Mutation: the source of all new alleles
Why variation matters
- Evolution is a change in the frequency of alleles in a population over generations.
- Selection can only change the proportions of alleles that already exist. It cannot invent a new one.
- So every evolutionary process depends on there being heritable variation to work with, and that variation has to come from somewhere.
Mutation is the only original source
-
A mutation is a change in the base sequence of DNA.
-
Mutation is the only process that creates genuinely new alleles. Everything else merely rearranges or redistributes alleles that already exist:
- Meiosis — independent assortment and crossing over — produces new combinations of existing alleles, not new alleles.
- Random fertilisation produces new combinations, not new alleles.
- Gene flow moves existing alleles between populations.
- Natural selection and drift change the frequency of existing alleles.
-
This is why mutation is listed first in the standard: without it, evolution would eventually stop as variation was used up.
Types of mutation
-
Gene (point) mutations change one or a few bases:
- Substitution — one base is replaced by another.
- Insertion or deletion — bases are added or removed, shifting the reading frame and usually altering every amino acid downstream.
-
Chromosome mutations change the number or structure of whole chromosomes:
- Polyploidy — a whole extra set of chromosomes. This matters enormously for speciation in plants, and has its own page later.
-
Only mutations in gametes or in the cells that produce them are heritable. A mutation in a body (somatic) cell affects the individual but is not passed to offspring, so it has no evolutionary consequence.
Mutations are random with respect to need
-
This is the single most important idea on the page, and the one most often got wrong.
-
Mutations occur at random. They are not produced in response to what the organism needs.
- An environment does not cause useful mutations to appear.
- An organism cannot mutate in order to adapt.
-
What the environment does is select among mutations that have already occurred by chance.
-
The effect of a mutation depends entirely on the environment it finds itself in:
- Harmful — most mutations that change a protein are harmful, because existing proteins are already well matched to their function.
- Neutral — many make no difference, often because the genetic code is degenerate, so a substitution may code for the same amino acid.
- Beneficial — rare, but these are the raw material of adaptation.
-
The same mutation can be all three in different environments. An allele conferring resistance to a pesticide is useless or slightly costly without the pesticide, and enormously advantageous with it. Nothing about the mutation changed — the environment did.
Why rare mutations still matter
- Mutation rates per gene per generation are very low, which makes students doubt mutation could supply enough variation. Two things resolve this:
- Genomes are large and populations are large. A low rate per gene, multiplied across thousands of genes and millions of individuals, produces many mutations per generation in the population as a whole.
- Mutations accumulate. Each generation adds new ones to those already present, so the total pool of variation builds up over time.
Selective advantage and mutation
- A beneficial mutation spreads by the standard mechanism:
- Individuals carrying the new allele survive or reproduce better in the current environment.
- They leave more offspring, which inherit the allele.
- Over generations the allele's frequency in the population rises.
- The mutation itself has no advantage in the abstract. Its advantage exists only relative to a particular environment, which is why the standard insists on linking processes to conditions.
Worked Example
Worked Example
An insect pest is sprayed with a new insecticide.
- Before spraying began, a survey of 10,000 insects found 3 individuals carrying an allele that confers resistance.
- After five years of annual spraying, 89% of the population carries the resistance allele.
- Resistant insects reared without insecticide grow more slowly and lay fewer eggs than non-resistant ones.
- A population on an island where the insecticide was never used still shows resistance in only about 0.03% of individuals.
Explain how the resistance allele arose and spread, and explain what the island population shows.
Answer:
How the allele arose.
The resistance allele arose by random mutation — a chance change in the DNA base sequence of a gamete-producing cell in some ancestral insect. The crucial evidence is that 3 individuals already carried it before spraying began. The allele therefore existed before the insecticide was ever applied, so the insecticide cannot have caused it.
This is the key point: mutations are random with respect to need. The insecticide did not induce a useful mutation. It merely selected among variation that was already present by chance.
How the allele spread.
Once spraying began, the environment changed and so did the allele's effect:
- Insects without the allele were killed by the insecticide, so most left no offspring.
- Insects with the allele survived the spraying, so they were a large proportion of the survivors that bred.
- Their offspring inherited the resistance allele.
- Repeating this each year, the allele's frequency rose from 0.03% to 89% in five years.
This is directional selection: one extreme of the variation is consistently favoured, so the population's allele frequencies shift steadily in that direction. The change is rapid because the selection pressure is extreme — non-resistant individuals do not merely do worse, they die — and because insects have short generation times, so many generations of selection occur in five years.
Why the allele was rare beforehand, and what the island shows.
The data state that resistant insects reared without insecticide grow more slowly and lay fewer eggs. So the allele carries a fitness cost in the absence of the insecticide, probably because the resistance mechanism diverts resources or impairs a normal protein.
Without insecticide, therefore, selection acts against the allele, keeping it rare — which is exactly why only 3 in 10,000 carried it initially. The island population, never sprayed, has remained at 0.03% for the same reason: there, selection still opposes the allele.
What this demonstrates. The island is effectively a control. It shows that the allele's frequency rises only where the insecticide is applied, confirming that the insecticide is the selective agent rather than some other difference between years.
More fundamentally, it demonstrates that an allele has no fixed value. The same allele is disadvantageous on the island and overwhelmingly advantageous on the mainland. Its effect depends entirely on the environment, and "beneficial mutation" is meaningful only in relation to a specified set of conditions.