Difficulty: Intermediate | Prerequisites: Basic understanding of alleles, genotypes, allele frequencies, and the concept of natural selection (BIOL 180 Lectures 14-15 or equivalent).
This lecture sits at a turning point in the course. Up to now, the focus has been on natural selection as the driver of evolution. This material asks a broader question: what other forces change allele frequencies in populations? The answer introduces mutation and genetic drift, two mechanisms that operate quite differently from selection. You need a solid grasp of allele and genotype frequencies, and the Hardy-Weinberg model, before this will click. If those are shaky, revisit Lectures 14-15 first.
Mutation changes one allele into another, but the per-generation shift in allele frequency is tiny (on the order of 10⁻⁴ to 10⁻⁵). Selection is not the only mechanism of evolution: mutation and genetic drift also count. Mutation's main role is as the ultimate source of all new genetic variation, not as a fast mover of allele frequencies on its own.
Allele frequency
The proportion of a specific allele among all copies of that gene in a population. Written as p or q for a two-allele system. In simple terms, this is how common a particular version of a gene is in the group.
Hardy-Weinberg equilibrium
A mathematical model predicting that allele frequencies stay constant across generations when five conditions hold: no selection, no migration, no mutation, no genetic drift (infinite population size), and random mating. Think of it as the "null hypothesis" for evolution: if nothing is pushing allele frequencies around, they stay put.
Mutation (as an evolutionary mechanism)
A heritable change in DNA sequence that converts one allele into another. In population genetics, it refers specifically to the process that shifts allele frequencies by creating new copies of an allele each generation. Think of it as the raw material factory for evolution, not a fast engine.
Mutation rate
The probability that a given allele mutates to a different allele in one generation per gene copy. Typical rates are around 5 x 10⁻⁵ mutations per gene copy per generation. In simple terms, this is how often copying errors happen at a particular spot in the genome.
Mutation-selection balance
The equilibrium point at which mutation introduces new copies of a deleterious allele at the same rate that selection removes them. This explains why harmful alleles persist in populations at low, stable frequencies rather than disappearing entirely.
Genetic drift
Random changes in allele frequency from one generation to the next, caused by sampling error in finite populations. Covered in detail in the companion notes on genetic drift.
Mechanisms of evolution
The four processes that can change allele frequencies in a population: natural selection, mutation, migration (gene flow), and genetic drift (sampling error). Any change in allele frequency, regardless of cause, counts as evolution.
The Hardy-Weinberg equilibrium principle is the starting point. Under five assumptions (no selection, no migration, no mutation, no sampling error, and random mating), allele frequencies do not change across generations.
Genotype frequencies can be calculated directly from allele frequencies: Fr(AA) = p², Fr(Aa) = 2pq, Fr(aa) = q²
The model cycles from adults to gene pool to zygotes to babies and back to adults, with allele and genotype frequencies staying constant at every stage
When any of those five assumptions is violated, allele frequencies can change, and that change is evolution
The four mechanisms of evolution are selection, mutation, migration, and genetic drift. This lecture focuses on mutation and drift.
Mutation converts copies of one allele into another during meiosis. To calculate the effect:
Start with the current frequency of the allele
Subtract the fraction lost to mutation (frequency x mutation rate)
The result is the new allele frequency
Worked example from lecture: allele A₁ has frequency 0.9, A₂ has frequency 0.1. If 1 in 10,000 copies of A₁ mutate to A₂ during meiosis:
New freq(A₁) = old freq(A₁) - (old freq(A₁) x mutation rate)
New freq(A₁) = 0.9 - (0.9 x 10⁻⁴) = 0.89991
This is evolution (allele frequencies changed), but the change per generation is vanishingly small.
Mutation rates are typically around 5 x 10⁻⁵ mutations per gene copy per generation. At that pace, mutation alone takes a very long time to move allele frequencies appreciably. The key insight is that mutation is the ultimate source of all genetic variation. Without it, selection, drift, and migration would have nothing to work on.
Some alleles are harmful but never fully disappear from the population. Achondroplasia illustrates this:
Genotype AA is inviable (lethal), Aa produces a person of short stature (relative fitness 0.2), aa produces typical stature (relative fitness 1)
Selection removes copies of the A allele every generation, yet the allele persists
The reason is that mutation regenerates new copies of A each generation
The equilibrium frequency is set by the balance between the rate at which selection removes the allele and the rate at which mutation creates it. For achondroplasia, the mutation rate of about 5 x 10⁻⁵ produces roughly 1.24 affected individuals per 10,000 live births (calculated as 2 x fr(A₁) x fr(A₂) = 0.000124).
