The Elements of Evolution (41-3) Mutation


A mutation is a change in a gene sequence. The term comes courtesy of Dutch botanist Hugo de Vries, who, unaware of Mendel’s work, reiterated his discoveries.

In 1889 de Vries presented a variant of Darwin’s 1868 pangenesis hypothesis. de Vries postulated that traits were carried in particles he termed pangenes.

To de Vries, different characters had different hereditary carriers. His mutations were plant varieties that suddenly appeared: a product, he supposed, of altered pangenes.

20 years later, Danish botanist Wilhelm Johannsen mutated pangenes into genes. Johannsen’s conceptualization matched the modern notion of genes as carriers of heredity.

Johannsen broke with Darwin in suggesting the evolutionary suddenness of saltation, as contrasted to adamant Darwinian gradualism.

In dropping the idea of epigenetics, which was rejected as Lamarckian woolliness, Johannsen led geneticists into an unrealistically simplistic direction that would hold for well over a half century. The tidiness of Johannsen’s gene concept was compelling.

Genetics is illustrative of a conceptual hypothesis coming well in advance of a supporting fact base: a long-standing mental construct awaiting confirmation. Evidence that did not fit the preconception was considered auxiliary or ignored altogether.

In this case, the concept of genes easily meshed with the model of DNA from Watson & Crick. The fit was so easy that there was no question by geneticists that the puzzle of heredity had been completely solved. This smugness held the discipline back for over half a century.

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Several types of mutation exist, at variable ranges of impact: from a sole nucleotide to an entire chromosome. A point mutation exchanges a single nucleotide for another. Insertions add 1 or more nucleotides. Deletions subtract. Translocations interchange genetic material. Inversions rearrange a gene sequence.

There are numerous provocations that can cause a mutation: from an error or deviation in the complex process whereby DNA is replicated, to mutagenic chemicals, radiation, or viruses. Stress drives evolution.

Proteins that regulate gene expression or DNA sequence let changes occur. These alterations tend to be adaptively responsive. Such coherent teleology is both obvious and empirically inexplicable.

Internally invoked mutations are called spontaneous. Induced mutations are caused by radiation or chemicals, commonly toxins.

A mutation may have no effect, alter a gene’s product, prevent a gene from properly functioning, or render a gene inoperable. In effect, mutations may be neutral (not affecting fitness), harmful (decreasing fitness), or beneficial (improving fitness). Overtly non-damaging mutations are commonly accepted by a cell.

Due to the damage that mutations may have, cells have several DNA repair mechanisms which are able to proofread and mend damaging deviations before they become permanent. If repair is not possible, organisms have techniques for eliminating permanently mutated cells.

Mutations can be immediately adaptive. Somatic hypermutation is the mechanism by which an immune system learns to confront a new foreign element, commonly a pathogenic microbe.