Evolution - concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution. Inorganic evolution, on the other hand, is concerned with the development of the physical universe from unorganized matter. Organic evolution, as opposed to belief in the special creation of each individual species as an immutable form, conceives of life as having had its beginnings in a simple primordial protoplasmic mass (probably originating in the sea) from which, through the long eras of time, arose all subsequent living forms.

History of Evolutionary Theory

Early Theories

Evolutionary concepts appeared in some early Greek writings, e.g., in the works of Thales, Empedocles, Anaximander, and Aristotle. Under the restraining influence of the Church, no evolutionary theories developed during some 15 centuries of the Christian era to challenge the belief in special creation and the literal interpretation of the first part of Genesis; however, much data was accumulated that was to be utilized by later theorists. With the growth of scientific observation and experimentation, there began to appear from about the middle of the 16th cent. glimpses of the theory of evolution that emerged in the mid 19th cent. The invention of the microscope, making possible the study of reproductive cells and the growth of the science of embryology, was a factor in overthrowing hampering theories founded in false ideas of the reproductive process; studies in classification (taxonomy or systematics) and anatomy, based on dissection, were also influential.

Linnaeus, in his later years, showed an inclination toward belief in the mutability of species as a result of his observations of the many variations among species. Buffon, on the basis of his work in comparative anatomy, suggested the influence of use and disuse in molding the organs of vertebrate animals. Lamarck was the first to present a clearly stated evolutionary theory, but because it included the inheritance of acquired characteristics as the operative force of evolution, his whole theory was ridiculed and discredited for many years.

Darwinism

Although special creation of each species was the prevalent belief even among scientists in the first half of the 19th cent., the evidence in favor of evolution had by that time been uncovered. It remained for someone to assemble and interpret the evidence and to formulate a scientifically credible theory. This was accomplished simultaneously by A. R. Wallace and Charles Robert Darwin, who set forth the concepts that came to be known as Darwinism. In 1859 appeared the first edition of Darwin's Origin of Species. The influence of this evolutionary theory upon scientific thought and experimentation cannot be overestimated. In the years following the promulgation of Darwin's theory of evolution, many accepted and many denied its validity.

The theory found an opposing force in some religious creeds that declared it incompatible with their basic tenets. For a time evolution, sometimes falsely interpreted as meaning human descent from monkeys rather than descent from an ancient and extinct ancestor, became a target for attack by both church and educational authorities. Feeling ran high even as late as the time of the Scopes trial. Nevertheless, the theory of evolution became firmly entrenched as a scientific principle, and in most creeds it has been reconciled with religious teachings. Some Christian fundamentalists, however, do not accept the theory and have striven to have biblical creationism taught in the schools as an alternative theory. (For the evolution of human beings, see human evolution.)

Modern Evolutionary Theory

Evolutionary theory has undergone modification in the light of later scientific developments. As more and more information has accumulated, the facts from a number of fields of investigation have provided corroboration and mutual support. Evidence that evolution has occurred still rests substantially on the same grounds that Darwin emphasized; comparative anatomy, embryology, geographical distribution, and paleontology. But additional recent evidence has come from biochemistry and molecular biology, which reveals fundamental similarities and relations in metabolism and hereditary mechanisms among disparate types of organisms. In general, both at the visible level and at the biochemical, one can detect the kinds of gradations of relatedness among organisms expected from evolution.

The chief weakness of Darwinian evolution lay in gaps in its explanations of the mechanism of evolution and of the origin of species. The Darwinian concept of natural selection is that inheritable variations among the individuals of given types of organisms continually arise in nature and that some variations prove advantageous under prevailing conditions in that they enable the organism to leave relatively more surviving offspring. But how these variations initially arise or are transmitted to offspring, and hence to subsequent generations, was not understood by Darwin. The science of genetics, originating at the beginning of the 20th cent. with the recognition of the importance of the earlier work of Mendel, provided a satisfactory explanation for the origin and transmission of variation. In 1901, De Vries presented his theory that mutation, or suddenly appearing and well-defined inheritable variation (as opposed to the slight, cumulative changes stressed by Darwin), is a force in the origin and evolution of species. Mutation in genes is now accepted by most biologists as a fundamental concept in evolutionary theory. The gene is the carrier of heredity and determines the attributes of the individual; thus changes in the genes can be transmitted to the offspring and produce new or altered attributes in the new individual.

Still prevalent misunderstandings of evolution are the beliefs that an animal or plant changes in order to better adapt to its environment—for example, that it develops an eye for the purpose of seeing—and that actual physical competition among individuals is required. Since mutation is a random process, changes can be either useful, unfavorable, or neutral to the individual's or species' survival. However, a new characteristic that is not detrimental may sometimes better enable the organism to survive or leave offspring in its environment, especially if that environment is changing, or to penetrate a new environment—such as the development of a lunglike structure that enables an aquatic animal to survive on land (see lungfish), where there may be more food and fewer predators.

Bibliography

See D. S. Bendall, Evolution from Molecules to Men (1983); P. Calow, Evolutionary Principles (1983); J. H. Birx, Theories of Evolution (1984); V. Grant, The Evolutionary Process (1985); H. Baltscheffsky et al., ed., Molecular Evolution of Life (1987); A. M. Clark, Understanding Science through Evolution (1987); F. E. Poirier, Understanding Human Evolution (1987); G. Richards, Human Evolution (1987); C. J. Avers, Process and Pattern in Evolution (1989); R. J. Berry, Evolution, Ecology, and Environmental Stress (1989); J. Weiner, The Beak of the Finch: A Story of Evolution in Our Time (1995); R. Fortey, Life: A Natural History of the First Four Billion Years of Life on Earth (1998); A. Jolly, Lucy's Legacy (1999); S. Jones, Darwin's Ghost: "The Origin of Species" Updated (2000); E. Mayr, What Evolution Is (2001).

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