Difficulty: Intermediate | Prerequisites: Biostatistics basics, understanding of alleles and genotypes, Mendelian genetics
This is the core of BIOL 180: how populations evolve over time. Population genetics gives you the maths (allele frequencies, mutation rates, genotype frequencies), while the four mechanisms of evolution (natural selection, genetic drift, mutation, and gene flow) explain why those numbers change. On top of that, you need to know how biologists define species (biological species concept, morphospecies concept) and how sexual selection produces differences between males and females. Everything here builds on the phylogenetics and statistics material and prepares you for the rest of the course.
Evolution happens when allele frequencies in a population change over generations. Natural selection, genetic drift, mutation, and gene flow are the four mechanisms that cause this. Natural selection favours traits that increase reproductive success; drift is random change that hits small populations hardest. Species can be defined by whether they interbreed (biological species concept) or by physical differences (morphospecies concept). Sexual selection explains why males and females of the same species can look very different.
Allele frequency
The proportion of a specific allele among all copies of that gene in a population. Calculated as: (number of copies of that allele) / (total number of allele copies). In simple terms, it is how common a particular version of a gene is in the population.
Genotype frequency
The proportion of individuals in a population with a particular genotype (e.g., SS, SN, NN).
Mutation (as a mechanism of evolution)
A change in the DNA sequence that creates new alleles. As an evolutionary force, mutation slowly shifts allele frequencies by converting one allele to another at a measurable rate. The formula for allele frequency after one generation of mutation is: f(next) = f(current) - f(current) x mutation rate.
Natural selection
The process by which individuals with heritable traits that improve survival or reproduction leave more offspring, causing those traits to become more common. It is the only mechanism of evolution that consistently produces adaptation.
Genetic drift
Random changes in allele frequencies from one generation to the next, caused by chance events in who survives and reproduces. Its effects are strongest in small populations. Think of it as sampling error in reproduction.
Gene flow
The movement of alleles between populations, usually through migration and interbreeding. It tends to make populations more similar to each other.
Hardy-Weinberg equilibrium
A mathematical model predicting that allele and genotype frequencies will remain constant from generation to generation in the absence of evolution (no selection, drift, mutation, gene flow, or non-random mating). Serves as a null model for detecting evolution.
Fitness
The relative reproductive success of an individual or genotype. Higher fitness means more offspring that survive to reproduce.
Adaptation / adaptive trait
A heritable trait that increases an organism's fitness in its environment. A trait is adaptive if individuals with it leave more surviving offspring than those without.
Biological species concept (BSC)
Defines a species as a group of organisms that can interbreed and produce fertile offspring, and are reproductively isolated from other such groups.
Morphospecies concept
Defines a species based on measurable physical (morphological) differences from other groups.
Sexual selection
A form of natural selection in which individuals with certain traits have an advantage in obtaining mates. It can operate through mate choice (intersexual selection) or competition between members of one sex (intrasexual selection).
Sexual dimorphism
Physical differences between males and females of the same species, often driven by sexual selection (e.g., brighter colours in males, larger body size in one sex).
Artificial selection
Selective breeding by humans (or researchers) for specific traits. It demonstrates that selection can cause rapid evolutionary change when heritable variation exists.
Mutation slowly converts one allele into another. If allele f has frequency 0.92 and the mutation rate from f to F is 5 x 10⁻⁴, the frequency of f in the next generation is:
f(next) = f(current) - f(current) x mutation rate
f(next) = 0.92 - (0.92 x 0.0005) = 0.92 - 0.00046 = 0.91954
Mutation alone causes very small changes per generation. It introduces variation but does not drive rapid evolutionary change on its own.
When different genotypes have different fitness (survival, mating success, or both), selection changes genotype and allele frequencies.
Worked example (steelhead trout): genotype frequencies are 0.81 SS, 0.18 SN, 0.01 NN. If all SS breed, half of SN breed, and no NN breed:
Survivors who breed: 0.81 SS + 0.09 SN + 0 NN = 0.90 total
New genotype frequencies among breeders: SS = 0.81/0.90 = 0.9, SN = 0.09/0.90 = 0.1, NN = 0/0.90 = 0
Allele frequencies among breeders: freq(S) = 0.9 + 0.5(0.1) = 0.95, freq(N) = 0 + 0.5(0.1) = 0.05
Next generation genotype frequencies (assuming Hardy-Weinberg after mating): SS = 0.95² = 0.9025, SN = 2(0.95)(0.05) = 0.095, NN = 0.05² = 0.0025
The mechanism at work here is natural selection, because individuals with certain genotypes have higher fitness (better ability to navigate home to breed).
