and amateur naturalists have collected hundreds of thousands of specimens of butterflies and moths from all over the world. As a result, the natural history museums of the world are stuffed with collections that illustrate various patters of variation in the phenotypes of butterflies and moths.

In the case of the peppered moth, there is an extensive museum collection that portraits a hundred and fifty years or more of the history of this moth in England. If you look to the collection of the species that were made near Manchester you’ll notice an interesting thing. Moths that were collected before the 1850’s were mostly light colored. They had light colored wings. There are a few examples that you find in these older collections of dark forms, but most of the moths were light colored.

By contrast, if you look at specimens that were collected a half-century later, around 1900, about 98% of the moths collected are uniformly dark colored. There are only a few light colored individuals represented. The typical wing color phenotype in the population of this moth found around Manchester shifted dramatically from mostly light individuals to mostly dark individuals over a fifty year period.

It is well established that wing color is a heritable trait in butterflies and moths. Therefore, the historical change in wing color observed in this population may be consistent with the hypothesis that the shift represents and evolutionary change. Interestingly, this transformation occurred during Darwin’s lifetime, but he never knew about it.

If this change in wing color is an evolutionary transformation, we would expect that natural selection must be acting on these individuals because of their wing colors. How might natural selection act in this way?

It was in the mid 1950’s when an English physician named Bernard Kettlewell proposed the following idea. He noticed that the change in moth coloration correlated with the onset of the industrial revolution in England. This was a time when coal burning around industrial centers like Manchester produced enormous amount of soot.

The peppered moth is a night-flying moth, and it usually spends the whole day resting on tree trunks. Normally, in the English countryside, these tree trunks that the moth would settle on are covered with light-colored lichens. What Kettlewell suggested was that the tree trunks around Manchester, because they became covered with soot (and they did), had become considerably darker.

What’s natural selection doing here? Kettlewell argued that visual predators, such as birds, were hunting these moths during the day. Moths were light-colored, he argued, because when they rested on light-colored lichens they couldn’t be seen. When the trees became soot-covered, the light colored individuals stood out and were easily found by predators. On the other hand, those few dark colored individuals that occurred would do much better, because their dark coloration would fit it with the now dark background.

Kettlewell tried to test his hypothesis the following way. He took an equal number of dark colored and light colored moths, and released them into two kinds of woods. First he would release an equal number of light and dark colored moths into woods that were darkened with soot. Then he would ask which of the moths got eaten more. When he did this, he found out that the light-colored moths were the ones that were getting eaten.

If you took the same number of light and dark colored moths and put them instead in a forest that was more distant from Manchester, where the trunks were still light colored, he got the opposite effect. In this case, the dark colored moths would stand out and the birds would eat them more.

The results of Kettlewell’s experiment are consistent with the idea that natural selection, caused by visual predators hunting these animals, is acting on wing color, such as over time, the darker individuals were favored. In this way, dark coloration spread through the population.

The case of the peppered moth is a famous example of natural selection because it was considered to be the first demonstrated example of natural selection occurring in a wild population. I must tell you, however, that this example is a bit infamous nowadays. In recent years it had been suggested that Kettlewell might not had done this experiment as neatly as he could. Specifically, it appears that Kettlewell didn’t just release these moths, he actually attached them to the tree trunks. This is problematic, because the moths weren’t choosing where to land, Kettlewell did. So, a number of people had argued that this was a very poorly-conducted experiment.

Certainly, his experiment isn’t conclusive evidence, but I still think that this is a useful example of natural selection that helps to illustrate some important points about it.

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Rui Monteiro said...

Why did you always forget Sexual Selection? Why it not makes part of your equation?

Fight the Status Quo and its Natural Selection exclusivity! See why here: http://nature-sucks.blogspot.com/

Marcus Sampson said...


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