A mutation is any change occurring in the message that a gene carries. Mutations mainly arise as copy errors when DNA is replicated at mitosis and meiosis.
Darwinian evolution requires a constant supply of variation: much of it is supplied by mutation, and a mutation-selection balance can maintain a genetic polymorphism.
The first major geneticist to study mutation was H.J. Muller, who demonstrated it can be induced by X-rays. He also recognized that the rate of mutation in nature is extremely low, and that they are almost always deleterious to the fitness of the organism. The accumulation of deleterious mutations places a mutational load on the population.
Mutations can occur at single base level or at chromosomal level. The effects of mutation can occasionally be very dramatic: some of these fruitflies have suffered mutations which alter the number of wings that develop..
Is mutation directed?