Unfortunately, Explore Evolution makes a confusing definition of "structural mutations" which significantly differs from normal scientific usage. Also, Explore Evolution remarkably fails to mention the major cause of mutations, errors in copying DNA.
A straightforward definition of mutation can be found in any genetics or evolution textbook. For example, the recently published Evolution textbook by Nick Barton and colleagues explains:
Mutation, formally defined as a heritable change in the genetic material (DNA or RNA) of an organism, is the ultimate source of all variation. Without mutation, there would be no evolution.Barton, et al., (2007) Evolution, p. 325
Unfortunately, Explore Evolution takes this straightforward concept and manages to make it incomprehensible:
As we have seen, there are scientists who doubt that natural selection can produce major evolutionary change. Specifically, they question whether there is a source of new information that can produce new genetic traits the variations needed to produce lasting biological change.
Defenders of the neo-Darwinian position dispute this critique by offering another argument for the creative power of natural selection. they say that the critics have underestimated the power of another type of variation, unknown in Darwin's time, called mutation.Explore Evolution, p. 98
How are mutations produced?
Mutations can occur when genes are exposed to heat, chemicals, or radiation.Explore Evolution, p. 98
Remarkably, Explore Evolution fails to mention that errors in copying DNA are a major source of mutations.
Explore Evolution's definitions of mutations are arbitrary and result in outright confusion. "Genetic mutations" are defined by Explore Evolution as:
a change in the sequential arrangement of the information-bearing bases the "letters" in the genetic text of the DNA molecule.Explore Evolution, p. 98
Explore Evolution explains that there are several types of "genetic" mutations including point mutations, gene duplications, and chromosomal inversions. Presumably, chromosomal translocations and insertions would also be "genetic" mutations, although they are not mentioned. In fact, Explore Evolution's definition of "genetic mutations" encompasses every type of mutation.
Why produce an arbitrary and unnecessary definition of "genetic mutation"? Explore Evolution seems to be distinguishing "genetic mutations" from something they define as "structural mutations":
For major changes to occur in more complex, multi-cellular animals, mutations must ultimately affect an animals shape or structure. Are there examples of such structural mutations? Evolutionary biologists say they are, and point to another striking example of the novel variation that mutation can produce: the four-winged fruit fly.Explore Evolution, p. 101
This distinction between "genetic" and "structural" mutations is found nowhere else in evolutionary biology, it misappropriates the legitimate term of "structural mutation," and it will generate considerable confusion among students. The long-standing legitimate definition of a "structural mutation" refers to a mutation in the protein coding portions of a gene that results in a change in amino acid sequence. An example of a structural mutation in the human serum cholinesterase gene that is a result of a mutation in the protein-coding region is described below.
A point mutation in the gene for human serum cholinesterase was identified that changes Asp-70 to Gly in the atypical form of serum cholinesterase. The mutation in nucleotide 209, which changes codon 70 from GAT to GGT, was found by sequencing a genomic clone and sequencing selected regions of DNA amplified by the polymerase chain reaction.McGuire et al., (1989), "Identification of the structural mutation responsible for the dibucaine-resistant (atypical) variant form of human serum cholinesterase," PNAS 86, p. 953
According to Explore Evolution, structural mutations "ultimately affect an animal's shape or structure", but this correctly labeled "structural mutation" in human serum cholinesterase does not result in a change in organismal structure. Instead, human serum cholinesterase is involved in controlling the stability of neurotransmitters.
What Explore Evolution refers to as "structural mutations" would be recognized by most biologists as mutations in genes that are used to build animal embryos, the genetic toolkit. For example, the four-winged fruit fly referred to by Explore Evolution is due to mutations of the Hox gene, Ultrabithorax. Are mutations in the protein-coding region of Ultrabithorax responsible for the four-winged fruit fly? No, the mutations are in non-coding regions of Ultrabithorax. Explore Evolution's own example of a "structural mutation" is the opposite of what biologists consider structural mutations.
If the intent of Explore Evolution was to prevent a clear understanding of mutations and their relationship to evolution, they succeed grandly. For teachers and students, however, this treatment of mutations promises headaches.