Evolutionary Theory
The concept of evolution first emerged in the early nineteenth century. Up until that time, the post-classical Western world had generally regarded the account of creation provided in the Biblical book of Genesis as literal fact. Thus, it was held that the world had been created relatively recently, that every species was created separately and distinctly, and that these species had remained unchanged over the centuries.
As the eighteenth-century scientific community began attempting to classify plants and animals systematically, however, the immense diversity and interrelatedness of living things cast doubt on this traditional model. The plant and animal fossils sought out by excavators were particularly at issue, since they implied that the history of earth extended back much further than previously thought, and that life developed only gradually and unevenly from simple to advanced organisms.
In 1809, the French botanist Jean Baptiste de Lamarck made an important attempt to formulate an explanation for this apparent complexity of organic life. Although he did not use the term "evolution," Lamarck argued that species gradually progressed over time from simpler to more complex types. To account for this change, Lamarck proposed that organisms possess an innate drive toward perfection and an ability to adapt to their environment.
Lamarck further believed that acquired characteristics could be passed on from generation to generation. For example, the ancestors of the giraffe, in reaching high leaves to eat, would have stretched and elongated their necks; this trait was then passed on to their descendants. Although this belief in the transmission of acquired traits has since been discredited, Lamarck was correct in assuming that traits could somehow be inherited, and that this process could lead, over long periods of time, to significant evolutionary changes.
It was the English naturalist Charles Darwin who initiated the understanding of evolution (in Darwin's own terms, "descent with modification") held today. Darwin's theories were based on the geological and biological studies he conducted in the 1830s during travels in South America and the South Sea islands. Darwin's detailed observations of plants and animals provided a considerable body of evidence for evolutionary change. Fossils indicated that some species had become extinct, although clearly related species survived. Darwin also recognized that similar but not identical species were now found in different geographical regions. In the Galapagos Islands, furthermore, Darwin observed that the beaks of different types of finch birds were evidently adapted to the food supplies of the geologically distinct islands they populated. Similar adaptations characterized the local species of giant tortoises.
Synthesizing his observations—ultimately presented to the world with the publication of On the Origin of Species in 1859--Darwin postulated that species were mutable and divergent rather than fixed in form. Darwin recognized that many species had common ancestors. Thus, lizards and rabbits are similar in their embryonic stage, and the bones in human arms and legs correspond to those in the limbs of dogs or horses. To explain these parallels, Darwin proposed that species had branched and evolved through natural selection.
Natural selection implies that, given the competition for limited resources such as food, those organisms best adapted to their specific environment are most likely to survive, reproduce, and transmit their traits to offspring. This process, occurring in conjunction with environmental changes, causes the advantageous traits of members of a particular species to predominate and the disadvantageous traits to be lost. Over thousands of years, certain forms of life accumulate enough changes for an apparently new and distinct species to emerge.
Darwin's theories, though bold and controversial, took place within the context of a dynamic nineteenth-century scientific and intellectual community. Thus, Darwin's Origin of Species was modeled on Charles Lyell's similarly groundbreaking Principles of Geology, published in the 1830s. Alfred Russel Wallace, working from independent studies in the East Indies, formulated his own theory of natural evolution contemporaneous with Darwin's.
It was Wallace who coined the phrase "the survival of the fittest"—a phrase that does not signify that the strongest species will necessarily flourish, but rather that those species best "fitted" to their environments have the best chances of survival. In addition, prominent intellectuals such as Herbert Spencer (who popularized the term "evolution") and T. H. Huxley (called "Darwin's Bulldog" for his promotion of Darwin's work) were vital in the process of debating, elaborating, and circulating Darwin's theories.
It was the presence of this community that allowed Darwin to publish his second important book, The Descent of Man, in 1871. This publication, which exposed Darwin to considerable religious opposition and public scorn, followed through on the implications of his earlier work by asserting that human beings had descended from, and were biologically related to, earlier life forms.
Even as the scientific community came to accept Darwin's principles of descent with modification and evolution through natural selection, they recognized that Darwin had failed to provide an adequate explanation of how biological variations were produced or passed on. Without this component, critics argued, the theory of natural selection accounted for very little.
This issue was essentially resolved by the Austrian botanist Gregor Mendel, who in the 1860s isolated the basic unit of heredity now known as the gene. Working in the garden of the monastery he had entered in 1843, Mendel discovered that differences among varieties of common garden peas (such as variations in shape, size, flowering, coloring, and seed characteristics) were due to paired units of heredity. Mendel was the first to discover that these molecular "blueprints," passed on from parents to offspring, determined which features a living organism will inherit. Mendel received credit for this contribution to evolutionary theory only in the early 1900s, when his work was confirmed by independent scientists such as Hugo de Vries. Mendel's discoveries, nonetheless, are now incorporated into our understanding of evolution.
The integration of Darwin's descent with modification and Mendel's work in genetics was substantially achieved by the Russian-American scientist Theodosius Dobzhansky, who published his influential book Genetics and the Origin of Species in 1937. Dobzhansky's experiments with fruit flies clarified the process of evolutionary adaptation by demonstrating the variability of genes and thus the possibility of rapid evolutionary change through genetic mutation and recombination. "Neo-Darwinism," as the synthesis of Darwinism and Mendelism genetics is often called, thus recognizes that evolution involves not only physical and behavioral traits, but also the genes that serve as the basis for those traits. This process of genetic evolution is known as genetic drift.
Continuous and substantial advances have been made in the study of evolution in the twentieth century. Evolutionary theory, in fact, has served as a point of reference for various specialists working within a complex network of scientific and mathematical disciplines that rely on increasingly sophisticated methodologies. Walter S. Sutton, Theodor Boveri, Wilhelm Johannsen, Thomas Hunt Morgan, and Hermann Muller have investigated the complex relationship between chromosomes, genes, and the laws of heredity. Biometricians such as Ronald Fisher, John Haldane, and Reginald Crundall Punnett have used mathematical and statistical techniques to analyze genetic changes, thereby establishing the field of population genetics. Julian Huxley, the grandson of T. H. Huxley, made important contributions to the field of embryology, among other areas. The paleonthologist George Simpson focused on the intercontinental migration patterns of ancient species. James Watson and Francis Crick introduced a model for deoxyribonucleic acid (DNA) to explain the chemical basis of genes, heredity, and evolution.
Today biologists continue to study the patterns, mechanisms, and pace of evolution, leading to some proposed changes in the original theory. For example, Darwin's model of evolution was based on gradualism, which assumes slow, steady rates of change. A newer model, called punctuated equilibrium, proposes that change may occur in relatively quick bursts, followed by longer periods of stasis. Rather than undermining evolution, such changes just underscore the vitality of evolution and the theory that explains it.
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