A mutation is an abrupt change in genes or chromosomes that affects an organism morphologically. Mutations can occur spontaneously or be induced by environmental factors, and, on occasion, they can even occur as the result of a change in chemistry within a cell. The process of creating a mutation is called mutagenesis. An organism that has a mutation is called a mutant, and something that creates a mutation is called a mutagen.
Mutations are the major cause of variability, which allows evolution to take place, although these random changes are far more likely to harm than to improve a complex organism. However, we can now deliberately select certain mutations (the favorable ones) to improve plants and animals. It is possible to create a specific mutation at will, rather than select among the array of random mutations for the infrequent useful ones. Mutation has been the most consistent way for geneticists to study and understand the nature of the gene. They are also crucial in giving geneticists clues about disease, since mutations either cause disease directly or predispose humans to many different disorders. As of 1998, scientists have identified over 13,000 mutations in more than 700 genes that can cause human inherited disease.
In 1859, Charles Darwin asserted that a species could evolve into distinctly different species because of small variations that changed each generation of offspring. The Dutch botanist Hugo de Vries disagreed, noting in his experiments with the evening primrose that new species would spontaneously appear, with characteristics undetected in previous generations. De Vries eventually formulated his own theory, based in part on Gregor Mendel's laws of heredity, but ultimately felt that something was missing. Mendel had explained how existing characteristics were redistributed to create variations, but not how new species originated. Harking back to his primrose experiments, de Vries termed the new varieties mutations. He theorized that evolution occurred in sudden jumps due to these mutations, not in gradual increments as suggested by Darwin. He believed a species produced mutants over relatively short time periods throughout its evolutionary life, and only the useful or "progressive" characteristics contributed to the evolution of the species. De Vries documented this theory in his Die Mutationstheorie (The Mutation Theory, 1901-1903).
Between 1910 and 1925, the American geneticist Thomas Hunt Morgan set out to test de Vries's mutation theory. The results of his famous experiments with the fruit fly proved, contrary to de Vries's belief, that Mendelian genetics could indeed explain the evolution of a new species. Morgan, however, did not entirely reject de Vries's hypothesis. He applied the Dutch scientist's term mutation to spontaneous changes in the genetic material, and while he viewed mutations as being less drastic than de Vries had originally thought, he saw them as being more abrupt and dramatic than Darwin's small variations.
Many others also became interested in studying mutations, including biologist Hermann Muller. In 1926, Muller was working with Morgan on his heredity experiments. But Muller grew tired of waiting for mutations to occur naturally and began looking for ways to artificially induce them. He eventually discovered that x-rays greatly increased the mutation rate within the genes, causing drastic molecular changes.
Mutations also caught the interest of British physiologist John Burdon Haldane. He was convinced that natural selection and not mutation was the driving force behind evolution. In 1922, he began investigating the variation of gene linkage and the mathematics of natural selection. Haldane was the first to estimate the rate of mutation of a human gene, enabling him to work out the effect of recurrent harmful mutations on a population.
Several years later, in 1937, Albert Francis Blakeslee (1874-1954) began carrying out experiments with mutations and found that the alkaloid colchicine could bring about mutations in plants. What distinguished Blakeslee's discovery from Muller's was that, rather than altering genes, colchicine caused chromosomes to double in number while preventing the cell from dividing. These findings represented the first direct chemical effect on heredity.
In 1937, the Russian-born American geneticist Theodosius Dobzhansky redefined the role played by mutations in natural selection. Prior to his work, scientists assumed that there were two types of genes: normal and abnormal (mutations). Thought to be generally disadvantageous, these mutations were quickly eliminated from the population and therefore contributed little to the evolution of the species. Dobzhansky, however, proved that "normal" genes did not exist. Each gene, he discovered, was capable of different degrees of variation--some more drastic than others. Mutations, therefore, were very common and could provide ample material for natural selection.
The presence of a higher than usual number of mutant organisms in an area can be a warning sign that something has gone wrong with the environment there. For example, in the late 1990s, scientists became alarmed by the high number of deformed frogs turning up in the midwestern, northeastern, southern, and western United States and in Canada. Researchers believe that something in the water--perhaps pesticides, metals, or chlorinated contaminants--is causing the mutations. While it is common for about 1 percent of frogs at any given site to be malformed, in some wetland hot spots, up to 75 percent of frogs from certain species are born deformed.
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