The gazelle is extremely fast, and the cheetah is even faster. These traits are adaptations-characteristics or behaviors that give an organism an edge in the struggle for survival. Darwinian theory holds that adaptations are the result of a two-stage process: random variation and natural selection. Random variation results from slight genetic differences. For example, one cheetah in a group may be slightly faster than the others and thus have a better chance of catching a gazelle. The faster cheetah therefore has a better chance of being well-fed and living long enough to produce offspring. Since the cheetah's young have the same genes that made this parent fast, they are more likely to be fast than the young of slower cheetahs. The process is repeated in each generation, and thereby great speed becomes an adaptation common to cheetahs. This same process of natural selection, in which the organisms best adapted to their environment tend to survive and transmit their genetic characteristics in increasing numbers to succeeding generations while those less adapted tend to be eliminated, also favors the fastest gazelles. Though evolution, in this case, may be thought of as an arms race, animals may also adapt to their environment in a process known as adaptive radiation, as the so-called Darwin's finches in the Galápagos have done. On the islands, one type of finch gradually gave rise to some 13 different species of birds with differently shaped beaks, each species having adapted to its varying food niches and feeding habits. And, though we seldom think of it, humans also have an impact on an organism's adaptation to its environment. For instance, because of the misuse of antibiotics, some disease-causing bacteria have rapidly adapted to become resistant to the drugs.

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