Variation — Mutation and Recombination

Natural selection, as described in the preceding two sections, can only act upon a population, and can only drive evolutionary change within that population, if the population actually contains heritable variation for natural selection to discriminate between in the first place — if every individual within a population were genetically identical to every other individual, there would be no differences in fitness between individuals for selection to act upon at all, and the population's characteristics could never change from one generation to the next through selection, however strong the selective pressure from the environment might be. Understanding where this essential heritable variation actually comes from is therefore just as important, to a complete theory of evolution, as understanding how natural selection then acts upon that variation once it exists.

Mutation is the single ultimate, original source of every new item of genetic variation that has ever existed in any population of any species: a mutation is a sudden, heritable change in the precise sequence of nucleotide bases making up an organism's DNA, and it can arise either spontaneously, through the occasional, naturally occurring error that creeps into the normally extremely accurate process of DNA replication, or it can be actively induced by external mutagenic agents, such as certain reactive chemicals or ionising radiation, that directly damage or alter DNA structure. A newly arisen mutation typically first appears in just a single individual, somewhere within an otherwise very large population, and — this point is worth stressing clearly, since it is a common source of confusion — the mere appearance of a single new mutation in one individual does NOT, by itself, constitute evolution in any meaningful biological sense. Evolution, properly understood as a population-level phenomenon, occurs only once that newly arisen mutant allele's overall frequency within the population as a whole subsequently changes across generations — whether because natural selection actively favours or actively disfavours the new mutant allele relative to the pre-existing alleles at that same gene, or, alternatively, because the mutant allele's frequency happens to rise or fall purely through the chance, random process of genetic drift (§6.10), entirely independently of whether the mutation itself is helpful, harmful, or selectively neutral. …