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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