Synthetic Theory of Evolution: Mutation

In order for a mutation to be inherited, it must occur in the genetic material of a sex cell. Estimates of the frequency of mutations in human sex cells generally are about 1 per 10,000-1,000,000 for any specific gene. Since humans have approximately 20,000-25,000 genes, it is to be expected that most sex cells contain at least one gene mutation of some sort. In other words, mutations are probably common occurrences even in healthy people. Most mutations probably do not confer a significant advantage or disadvantage because they occur in non-gene coding regions of DNA molecules. They are relatively neutral in their effect. However, some mutations are extremely serious and can result in death before birth, when an individual is still in the embryonic or early fetal stages of development.

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.

Double Immunity--a mutation selected for by plague may provide immunity to HIV
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