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14

The Nature of Living Things

another, whereas in species with a large number of chromosomes, randomization is

more effective. There are also mechanisms whereby chromosome fission and fusion

can occur, leading to aneuploidy (cf. Sect. 14.4.1), which is a hallmark of cancer

(Sect. 14.5).

14.7.4

Summary of Sources of Genome Variation

Single-site mutations, common to all life-forms, may be due to mistakes in duplica-

tion (possibly caused by damage to the template base; e.g., due to ionizing radiation).

A point mutation is a change in a single base (pair). Note that single insertions or

deletions will change the reading frame; that is, all subsequent triplets will be mis-

translated.

Microchromosomal and macrochromosomal rearrangements refer to large-scale

changes involving many blocks of nucleotides. Tandem gene duplications may arise

during DNA replication but, otherwise, the main source for chromosome rearrange-

ment is meiosis.

Prokaryotes mostly do not reproduce sexually and, hence, do not undergo meiosis

but, on the other hand, they are rather susceptible to “horizontal transfer” (i.e., the

acquisition of genetic material from other bacteria, and viruses). ⁴⁷

The question of bias in single-site mutations is one of great relevance to evolution.

The null hypothesis is that any mutation will occur with equal probability. If the

mutation is functionally deleterious, according to the Darwinian principle it will not

be fixed in the population, and the converse is true for functionally advantageous

mutations. Kimura’s “neutral” theory of evolution asserts that functionally neutral

(i.e., neither advantageous nor deleterious) mutations will also become incorporated

into the genome (leading to the phenomenon of “genetic drift”).

A similar, but even more intriguing, question can be posed regarding bias in sites

of chromosome breakage and crossover. At present, although it is recognized that the

likelihood of DNA duplication or moving is sequence-dependent, there is no overall

understanding of the dependency.

Non-Darwinian evolution ascribes the major rôle in molecular evolution to

“genetic drift”—random (“neutral”) changes in allele frequency (cf. Sect. 14.7.4).

Classically, it is questionable whether genotypic differences without an effect on

phenotype can affect fitness (in any sense relevant to evolution). One should bear in

mind that one of the engines of evolution, natural selection, operates on phenotype

not genotype (to a first approximation at least) and, therefore, genes on their own

are only the beginning of comprehending life; it is essential to understand how those

genes are transformed into phenotype. To survive, however, a species or population

needs adaptedness (to present conditions), (genetic) stability, and (the potential for)

variability. Without stability, reproductive success would be compromised. Genetic

variability is, of course, antithetical to stability, but phenotypic variability, reflecting

47 See Arber (1998).