Evolution is the change of species over time. This theory is the central unifying theme of biology. This change over time is well documented by extensive evidence from a wide variety of sources. Students need to know that in sexually reproducing organisms, only changes in the genes of sex cells can become the basis for evolutionary change that these evolutionary changes may occur in structure, function, and behavior over time. Students need to be able to distinguish between evolutionary change and the changes that occur during the lifetime of an individual organism.
According to many scientists, biological evolution occurs through natural selection. Natural selection is the result of overproduction of offspring, variations among offspring, the struggle for survival, the adaptive value of certain variations, and the subsequent survival and increased reproduction of those best adapted to a particular environment. Selection for individuals with a certain trait can result in changing the proportions of that trait in a population.
The diversity of life on Earth today is the result of natural selection occurring over a vast amount of geologic time for most organisms, but over a short of time for organisms with short reproductive cycles such as pathogens in an antibiotic environment and insects in a pesticide environment.