Natural selection: Differential survival and reproduction based on heritable traits. Requires heritable variation and differential fitness. The only mechanism that produces adaptation (a good fit between organism and environment).
Genetic drift: Random fluctuations in allele frequency, especially strong in small populations. Can cause alleles to be lost or fixed by chance. Explains short-term ups and downs in traits even when selection is operating.
Mutation: Creates new alleles. Very slow on its own, but essential because it is the ultimate source of all genetic variation.
Gene flow: Migration and interbreeding between populations. Can introduce new alleles or shift frequencies. Tends to homogenise populations.
If a trait increases fitness and its frequency rises over generations, natural selection is the explanation.
If allele frequencies fluctuate randomly from generation to generation (particularly in small populations), genetic drift is likely.
If running distance increases in "selection lines" but stays flat in "control lines" (random breeding), the difference is caused by artificial selection, which mimics natural selection.
Short-term fluctuations within selection lines (running goes up and down even while the overall trend is upward) can be explained by genetic drift or by environmental variation between generations.
A plateau in a selection experiment (the trait stops changing after many generations) suggests that genetic variation for the trait has been exhausted. Selection needs heritable variation to work.
When a cross between two populations produces F₂ offspring with a continuous, bell-curve distribution of a trait (like skull length), the trait is polygenic, controlled by many genes.
A continuous distribution rules out a single-gene trait (which would produce discrete size classes in the F₂).
If the size difference persists when both populations are reared in identical conditions, the difference has a genetic basis (not purely environmental).
Biological species concept (BSC): Are they reproductively isolated? If two populations can interbreed and produce fertile offspring, they are the same species under the BSC. Gough Island mice and mainland mice can interbreed, so they are not separate species under this definition.
Morphospecies concept: Are they physically distinguishable? If Gough Island mice are measurably larger even when reared in common conditions, they could be considered a distinct species under this concept.
Additional evidence that could help: molecular phylogenetics (how genetically distinct are they?), mate-choice experiments, checking for reduced hybrid fitness in F₁ or F₂ offspring.
A common student error is Lamarckian reasoning: "Birds lost flight because they didn't use their wings." Evolution does not work by use and disuse. Individuals do not change their genes based on need.
Another error is teleological thinking: "selective pressure worked on them to cause their wings to become flightless." Selection does not have intent. It does not work toward a goal.
A better explanation: In populations of birds living on islands with no predators and abundant ground-level food, individuals that allocated fewer resources to flight muscles and wing maintenance may have had higher fitness (more energy for reproduction). Over many generations, natural selection favoured reduced wings. Heritable variation in wing size already existed in the population; selection acted on that existing variation.
Bronikowski's mouse experiment: four lines selected for high running, four control lines (random breeding). Over generations, running distance increased in the selection lines and stayed flat in controls.
This demonstrates that running behaviour has a heritable genetic basis (if it did not, selection could not change it).
Fluctuations within lines from generation to generation are expected from drift or environmental variation.
A plateau after ~20 generations suggests the available genetic variation for running has been used up.
If selection is relaxed (breeders chosen at random from all 8 lines after generation 27), the selection lines would be expected to drift back toward the control level over time. Without selection maintaining high running, random mating would erode the gains.
In dark-eyed juncos, males are showier (darker) than females. The graph shows that males with more mates produce more offspring (a positive relationship between mating success and reproductive success).
For females, reproductive success is less dependent on mating success because females invest heavily in each offspring. The predicted line for females would be relatively flat: whether a female mates once or twice, she produces a similar number of offspring.
Males benefit from multiple mates, so traits that attract additional mates (showier colours) increase male fitness. Females do not benefit in the same way, so selection does not favour showiness in females.
To test whether dark colours are adaptive for males, you could experimentally alter male coloration (e.g., dye some males darker and others lighter) and then compare their mating success and offspring number.
Male juncos are less showy than peacocks because showiness carries costs (predation, energy). The equilibrium between sexual selection favouring showiness and natural selection penalising it differs between species. In juncos, the costs of extreme ornamentation outweigh the mating benefits.
Students often use Lamarckian reasoning: "The birds stopped flying, so their wings shrank." Organisms do not evolve because they need to. Evolution acts on variation that already exists in the population.
Students sometimes say natural selection "causes" mutations or "creates" new traits. Selection only acts on variation that mutation and recombination have already produced.
Students confuse genetic drift with natural selection. Drift is random; selection is directional and tied to fitness. Both change allele frequencies, but only selection produces adaptation.
Students forget that natural selection requires heritable variation. If a trait difference is entirely environmental, selection cannot change it across generations.
