evolution: Modern Evolutionary Theory

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.

Sections in this article:

Introduction
Modern Evolutionary Theory
Darwinism
Early Theories
Bibliography

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