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Adaptive Evolution
| Article
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16325 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
3 / 1989 |
4,551 Words |
| Author
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Barry G. Hall
Barry G. Hall, professor of molecular and cell biology at the
University of Connecticut, will soon take up a position as
professor of biology of the University of Rochester. He is an
associate editor of the Journal of Molecular Biology and
Evolution and is on the editorial board of the Journal of
Molecular Evolution. |
Any casual observer of the biological world is immediately struck by the enormous amount of variation in form that distinguishes organisms from one another. There is both variation between species (i.e. although humans and apes have basically similar forms, they are quire different from each other in detail) and variation within populations of the same species (the variation that permits us to recognize each other as individuals). A great deal of that variation is obviously adaptive in that it confers some special advantage that permits the organism to better exploit its environment. The long neck of the giraffe allowing it to consume of tall trees is an example of adaptive variation. Biology students learn that evolution proceeds by causing better adapted individuals to increase in frequency from one generation to the next, a process called "selection." It is believed, for example, that among primitive giraffes, rare individuals with somewhat longer necks were able to reach higher into trees than could the normal individuals and therefore were able to be better nourished. As a consequence of better nourishment, they had an advantage either in reproduction or in escape from predators; thus genes for long necks were selectively advantageous in the long run.
Mutations Are The Source of Biological Variation
The ultimate source of biological variation is mutations that alter the DNA of an organism. DNA, or deoxyribonucleic acid, is the molecule containing all of the information that directs the complex processes of metabolism, growth, and reproduction for the cell and that ultimately determines the properties of an organism: its shape, average lifespan, and so forth. The fundamental problem of evolutionary biology is to understand, in detail, the processes by which mutations in DNA have led to the variation that is observed today. The structure of DNA is defined by the sequence of the nucleotide bases that make up the DNA chain. In the common intestinal bacterium Escherichia coli, commonly called E. coli, that chain is about 4.7 million bases long (DNA is made up of sequences of nucleotide bases, just as words are made up of sequences of letters.) Mutations are changes in the DNA chain, and most changes will affect one or more vital cellular processes, just as changing a letter in a word can completely alter the meaning of that word. It is not surprising that most mutation will actually improve things. Random mutations--random changes in the DNA--are very much like whacking the engine of a Ferrari with a sledgehammer in order to make it run better. While there may be a faint possibility that the tuning will be improved, the odds greatly favor a trip to the repair shop. It would be enormously beneficial to an
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