Darwin's Revolution in Biological Thought Darwin brought about the greatest revolution in biological thought by proposing not only a theory of branching evolution but also a mechanism of natural selection to explainhowit occurred.Much of Darwin's evidence was gathered during his voyage around the world aboard HMS Beagle. Darwin's stop in the Galapagos Islands and his study of tortoises and finchlike birds on these islands are usually credited with convincing him that evolution was a branching process and that adaptation to local environments was an essential part of the evolutionary process. Adaptation, he later concluded, came about through natural selection, a process that killed the maladapted variations and allowed only the well-adapted ones to survive and pass on their hereditary traits. After returning to England from his voyage, Darwin raised pigeons, consulted with various animal breeders about changes in domestic breeds, and investigated other phenomena that later enabled him to demonstrate natural selection and its power to produce evolutionary change. Darwin's greatest contribution was that he proposed a suitable mechanism by which permanent organic change could take place. All living species, he said, were quite variable, and much of this variation was heritable. Also, most organisms produce far more eggs, sperm, seeds, or offspring than can possibly survive, and the vast majority of them die. In this process, some variations face certain death while others survive in greater or lesser proportion. Darwin called the result of this process "natural selection," the capacity of some hereditary variations (now called genotypes) to leave more viable offspring than others, with many leaving none at all. Darwin used this theory of natural selection to explain the form of branching evolution that has become generally accepted among scientists. Darwin delayed the publication of his book for seventeen years after he wrote his first manuscript version. He might have waited even longer, except that his hand was forced. From the East Indies, another British scientist, Alfred Russel Wallace, had written a description of the very same theory and submitted it to Darwin for his comments. Darwin showed Wallace's letter to Lyell, who urged that both Darwin's andWallace's contributions be published, along with documented evidence showing that both had arrived at the same ideas independently. Darwin's great book, On the Origin of Species by Means of Natural Selection, was published in 1859, and it quickly wonmost of the scientific community to a support of the concept of branching evolution. In his later years, Darwin also published The Descent of Man and Selection in Relation to Sex (1871), in which he outlined his theory of sexual selection. According to this theory, the agent that determines the composition of the next generationmayoften be the opposite sex. An organism may be well adapted to live, but unless it can mate and leave offspring, it will not contribute to the next or to future generations.
Innate Versus Learned Behavior That imprinting behavior occurs is not debatable, but controversy has arisen pitting Lorenz and his followers, who believe that the process is wholly innate, against a host of later investigators who conclude that imprinting is actually a form of learning. Those who say that imprinting is innate base their opinion on observations that the drive to imprint is strikingly different from other instinctive behavior patterns, whose releasing schemata are not innately determined but are acquired like conditioned reflexes. Lorenz states that imprinting is different from learning in that imprinting can occur only during a very definite, and perhaps short, period in the animal's life and, therefore, is dependent on a specific physiological developmental condition in the young bird. He further asserts that imprinted recognition, even after the critical period, is the same as innate behavior because the recognition response cannot be forgotten (as opposed to learned processes, which can be forgotten). Others, however, believe that imprinting involves a behavior that is absolutely necessary for the survival of the animal under normal conditions. In the case of both social and food imprinting, this is clear. The desire to follow the parent or to want to eat a particular food (as well as to learn what objects are the targets) motivates these behaviors. The urge to learn these objects is so strong that the associated processes which fulfill this learning are contradictory to those associations usually involved in this rotetype learning. If imprinting is a learned behavior, then can it be considered conditioning? Conditioning involves the building of associations between stimuli and responses. A wide variety of stimuli may initiate the imprinting process by innate unconditioned approach responses. The particular stimulus continues to elicit filial responses, but eventually any new stimuli begin to be ignored and later even feared. There is no selective pairing of stimuli and responses, as in conditioning. In imprinting, the primary bond between stimuli and responses continues to be strengthened and becomes exclusive.
