Part 1: Darwin's Theory

Let's continue with the series on Evolution by Dr. Jay Wile and Marilyn Durnell...

Word count: 1,459
Estimated reading time: 5-10 minutes

During his time on the HMS Beagle, Darwin had a chance to investigate a small chain of islands called the Galapagos. The thirteen islands of the archipelago are the result of volcanic activity, and these islands still exist about 600 miles west of the South American nation of Ecuador (on the equator). Although Darwin was pleased to study a wide variety of plant and animal life on these islands, he concentrated on the finches that lived there. Many science historians credit these birds, now known as “Darwin's finches,” as inspiring Darwin's theory of evolution through natural selection.

You see, there were (and still are) many different species of finches living in the Galapagos. These species have several common characteristics, but there are specific differences between each species that were of great interest to Darwin.

Certhidea olivacea has a small, slender beak, while Geospiza difficilis has a larger, stouter beak, and G. magnirostris has the largest, stoutest beak of the three. Why do these finches have such different beaks? Well, let's look at what they eat. C. olivacea eats tiny insects. G. difficilis eats eggs that it steals from nests, but it does not use its beak to break them. Instead, it grabs onto something with its beak and then kicks the eggs into a rock. Finally, G. magnirostris eats hard seeds that it must crack with its beak. Each finch seems to have just the right beak for its food source: a small, slender beak for the finch that eats soft insects; a larger, sturdier beak for the one that uses it as an anchor; and the largest, sturdiest beak for the one that uses it to crack hard seeds.

Differences like these fascinated Darwin. You see, the scientists of Darwin's day would have looked at each of these species and assumed that God had designed each one individually and gave each one exactly the beak that it needed to eat the food it was supposed to eat. Darwin, however, imagined something else. He said that other than the beak and a few other differences (size and plumage color, for example), these finches were all remarkably similar. Since they were all so similar, he imagined that they all came from a common ancestor long ago. As the feeding needs of the finches changed, however, this common ancestor began to give rise to many different species of finch, each with a unique beak.

How did Darwin propose that this could happen? Well, he said, look at what happens when any species reproduces. When two people have a baby, for example, the baby has many characteristics in common with his parents. The baby's eye color might be the same as his mother's, and his hair color might be the same as his father's. Nevertheless, the baby usually has some characteristics that do not seem to come from either parent. Some tall professional basketball players, for example, have short parents. Thus, although offspring do tend to resemble their parents, they also have a few characteristics that are quite different from the corresponding characteristics in their parents. It is these differences, Darwin thought, which could be responsible for all of the finches in the Galapagos.

Suppose that long ago, there was only one species of finch living on the islands. If food supplies were to grow scarce, the finches that made up the population of this species would compete with one another for the dwindling food supplies, just as Malthus predicted. When this competition began occurring, any finch that had an advantage would be more likely to win the competition than one who didn't. Thus, suppose a finch was born that had a beak which was stronger than the typical finch. Well, that finch might be able to find a new source of food (hard nuts that other finches couldn't break open, for example). With this new source of food, this strong-beaked finch would most likely win the competition for survival. As it reproduced, then, it would most likely pass on this new, strong beak to at least some of its offspring. Over many, many generations, each time one of these finches was born with an even stronger beak, it would be more likely to survive, because it could continue to find more food than the finches with which it was competing. This competition, combined with the natural differences that arise between parent and offspring, could, over generations, produce a finch whose beak was short and stout, like that of G. magnirostris, even if the original species of finch had a small, slender beak, like that of C. olivacea.

During that same time (or perhaps later), another finch might have been born whose beak was short and stout, but not short and stout enough to break open new seeds. However, perhaps it was short and stout enough to act as an anchor while the finch broke open eggs with its feet. This would make it easier for that finch to get to the nutrition inside of eggs, giving it an edge in the competition to survive. As time went on, this finch would survive and would pass on its basic beak shape to its offspring. Each time a finch was born whose beak was more ideally suited for the task of anchoring the bird while it broke open eggs, the finch would have a better edge in the competition for survival, making it more likely to live and pass its new characteristic on to more and more offspring. Thus, as time went on, the original finch species that lived in the Galapagos and had a beak ideal for eating insects would eventually give rise to two new species of finch: one with a beak ideal for grasping onto objects and another with a beak that was ideal for breaking open hard seeds.

This is the mechanism by which Darwin imagined that all species of finch he observed could have originated from a single type of finch long ago. He called this mechanism “natural selection,” because he said that due to the fierce competition which occurred between members of a species, any individual that had a unique characteristic making it more likely to win the competition would be selected by nature to survive. As time went on, these unique characteristics would continue to “pile up” on one another until eventually, a new species was formed.

Darwin, of course, did not stop there. After all, he imagined, if such a mechanism could be responsible for causing finches to develop new beaks, why couldn't that same mechanism allow them to develop longer, stronger wings, longer, sharper talons, and keener eyesight? If that was so, finches could eventually give rise to eagles! These ideas led Darwin to his overall theory of evolution. At one time, Darwin believed, there was a relatively simple (most likely aquatic) life form that existed on earth. Darwin made no speculations about how that life form developed, but others who followed have constructed wild scenarios that try to explain the formation of this organism without the intervention of a supernatural creator. This life form, Darwin assumed, would begin to reproduce and, as is the case today, variations would occur in the reproduction. These variations, guided by the process of natural selection, would eventually “pile up” so as to form new species. These species would, in the same way, give rise to other species. Thus, over eons of time, Darwin believed that this mechanism could explain the existence of all life forms on the planet.

Hopefully you can see how Malthus and Lyell influenced Darwin's thinking. After all, Malthus gave Darwin the idea that individuals within a species compete with one another in order to survive. This led to Darwin's idea of natural selection. Lyell's concept that the present is the key to the past allowed Darwin to speculate that the same variations which we see in reproduction today could, over vast ages of time, be responsible for all of the variations among all of the species that exist on the planet. In other words, Darwin did not dream up this theory on his own. He was influenced by the works of others.

If you remember our discussion of the scientific method, you will recognize that at this point in the story, Darwin's idea was really no more than a hypothesis. Darwin made a bunch of observations and then proceeded to develop an explanation for those observations. Did the concept of evolution through natural selection ever make it past the stage of being a hypothesis? Well, the answer to that is both yes and no. Hopefully you will see what we mean in the next section.

Restrictions on comments state at the end of the introductory post still apply, but comments are ALWAYS welcome!