A mutation is a random, permanent change in DNA. If a mutation is a germline mutation it can be heritable. Mutation allows for variation from individual to individual within a species and therefore provides the raw material upon which natural selection acts.
Mutations may be beneficial, deleterious, or (most commonly) neutral toward the affected organism's chances of survival. Beneficial mutations are likely to propagate within a species. Deleterious mutations usually result in the death of the organisms in which they occur, and thus they are not likely to become common in the species. Whether a specific mutation is beneficial or deleterious can depend upon conditions present in the organism's environment.
There are an estimated 1.6 to 4.2 mutations per sperm cell in humans. The average human zygote has about 64 mutations. Heritable mutations are usually the result of vertical genetic transfer, which occurs when genes are copied during reproduction.
Types of mutations
A point mutation occurs when the code at a single position on a gene is replaced by another. Alternately a segment of DNA may be deleted or inserted, a chromosome may break into two pieces, or two chromosomes may be joined.
In chromosomal duplication the offspring gets extra copies of one or more duplicated chromosomes. The duplicated genes are then free to undergo further mutation, creating families of related genes with different functions.
Retroviruses have the ability to insert a copy of themselves into the genes of a host. Plasmids are pieces of DNA that are passed from bacterium to bacterium in an example of horizontal genetic transfer. In bacterial DNA exchange, bacteria can exchange DNA directly. Parasites can carry genetic material from one organism to another. In symbiotic transfer an organism may borrow genes from another with which it exists in a close symbiotic relationship. Transposons are genes that can move from one place to another.
In an experiment begun in 1988, MSU biologist Richard Lenski observed a single E. coli microbe as it began reproducing. He soon split the resulting population into 12 lines and by 2008 had logged over 44,000 generations, having frozen a sample every 500 generations for future reference.
At 33,127 generations one line of the E. coli developed the novel ability to metabolize citrate. This splendid example of evolution was determined to be the result of two mutations, one of which first occurred sometime before 31,500 generations. Upon replaying the incident from the line's frozen "fossil record" only generations after 27,000 ever re-developed the mutations, and then only rarely.
Examples of beneficial mutations
- Antibiotic resistance in bacteria
- Pesticide resistance in insects
- Bacteria that eat nylon
- Sickle cell resistance to malaria
- Lactose tolerance
- Immunity to HIV
Creationists refuse to acknowledge that mutations can provide raw material for evolution, and they present a variety of claims in an attempt to provide evidence for their beliefs.
Creationists claim that successive mutations are mathematically impossible. Answers in Genesis (AiG) notes, "The mathematical problem for evolution comes when you want a series of related mutations. The odds of getting two mutations that are related to one another is the product of the separate probabilities..." Note the model in this claim assumes both of the mutations must initiate in the same individual, which is not the way mutations propagate throughout a population.
AiG also claims, "Contrary to popular opinion, drug resistance in bacteria does not demonstrate evolution...It does demonstrate natural selection (or a sort of artificial selection, in this case), but only selection among already existing variations within a kind." In fact mutation in bacteria has been observed in populations grown from a single individual, where there should have been no variation whatsoever.
Creationists make the claim that mutations are nearly always harmful and therefore cannot be depended upon to "improve" an organism. AiG notes that "mutations are 'going the wrong way' as far as evolution is concerned. Almost every mutation we know is identified by the disease or abnormality that it causes...In other words, time, chance, and random changes do just what we normally expect: tear things down and make matters worse." The existence of any beneficial mutations, such as those listed above, falsifies this claim. Note also in this claim the implication that evolution is working toward a goal, which it certainly is not.
Creationists claim that the cumulative effects of numerous mutations would be burdensome to a species. AiG notes, "Geneticists, even evolutionary geneticists, refer to the problem as 'genetic load' or 'genetic burden.' In their textbook on evolution, Dobzhansky et al. state clearly that the term is meant to imply a burden that 'weighs down' a species and lowers its genetic quality...When genetic burden gets too great, offspring are so likely to have serious hereditary defects that the ability of the species to survive is threatened." This claim ignores the fact that any deleterious mutations will be selected against and will not propagate throughout a population. Thus equilibrium is achieved between selection and genetic load in a given species.
Creationists also claim that mutations cannot result in the increase of information necessary for evolution to produce more complicated organisms from less complicated ones. AiG notes that "evolutionists need some kind of 'genetic script writer' to increase the quantity and quality of genetic information." Much could be said about the straw man of evolution presented in this claim in addition to the misuse of the term "information." Also this claim ignores the phenomenon of gene duplication, which surely constitutes an increase in information.
Creationists claim that genes are analogous to the information contained in words and that therefore a mutation would render a gene unreadable. This is a bad analogy as it suppresses the fact that proteins' sequences are flexible and that a mutation in a gene that codes for a new protein, transcription to RNA, or regulatory process may be just as viable.
- Zimmer, Carl (June 2008). "A New Step in Evolution." ScienceBlogs.
- Parker, Gary. "Mutations." Answers in Genesis.
- See Lederberg, J. and E. M. Lederberg, 1952. Replica plating and indirect selection of bacterial mutants. Journal of Bacteriology 63: 399-406.; Francis, J.E., & Hansche, P.E., 1972. Directed evolution of metabolic pathways in microbial populations. I. Modification of the acid phosphatase pH optimum in Saccharaomyces cervisiae. Genetics, 70: 59-73.; Lenski, above.
- Hodge, Bodie (Feb. 2010). "Chapter 7: Are Mutations Part of the "Engine" of Evolution? Answers in Genesis.