Darwin made two major points in Origin: that living things had descended with modification from common ancestors and that the main mechanism resulting in evolution was the mechanism he had discovered, which he called natural selection (see chapter 2). As described by the historian Ronald Numbers (1998), in the late nineteenth and early twentieth centuries, scientists in the united States largely responded positively to Darwin's ideas. The idea of evolution itself was less controversial than Darwin's mechanism of natural selection to explain it.
The scientific knowledge of the time was insufficient to provide support for a full-fledged theory of natural selection, primarily because of a lack of understanding of heredity. Although the Austrian monk Gregor Mendel had discovered the basic principles of heredity, he labored in obscurity, his insights unknown to other scientists of his time. How organisms passed information from generation to generation was a puzzle. Many theories of the day involved the idea that some activity of the individual animal caused organic change that was subsequently passed to offspring—by mechanisms only guessed at. Darwin himself favored a blending type of inheritance in which particles (which he called gemmules) from all parts of the parents' bodies would flow to the reproductive organs, where they would be blended and passed on to offspring.
But natural selection could not be combined with blending inheritance or various models on which acquired characteristics are inherited because such mechanisms would reduce genetic variation each generation. Natural selection is based on the fact that individuals in a population vary in hereditary characteristics, and that organisms that have characteristics most suitable to a particular environment are the ones that tend to survive and reproduce. Natural selection thus requires that variation be continually renewed each generation; both blending inheritance (if true) and natural selection itself would reduce variation. Adaptation would be unlikely to occur. In Darwin's day, many (though not all) scientists concluded that there were critical problems with natural selection as a mechanism of evolution because there was no consensus among scientists on how new variation could be produced every generation.
It was not until the early twentieth century that it became clear that variability does not reduce each generation and that a mechanism to explain it was postulated. Gregor Mendel's rediscovered (and confirmed) research on pea plants showed that whatever it was that was passed on from generation to generation (later to be called genes, and even later to be recognized as DNA-encoded instructions), it did not blend in the offspring but remained separate, even if it was hidden for one or more generations. Heredity material acts like particles and does not blend each generation. Furthermore, genetic information is shuffled each time a sperm fertilizes an egg. Given the particulate nature of inheritance, the mixing up of genes among sexually reproducing organisms, and the existence of phenomena such as dominance and recessiveness, it was clear that natural selection would have sufficient variation on which to operate.
In the late nineteenth and early twentieth centuries, natural selection nonetheless competed with alternate explanations of evolution (Bowler 1988: 7), including a brief revival in popularity of Jean-Baptiste Lamarck's views of the inheritance of acquired characteristics. Lamarckism pointed to observable change: the activities in which an individual engaged during its life could affect its size, shape, and even other characteristics. If these characteristics could be passed on to its offspring, a mechanism would exist to bring about adaptive change. A rabbit living in a cold climate grew a thicker coat; did it pass on its thicker coat to its offspring? There seemed to be evidence of such things: the blacksmith developed large muscles, and the blacksmith's son also tended to be well muscled—but was this a result of the blacksmith's passing down the big muscles acquired from swinging a hammer at the forge? or was there another explanation, such as the son's going into the family business (and having inherited the potential to develop large muscles under conditions of strenuous exercise)? Without a better knowledge of how heredity operated, evolution by natural selection seemed no more plausible than Lamarckism and other teleological explanations.
In the 1890s, the German biologist August Weismann performed an experiment that was instrumental in convincing most scientists that Lamarckian evolution was untenable. First, he cut the tails off of a number of rats and then bred them with one another. When the rat pups were born, all of them had normal tails; so he cut them off and again bred the offspring with one another. The next generation of rats was also born with normal-length tails. Weismann continued his experiment for twenty generations of rats, and in each and every new generation, there was no inheritance of the acquired trait of cropped tails. The combination of reduced confidence in Lamarckism together with experimental demonstration of Mendelian principles of heredity moved Mendelian genetics to the forefront of heredity studies during the 1930s.
In the 1940s, Darwinian natural selection and Mendelian genetics came together as scientists recognized the powerful support that Mendelian genetics provided to the basic Darwinian model of evolution by natural selection. Called the neo-Darwinian synthesis or neo-Darwinism, it remains a basic approach to understanding the mechanisms of evolution. Neo-Darwinism further has been expanded by the second genetic revolution of the twentieth century, the discovery of the molecular basis of heredity. Since the 1953 discovery by James Watson and Francis Crick of the structure of DNA, the hereditary material of cells, investigation of the molecular basis of life has expanded almost exponentially to become perhaps the most active—and certainly the best funded—area of biological research. Such knowledge has also informed our understanding of the relationships among living things. The big idea of descent with modification—that the more recently two forms have shared a common ancestor, the more similar they will be—is reflected not only in anatomy and behavior but also in proteins.
It is safe to say that by the mid-twentieth century, mainstream science in both Europe and the United States was unanimous in accepting not only the common ancestry of living things but also natural selection as the main—though not the only— force bringing about evolution. The late-twentieth-century advances in biochemistry and molecular biology have further substantiated these conclusions.
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