Darwinism the view that biological species evolve primarily by means of chance variation and natural selection. Although several important scientists prior to Charles Darwin (1809–82) had suggested that species evolve and had provided mechanisms for that evolution, Darwin was the first to set out his mechanism in sufficient detail and provide adequate empirical grounding. Even though Darwin preferred to talk about descent with modification, the term that rapidly came to characterize his theory was evolution. According to Darwin, organisms vary with respect to their characteristics. In a litter of puppies, some will be bigger, some will have longer hair, some will be more resistant to disease, etc. Darwin termed these variations chance, not because he thought that they were in any sense ‘uncaused,’ but to reject any general correlation between the variations that an organism might need and those it gets, as Lamarck had proposed. Instead, successive generations of organisms become adapted to their environments in a more roundabout way. Variations occur in all directions. The organisms that happen to possess the characteristics necessary to survive and reproduce proliferate. Those that do not either die or leave fewer offspring. Before Darwin, an adaptation was any trait that fits an organism to its environment. After Darwin, the term came to be limited to just those useful traits that arose through natural selection. For example, the sutures in the skulls of mammals make parturition easier, but they are not adaptations in an evolutionary sense because they arose in ancestors that did not give birth to live young, as is indicated by these same sutures appearing in the skulls of egg-laying birds. Because organisms are integrated systems, Darwin thought that adaptations had to arise through the accumulation of numerous, small variations. As a result, evolution is gradual. Darwin himself was unsure about how progressive biological evolution is. Organisms certainly become better adapted to their environments through successive generations, but as fast as organisms adapt to their environments, their environments are likely to change. Thus, Darwinian evolution may be goal-directed, but different species pursue different goals, and these goals keep changing.
Because heredity was so important to his theory of evolution, Darwin supplemented it with a theory of heredity – pangenesis. According to this theory, the cells throughout the body of an organism produce numerous tiny gemmules that find their way to the reproductive organs of the organism to be transmitted in reproduction. An offspring receives variable numbers of gemmules from each of its parents for each of its characteristics. For instance, the male parent might contribute 214 gemmules for length of hair to one offspring, 121 to another, etc., while the female parent might contribute 54 gemmules for length of hair to the first offspring and 89 to the second. As a result, characters tend to blend. Darwin even thought that gemmules themselves might merge, but he did not think that the merging of gemmules was an important factor in the blending of characters. Numerous objections were raised to Darwin’s theory in his day, and one of the most telling stemmed from his adopting a blending theory of inheritance. As fast as natural selection biases evolution in a particular direction, blending inheritance neutralizes its effects. Darwin’s opponents argued that each species had its own range of variation. Natural selection might bias the organisms belonging to a species in a particular direction, but as a species approached its limits of variation, additional change would become more difficult. Some special mechanism was needed to leap over the deep, though possibly narrow, chasms that separate species.
Because a belief in biological evolution became widespread within a decade or so after the publication of Darwin’s Origin of Species in 1859, the tendency is to think that it was Darwin’s view of evolution that became popular. Nothing could be further from the truth. Darwin’s contemporaries found his theory too materialistic and haphazard because no supernatural or teleological force influenced evolutionary development. Darwin’s contemporaries were willing to accept evolution, but not the sort advocated by Darwin. Although Darwin viewed the evolution of species on the model of individual development, he did not think that it was directed by some internal force or induced in a Lamarckian fashion by the environment. Most Darwinians adopted just such a position. They also argued that species arise in the space of a single generation so that the boundaries between species remained as discrete as the creationists had maintained. Ideal morphologists even eliminated any genuine temporal dimension to evolution. Instead they viewed the evolution of species in the same atemporal way that mathematicians view the transformation of an ellipse into a circle. The revolution that Darwin instigated was in most respects non-Darwinian. By the turn of the century, Darwinism had gone into a decided eclipse. Darwin himself remained fairly open with respect to the mechanisms of evolution. For example, he was willing to accept a minor role for Lamarckian forms of inheritance, and he acknowledged that on occasion a new species might arise quite rapidly on the model of the Ancon sheep. Several of his followers were less flexible, rejecting all forms of Lamarckian inheritance and insisting that evolutionary change is always gradual. Eventually Darwinism became identified with the views of these neo-Darwinians. Thus, when Mendelian genetics burst on the scene at the turn of the century, opponents of Darwinism interpreted this new particulate theory of inheritance as being incompatible with Darwin’s blending theory. The difference between Darwin’s theory of pangenesis and Mendelian genetics, however, did not concern the existence of hereditary particles. Gemmules were as particulate as genes. The difference lay in numbers. According to early Mendelians, each character is controlled by a single pair of genes. Instead of receiving a variable number of gemmules from each parent for each character, each offspring gets a single gene from each parent, and these genes do not in any sense blend with each other. Blue eyes remain as blue as ever from generation to generation, even when the gene for blue eyes resides opposite the gene for brown eyes. As the nature of heredity was gradually worked out, biologists began to realize that a Darwinian view of evolution could be combined with Mendelian genetics. Initially, the founders of this later stage in the development of neo- Darwinism exhibited considerable variation in their beliefs about the evolutionary process, but as they strove to produce a single, synthetic theory, they tended to become more Darwinian than Darwin had been. Although they acknowledged that other factors, such as the effects of small numbers, might influence evolution, they emphasized that natural selection is the sole directive force in evolution. It alone could explain the complex adaptations exhibited by organisms. New species might arise through the isolation of a few founder organisms, but from a populational perspective, evolution was still gradual. New species do not arise in the space of a single generation by means of ‘hopeful monsters’ or any other developmental means. Nor was evolution in any sense directional or progressive. Certain lineages might become more complex for a while, but at this same time, others would become simpler. Because biological evolution is so opportunistic, the tree of life is highly irregular. But the united front presented by the neo-Darwinians was in part an illusion. Differences of opinion persisted, for instance over how heterogeneous species should be.
No sooner did neo-Darwinism become the dominant view among evolutionary biologists than voices of dissent were raised. Currently, almost every aspect of the neo-Darwinian paradigm is being challenged. No one proposes to reject naturalism, but those who view themselves as opponents of neo-Darwinism urge more important roles for factors treated as only minor by the neo-Darwinians. For example, neo- Darwinians view selection as being extremely sharp-sighted. Any inferior organism, no matter how slightly inferior, is sure to be eliminated. Nearly all variations are deleterious. Currently evolutionists, even those who consider themselves Darwinians, acknowledge that a high percentage of changes at the molecular level may be neutral with respect to survival or reproduction. On current estimates, over 95 percent of an organism’s genes may have no function at all. Disagreement also exists about the level of organization at which selection can operate. Some evolutionary biologists insist that selection occurs primarily at the level of single genes, while others think that it can have effects at higher levels of organization, certainly at the organismic level, possibly at the level of entire species. Some biologists emphasize the effects of developmental constraints on the evolutionary process, while others have discovered unexpected mechanisms such as molecular drive. How much of this conceptual variation will become incorporated into Darwinism remains to be seen. See also MECHANISTIC EXPLANATION , MENDEL , PHILOSOPHY OF BIOLOGY, TELEOL – OGY. D.L.H.