Mutations are classified as beneficial, harmful or neutral. Harmful mutations will be lost if they reduce the fitness of the individual. If fitness is improved by a mutation, then frequencies of that allele will increase from generation to generation. The mutation could be a change in one allele to resemble one currently in the population, for example from a dominant to a recessive allele. Alternatively, the mutation could generate an entirely new allele. Most of these mutations though will be detrimental and lost. But if the environment changes, then the new mutant allele may be favored and eventually become the dominant alelle in that population. If the mutation is beneficial to the species as a whole, migration from the population in which it initially arose must occur for it to spread to other populations of the species.
The most basic type of mutation is the change in a single nucleotide in the gene. Mutations are generally deleterious and are selected against. But the genome of a species can undergo another type of change, gene duplication, which actually favors mutational events. If a single gene that is important undergoes a duplication, mutation in the duplicated copy would not necessarily reduce the fitness of the individual because it still would have a functioning copy of the original gene. With this adaptive constraint removed, further changes can occur that generate a new gene that has a similar function in the organism, but may function at a specific time in development, or in a unique location in the individual. This type of evolution generatesmultigene families. Many important genes such as hemoglobin and muscle genes in humans, and seed storage and photosynthetic genes in plants are organized as multigene families.