The Role of Natural Selection in Evolution - In many cultural contexts, the word evolution means progressive change. We talk about the evolution of economies, fashion, or musical tastes. From a biological perspective, the word has a more specific meaning. Evolution is the continuous genetic adaptation of a population of organisms to its environment over time. Evolution results when there are changes in genes present in a population. Individual organisms can not evolve—only populations can. Although evolution is a population process, the mechanisms that bring it about operate at the level of the individual.
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There are three factors that interact to determine how a species changes over time: environmental factors that affect organisms, sexual reproduction among the individuals in the gene pool, and the generation of genetic variety within the gene pool. The success of an individual is determined by how well its characteristics match the demands of the environment in which it lives. There is a fit between the characteristics displayed by a species of organism and the surroundings the species typically encounters. Biologists refer to this match between characteristics displayed, the demands of the environment, and reproductive success as the fitness of the organism. Those individuals whose characteristics best fit their environment will be likely to live and reproduce. Since the various processes that encourage the passage of beneficial genes to future generations and discourage the passage of harmful or less valuable genes are natural processes, they are collectively known as natural selection.
The idea that some individuals whose gene combinations favor life in their surroundings will be most likely to survive, reproduce, and pass their genes on to the next generation is known as the theory of natural selection. The theory of evolution, however, states that populations of organisms become genetically adapted to their surroundings over time.
Natural selection is the process that brings about evolution by “selecting” which genes will be passed to the next generation. The processes of natural selection do not affect genes directly but do so indirectly by selecting individuals for success based on the phenotype displayed. Recall that the characteristics displayed by an organism (phenotype) are related to the genes possessed by the organisms (genotype). It is also important to recognize that when we talk about the characteristics of an organism that we are not just talking about structural characteristics. Behavioral, biochemical, or metabolic characteristics are also important. However, when looking at evidence of the past evolution of species of organisms it is difficult to assess these kinds of characteristics, so we tend to rely on structural differences.
Recall that a theory is a well-established generalization supported by many different kinds of evidence. The theory of natural selection was first proposed by Charles Darwin and Alfred Wallace and was clearly set forth in 1859 by Darwin in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life. Since the time it was first proposed, the theory of natural selection has been subjected to countless tests and remains the core concept for explaining how evolution occurs.
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