Examples of Natural Selection - Part 2

Here I will show you more examples of natural selection. Darwin’s thinking about natural selection and evolution was profoundly influenced by his observations of island organisms. Among them was a remarkable group of birds that he observed in the Galapagos Islands, collectively now known as Darwin’s finches. One of the best studied examples of natural selection occurring in the wild comes from the work of Peter and Rosemary Grant. They’ve been studying one species of Darwin’s finches for over three decades now.

The different species of finches were all very similar in their size, shape and color, but they differed specially in the size and shape of their beaks. This is consistent with the idea that natural selection has adapted the beaks of these different species to be specialized for eating different kinds of foods. There is also considerable variation within species in the size and shape of individuals’ beaks. If natural selection is responsible for the evolution of beak shape in this group of birds, then we should also be able to detect differences in the survivorship and reproductive success of individuals within a species.

It might seem impossible to think that we can detect this kind of differential reproductive success occurring in a natural population of birds, but this is exactly what the work of the Grants have revealed.

The Grants have focused much of their work on one population of a single species of Darwin’s finches, of the so-called medium-ground finch. The population of this finch is ideal for this kinds of long-term study because it occurs in a relatively small island, and the population size never gets above a thousand individuals. This makes it possible to capture, mark and measure almost every bird in the population.

Like many of the Darwin’s finches, the medium-ground finch’s diet consists primarily of seeds, which they crack open with its beaks. The Grants and many of their students have shown that both within this particular species as well across different species, the size of the beak actually corresponds to the size and the hardness of the seeds that they usually eat.

Within the population of the medium-ground finch, there is considerable variation in beak size. Most individuals have sort of average size beaks. Some birds in the population have beaks with depths as large as 13 or 14 millimeters. Other individuals have beaks with depths that are small as only 6 or 7 millimeters.

The evidence pointed to the possibility that natural selection may be occurring on beak shape as an adaptation for feeding. How could the Grants actually test this? This is where a bit of serendipity and bad weather came into play. In 1977, after the Grants had been studying this population for a number of years, there was a severe drought in Galapagos, brought on by an El Niño weather pattern. This drought had a profound effect on the population. Over 80% of the population died, leaving less than 200 birds as survivors.

The reason for this decrease in population size clearly was because the finches didn’t have enough food to eat. Specifically, the drought caused many of the plants that produce the seeds they normally eat to cease flowering. The plants didn’t produce seeds and the finches simply didn’t have enough to eat. Many emaciated dead birds were found.

The Grants’ most important observation, however, was that the individuals who survived the drought differed from those who didn’t survive. Specifically, they differed in the size of their beaks. The individuals who made it had larger beaks than the individuals who didn’t make it. Why should this be?

This was because the types of seed produced also were affected by the drought. One kind of plant proved to be drought-resistant, and thus did flowered and produced some seeds. This plant produced very large and hard seeds. So, the food that was available was larger seeds that only the birds with larger beaks were able to eat efficiently.

Following this drought in 1977, the distribution of beak size in this population of finches shifted dramatically. The average individual after 1977 had a much larger beak than the average individual before the drought. In other words, the selection brought on by this drought had changed the populations mean characteristics, and caused evolution to occur.

The 1977 drought wasn’t actually a unique event. The Galapagos are subject to periodic droughts. The Grants observed that following a drought, the population mean beak size would shift to larger sizes. Following a wet year, however, when a lot of small soft seeds were produces, the population would have its mean beak size actually shift back down. So, selection is pushing the characteristics of this population in a way that is predicted by the particular adaptation of beak size to the type of food. I think that this is one of the most compelling and complete examples of natural selection at work.

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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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