Darwin And The Theory Of Natural Selection (1967)

[Music] September 1835. Her Majesty ship Beagle arrived at the Galopagus Archipelago, 500 m west of South America. The voyage was in its third year. The principal objective was to survey South American shorelines and islands, but incidentally, the ship carried a young scientist to report on the natural history of visited lands. He was Charles Darwin. Darwin studied land forms. He observed and classified species of plants and animals. These observations were later to become a basis for his theory about the origins of species. A work that not only was to dwarf the original purposes of the voyage, but that was to have a profound effect on nearly every field of human knowledge. But as he studied the Galopagus, young Darwin had little intention of proposing revolutionary theories. Unlike some other scientists of his time, he had not yet questioned a widely held belief that every species of life had sprung forth on Earth at one moment. He was a scientist examining nature. Among the first things Darwin noticed was that the Galopagus were volcanic islands. Never attached to the mainland. They had been thrust up from the bottom of the sea. Yet the islands had life. One kind of plant he noticed was a low-lying bush, another a great odd-looking cactus. Several species of huge tortoises found nowhere else in the world inhabited the islands. And there were two species of lizards, one of these the world's only swimming lizards. Young Darwin studied birds of the islands, particularly numerous finches, which he classified into 13 separate species. But Darwin saw more than separate groups of plants and animals. He observed patterns in the distribution of life. He noticed, for example, that on the islands there were far fewer living things than on the mainland. He noticed some unusual species, yet they all seemed more or less related to those on the mainland. What astonished Darwin was that each island had several unique animal and plant species, absent from other islands in the archipelago. He had never dreamed that islands so close together and so similar would have such different forms of life. Was it possible that all species had not sprung into being at one time? Was it possible that these individuals ancestors had been carried to the islands from the mainland and that in isolation they came to differ from mainland species? And was it possible that a single species could somehow over many generations give rise to a number of different species, basically alike, but somewhat different from one another. These were some of the questions Darwin may have pondered over as he returned to England aboard the Beagle. In England, he noted the existence of many breeds of sheep and cattle. It was well known to breeders that new varieties were being produced. Similarly, he noted the existence of many breeds of dogs. It seemed unlikely to Darwin that each could have been separately created. He made a thorough study of the many breeds of domestic pigeons and he found that breeders agreed all existing breeds had descended from a single ancestral type. Observing many more living species and collecting much more of this sort of evidence, Charles Darwin came to agree with the belief of several other scientists of his time. The many varieties of living things in the world have not been here from the beginning of time. They have descended from a lesser number of common ancestors. Perhaps the remains of some of these ancestors are still on Earth. Perhaps some are represented by fossils constantly being unearthed. And perhaps many species on the Galopagus are descended from a few mainland individuals that happened to be carried across the sea to the islands. But this was only speculation. Evidence was required and Darwin tried to supply it. He placed seeds in salt water. He cooled it to the temperature of the sea. After several weeks, the seeds were planted and in time they grew. So, some plant life might have been carried to the Galopagus by the sea. Darwin held a duck's feet in water where freshwater shellfish were hatching, and he observed shellfish clinging to the bird's body. In this way, some small animals might have been carried to the Galopagus by flying birds. But could a few individuals arrived on isolated islands give rise to a number of different species? Darwin believed they could. And as evidence, he cited the many species of finches he had found. On these islands, nearly devoid of other species of small birds, there were finches that fed close to the ground and finches that fed in tall cactus plants or trees. There were finches that, like all other finches in the world, ate seeds. But remarkably, here there were also finches that ate insects and finches that ate both seeds and insects. Colorings of the many species varied according to habitat. Bills varied according to diet. Yet all the birds were finches. And it seemed fantastic to believe that each type could have been separately created. It seemed more logical to believe that centuries before the only birds that reached these isolated islands were a few finches. Assuming that these finches lived and reproduced, it seemed logical to Darwin that their many descendants would compete for nesting space and food. Perhaps during this struggle for existence, an accident of heredity made it possible for a particular bird to feed in a different place or to eat a different food. For such an individual, competition would be lessened. The individual would mate and reproduce, resulting in more birds able to take advantage of the new conditions. And in time, the descendants of these might come to be a new species. The reason for survival would simply be fitness and Darwin called survival of the fit natural selection. He proposed that natural selection had occurred in the origins of all species and his belief has persisted to the present day. Modern scientists revisiting the Galopagus seldom fail to be impressed by the accuracy of Darwin's observations and the soundness of his conclusions. They explore the islands more intensively than Darwin did, and they described their observations in terms Darwin might have found strange but fitting. As they see it, the material of natural selection is any population of organisms of the same species. Within a population, any inheritable change that occurs is a mutation. The first swimming lizard was probably the result of one or more mutations. A mutation that increases fitness to survive as swimming does is an adaptation. And the phenomenon of several new species arising from one ancestral species is known as adaptive radiation. In the language of biology, a particular combination of food, shelter, and other living conditions is a niche. If a niche exists, chances are good that after centuries of mutation, adaptation, and adaptive radiation, some population will happen to fill it. Thus, the islands have a species of finch that fills the niche warblers occupy on the mainland. These finches eat foods ordinarily chosen by warblers, and they resemble warblers in structure, plumage, and song. Another sort of finch, one that escaped Darwin's notice, fills the insect eating niche woodpeckers occupy on the mainland. Lacking a long bill, this so-called tool using finch uses cactus needles to extract insects from crevices in trees. Still another niche is filled by a species of flightless corrant. Though most corrants fly, this species is adapted for underwater predation. Plant adaptations are equally common. On islands with no tortoises, the opanchia cactus grows as a short plant. But on islands with tortoises, the cactus grows tall and treelike. Perhaps the treelike form was a mutation that by chance kept tortoises from eating the plant. We cannot say with absolute certainty, but certainty often is not the material of science. Instead, scientists of today like Darwin rely upon a huge growing store of evidence gathered through observation and experimentation. The evidence indicates in countless cases that living things on Earth have a common ancestry. It indicates that living species have originated and will continue to originate through natural selection.