Artificial selection is the identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations. 

Environmental change occurs as a result of both natural and human processes. Environmental systems and human activities contribute to environmental changes through the transformation and transportation of large quantities of energy and materials. 

Any change in the DNA sequence of a cell. Mutations may be caused by mistakes during cell division, or they may be caused by exposure to DNA-damaging agents in the environment.