Hardy-Weinberg genotype frequencies (two-allele system)
Fr(AA) = p², Fr(Aa) = 2pq, Fr(aa) = q², where p = freq(A) and q = freq(a), and p + q = 1.
Allele frequency change due to mutation (one direction, A₁ to A₂)
New freq(A₁) = old freq(A₁) - (old freq(A₁) x mutation rate)
Carrier frequency at mutation-selection balance
2 x fr(A₁) x fr(A₂), where fr(A₁) is the equilibrium frequency of the mutant allele.
Achondroplasia is a real condition caused by a dominant mutation in the FGFR3 gene. It persists in the human population at a predictable frequency because of mutation-selection balance. The same principle applies to other genetic conditions where affected individuals have reduced fitness but the allele never fully disappears, such as haemophilia and some forms of muscular dystrophy.
Students often assume mutation is a fast mechanism of evolution. It is not. The per-generation change in allele frequency due to mutation alone is extremely small (on the order of 10⁻⁴ or less). Mutation matters because it is the only source of new alleles, not because it shifts frequencies quickly.
Students sometimes confuse "mutation" in everyday language (a sudden, dramatic change) with mutation in population genetics (a slow, incremental change in allele frequency each generation).
A common error is thinking that harmful alleles should eventually be eliminated from a population by selection. Mutation-selection balance explains why they persist at low frequencies.
Students occasionally forget that any change in allele frequency counts as evolution, even a tiny shift caused by mutation. If the frequency moved, evolution happened.
⚠️ You will likely be asked to calculate the new allele frequency after one generation of mutation. Know the formula and be comfortable plugging in numbers.
⚠️ Expect a question about whether a small change in allele frequency due to mutation counts as evolution. The answer is yes.
⚠️ Mutation-selection balance is a classic exam topic. Be prepared to explain why deleterious alleles persist in populations, using achondroplasia as the worked example.
⚠️ Know all four mechanisms of evolution (selection, mutation, migration, genetic drift) and be able to compare their relative speeds and roles.
True or false: Mutation is the fastest mechanism of evolution. (False. Mutation changes allele frequencies very slowly.)
Fill in the blank: The ultimate source of all genetic variation is ________. (Mutation.)
True or false: If allele frequencies change due to mutation, that counts as evolution. (True.)
Fill in the blank: The equilibrium at which mutation creates copies of a harmful allele at the same rate selection removes them is called ________. (Mutation-selection balance.)
True or false: Under Hardy-Weinberg conditions, allele frequencies change across generations. (False. They remain constant.)
Q: A population has allele A₁ at frequency 0.7 and allele A₂ at frequency 0.3. The mutation rate from A₁ to A₂ is 2 x 10⁻⁴ per generation. What is the frequency of A₁ after one generation of mutation only?
A: New freq(A₁) = 0.7 - (0.7 x 2 x 10⁻⁴) = 0.7 - 0.00014 = 0.69986.
Q: Achondroplasia is caused by a dominant allele A. Individuals with genotype AA are inviable, Aa individuals have reduced fitness (0.2), and aa individuals have typical fitness (1.0). Why does the A allele persist in the population?
A: Mutation-selection balance. Selection removes copies of A each generation because AA is lethal and Aa has low fitness, but mutation generates new copies of A at a rate sufficient to maintain a low equilibrium frequency.
Q: Name the four mechanisms of evolution.
A: Natural selection, mutation, migration (gene flow), and genetic drift (sampling error).
Q: A student says, "Mutation is the most important mechanism of evolution because without it, there would be no variation." Evaluate this claim.
A: The student is partly right: mutation is the ultimate source of all genetic variation, so without it, none of the other mechanisms would have raw material to work on. However, calling it the "most important" mechanism is misleading in terms of speed. Mutation changes allele frequencies extremely slowly. Selection and drift are typically the dominant forces driving allele frequency change within populations over shorter timescales.
Q: Is a change in allele frequency of 0.00009 in one generation due to mutation considered evolution?
A: Yes. Evolution is defined as any change in allele frequency in a population, regardless of the magnitude or cause.
This material connects directly to Hardy-Weinberg equilibrium (Lectures 14-15), which provides the null model that mutation violates. It also sets up the genetic drift material covered in the second half of this same lecture (see companion notes). Later in the course, mutation-selection balance will resurface when discussing the persistence of genetic diseases in human populations and the maintenance of genetic variation more broadly.
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