In 1859, Charles Darwin published the book The Origin of Species that presented a theory to explain life's great diversity of organisms. Much of the knowledge found in modern biology would not make sense without Darwin's theory.
Evolution is the process by which species come to possess genetic adaptations to the environment. Its working mechanism is natural selection. The mechanism of natural selection acts through individuals as it determines which individuals have the best adaptations for reproductive success. Because the nature of environment is always changing, natural selection is continually influencing the genetic characteristics of the population through time. As a result, natural selection acts to keep most populations of organisms in sync to their ever changing surroundings.
The evolution of a population also involves a change in gene frequency. A changing pattern of genetic variation from one generation to the next occurs as natural selection determines the fittest individuals. Natural selection controls the future frequency of a population's genes, because it determines which individuals will become reproductively successful. Through mutations new genes are always entering the species gene pool. Most of these mutations are unsuccessful in making a species better adapted to its environment. However, few mutation do produce favorable genetic adaptations. Consequently, its the appearance of these few favorable mutations together with the change in gene frequency because of natural selection that results in evolution.
Another important condition for evolution is the spatial isolation of sub-populations from the rest of the population. If a remote sub-population cannot exchange genes to the main population because of isolation, natural selection will act differently in each population causing divergent evolution. Overtime, differences in gene frequency will arise between the two populations because different mutations will occur in the two population and because no two patches of habitat are absolutely the same. As a result of the habitat differences, natural selection will favor different adaptations in each patch. The isolation of populations can occur simply because of distance or due to the preesence of physical barriers like mountains and rivers.
One of the best known examples of natural selection operating in a modern species is the development of pesticide resistance in select insect species. Prior to the use of pesticides in the 1940s, crop pest insect populations only contained a limited amount of genetic variability for resistance to these chemicals. In the absence of pesticides, natural selection cannot cause changes in the frequencies of genes causing resistance to chemical pesticides. Once the application of pesticides became widespread, insects that possessed chemical resistance genes became more frequent as they survived the applications. As a result, these individuals were able to pass on a greater percentage of their genes to the gene pool of subsequent generations.