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            Natural selection, in biology, is the process by which environmental effects lead to varying degrees of reproductive success among individuals of a population of organisms with different hereditary characters, or traits, as defined by Microsoft Encarta Encyclopedia. For billions of years, natural selection has been taking place and changing life on this planet. Survival of the fittest is a term used to describe how and why natural selection occurs. An organism better equipped to survive in a specific environment will survive longer and have a greater chance of producing offspring than an organism that does not have favorable traits. Charles Darwin and Alfred Wallace first proposed this concept in 1858. Although there are many arguments against the theory of evolution and natural selection, evidence of this process can be seen in several animals including peppered moths and the finches Darwin observed early in his studies. .
             About 3.5 billion years ago the earth's environment was very different than it is today. There was not a significant amount of free oxygen in the atmosphere. According ("Evolution," Microsoft ® Encarta ® Encyclopedia 99. © 1993-1998 Microsoft Corporation) experiments have shown that complicated organic molecules, including amino acids, can occur spontaneously under certain conditions believed to simulate the earth's primitive environment. Other experiments indicate that concentration of organic molecules may have led to the synthesis of complex molecules, such as proteins, nucleic acids, and carbohydrates, and eventually to interactions among these molecules. A basic genetic system was developed and elaborated by natural selection into the more complex genetic system of today. Scientists believe that the early organisms fed on inorganic or organic molecules dissolved in the waters of the earth. Later, evolution led to organisms that used chemical and solar energy sources to produce organic molecules.

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