In some environments, mutations that were once neutral or a disadvatage may become beneficial. This is called a selective advantage. it gives a genetic advantage to the organism over its competitors. For example look at the water flea Daphnia. it normally lives in water that is 20 degrees celsius and it cannot survive in water 27 degrees celsius or higher. But some mutations can survive in temperatures between 25 and 30 degrees celsius.
Other organisms like bacteria viruses and many insects reproduce very very rapidly. Some bacteria populations can double in under ten minutes. In populations that reproduce that quickly random mutations can be the key to survival. If the organism lives long enough to reproduce and pass on the genetic information then in time the whole population might gain the selective advantage. One mutation can mean the survival of an entire poulation.
Variation within species & mutations and selective advantage
Variation Within Species & Mutations and Selective Advantage<br />Patrick Zisser and Peter Dyloco<br />
A group of organisms that can interbreed in nature and produce fertile offspring<br />Species<br />
A part of a chromosome that governs the expression of a trait and is passed on to the offspring; it has a specific DNA sequence<br />Gene<br />
Environmental: Mutagens - Transposons<br />A segment of DNA that can become integrated at many different sites along a chromosome (especially a segment of bacterial DNA that can be translocated as a whole)<br />