Mutation:

 Most biological molecules have a limited lifetime. Many proteins, lipids and RNAs are degraded once they are not any longer needed or damaged, and smaller molecules like sugars are metabolized to compounds to form or store energy. In contrast, DNA is that the most stable biological molecule known, befitting its role in storage of genetic information. The DNA is passed from one generation to a different , and it's degraded only cells die. However, it can change, i.e. it's mutable. Mutations, or changes within the nucleotide sequence, may result from errors during DNA replication, from covalent changes in structure due to reaction with chemical or physical agents within the environment, or from transposition. Most of the sequence alterations are repaired in cells. a number of the main avenues for changing DNA sequences and repairing those mutations are going to be discussed during this chapter. Sequence alteration within the genomic DNA is that the fuel driving the course of evolution. Without such mutations, no changes would occur in populations of species to permit them to adapt to changes within the environment. Mutations within the DNA of germline cells fall under three categories with reference to their impact on evolution. Most haven't any effect on phenotype; these include sequence changes within the large portion of the genome that neither codes for protein, or is involved in gene regulation or the other process. a number of these neutral mutations will become prevalent during a population of organisms (or fixed) over long periods of your time by stochastic processes. Other mutations do have a phenotype, one that's advantageous to the individuals carrying it. These mutations are fixed in populations rapidly (i.e. they're subject to positive selection). Other mutations have a detrimental phenotype, and these are cleared from the   population quickly. they're subject to negative or purifying selection.