Competition.

Competition is a major driving force in evolution, the process by which living organisms change over time, with better-adapted species surviving and less well-adapted species becoming extinct. Evolution begins with mutation, changes in the nucleotide sequence of a gene or genes, resulting in the production of slightly altered genes which encode slightly different proteins. These altered proteins are the expressed traits of an organism and may give the organism an advantage over its competitors. The organism outcompetes its rivals in the environment, and hence the environment favors the better-adapted, fitter organism, a process called natural selection . A mutation may help an organism in one environment but may hurt it in a different environment (for example, an albino squirrel may flourish in snowy regions but may not do as well in warm regions). Mutations are random events whose occurrence can be increased by chemicals called mutagens or by ionizing radiation such as ultraviolet light, X rays, and gamma radiation.