Reply #4: Some causes of mutations

[Zenlitened]

...Mutations can occur naturally as a result of occasional errors in DNA replication during cell division. Replication is not always 100% accurate in copying the millions of base units in a DNA molecule. Mutations also can be caused by exposure to radiation, alcohol, lead, lithium, organic mercury, and some other chemicals. Viruses and other microorganisms may also be responsible for them. Even some commonly prescribed drugs are thought to be teratogens . In this group are androgens, ACE inhibitors, streptomycin, tetracycline, and vitamin A. However, mutations appear to be spontaneous in most instances. That does not mean that they occur without cause but, rather, that the specific cause is almost always unknown.

The great diversity of life forms that have been identified in the fossil record is evidence that there has been an accumulation of mutations producing a more or less constant supply of both small and large variations upon which natural selection has operated for billions of years. Mutation has been the essential prerequisite for the evolution of life.

In order for a mutation to be subject to natural selection, it must be expressed in the phenotype of an individual. Selection favors mutations that result in adaptive phenotypes and eliminates nonadaptive ones. Even when mutations produce recessive alleles that are seldom expressed in phenotypes, they become part of a vast reservoir of hidden variability that can show up in future generations. Such potentially harmful recessive alleles add to the genetic load of a population.

http://anthro.palomar.edu/synthetic/synth_4.htm