⚠️ Expect a calculation question: allele frequency change due to mutation, or genotype frequencies after selection. Show your working.
⚠️ Be ready to identify which mechanism of evolution is operating in a described scenario and explain your reasoning.
⚠️ Know the difference between the biological species concept and the morphospecies concept. Be able to apply each to a given example.
⚠️ Watch for Lamarckian reasoning in your own answers. If you write "the organism evolved because it needed to," you have made an error.
⚠️ Sexual selection questions will ask you to explain dimorphism. Connect the answer to differential mating success between the sexes.
True or false: Mutation is the fastest mechanism of evolution.
False. Mutation changes allele frequencies very slowly. Natural selection and genetic drift typically change them faster.
Fill in the blank: The only mechanism of evolution that produces adaptation is ___.
Natural selection
True or false: Under the biological species concept, two populations that can interbreed and produce fertile offspring are the same species.
True.
Fill in the blank: When selection is relaxed in an artificial selection experiment, the selected trait is expected to ___ over subsequent generations.
Drift back toward the ancestral (control) level
True or false: A continuous, bell-curve distribution of a trait in the F₂ generation suggests the trait is controlled by a single gene.
False. A continuous distribution indicates a polygenic trait (many genes).
Q: The frequency of allele f is 0.92. The mutation rate from f to F is 5 x 10⁻⁴. What is the frequency of f in the next generation?
A: f(next) = 0.92 - (0.92 x 0.0005) = 0.92 - 0.00046 = 0.91954.
Q: In a steelhead population, genotype frequencies are 0.81 SS, 0.18 SN, and 0.01 NN. All SS, half of SN, and no NN breed. What are the genotype frequencies among the next generation?
A: Breeders: 0.81 SS + 0.09 SN = 0.90 total. Adjusted frequencies: SS = 0.9, SN = 0.1, NN = 0. Allele freq: S = 0.95, N = 0.05. Next generation (Hardy-Weinberg): SS = 0.9025, SN = 0.095, NN = 0.0025.
Q: What mechanism of evolution is at work in the steelhead population described above? Explain.
A: Natural selection. Individuals with certain genotypes (SS) have higher fitness because they are better at navigating home to breed. The S allele increases in frequency because it confers a survival and reproductive advantage.
Q: Gough Island mice are much larger than mainland mice, and the difference persists in common-garden conditions. Name two different mechanisms of evolution that could explain the large size.
A: Natural selection (larger size may confer higher fitness on Gough Island, e.g., for competing for food or surviving conditions) and genetic drift (the founding population on Gough Island may have been small, and large-size alleles may have become common by chance).
Q: Are Gough Island mice a distinct species under the biological species concept? Under the morphospecies concept?
A: Under the BSC, no, because they can interbreed with mainland mice and produce offspring. Under the morphospecies concept, a case could be made for yes, because they are measurably and consistently larger, even when reared in the same environment.
Q: A student writes that flightless birds lost flight because they did not need to fly. Identify the error and give a better explanation.
A: The error is Lamarckian reasoning: evolution does not respond to an organism's needs. A better explanation is that in populations where flight was not necessary (e.g., islands without predators), individuals with smaller wings may have had higher fitness because they spent less energy on maintaining flight muscles. Natural selection favoured reduced wings over many generations, acting on heritable variation already present in the population.
Q: In Bronikowski's mouse experiment, why did the selection lines plateau after about 20 generations?
A: The most likely explanation is that the genetic variation for running behaviour was depleted. After many generations of selection, most individuals in the population carried the high-running alleles, and there was little remaining variation for selection to act on.
Q: In dark-eyed juncos, why did males evolve showier colours than females?
A: Males with more mates produce more offspring (a steep positive relationship between mating success and reproductive success). Showier colours help males attract more mates. Females do not gain the same fitness benefit from additional mates because their offspring number is limited by egg production and parental care, not by mate number. So sexual selection drives showiness in males but not females.
Q: How would you test whether darker coloration is adaptive for male juncos?
A: Experimentally alter male coloration (e.g., use dye to make some males darker and others lighter than normal) and measure the number of mates each group obtains and the number of offspring produced. If darker males gain more mates and produce more offspring, the trait is adaptive.
Population genetics underpins everything in evolutionary biology. The allele-frequency calculations here connect to Hardy-Weinberg equilibrium problems you will see later. The mechanisms of evolution (selection, drift, mutation, gene flow) explain the patterns you read off phylogenies. Species concepts tie into speciation, which is how new branches form on those trees. Sexual selection links to ecology and behaviour.
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