Educerie · AP · Biology
AP Biology 7.4 Population Genetics
In one breath
- Evolution means a change in allele frequencies over generations; count gene copies, two per diploid individual.
- Selection changes frequencies in a direction set by the environment, but random processes change them too.
- Mutation is random and is the only source of new alleles, giving selection something to act on.
- Genetic drift is chance change, strong in small populations; it can fix or lose an allele and lets a small population diverge from the rest of its species.
- A bottleneck is drift after a population is cut down where it lives; a founder effect is drift in a new population started by a few individuals.
- Gene flow moves alleles between populations and keeps them alike; stop it and they can diverge.
- The null hypothesis is that allele frequencies do not change.
What you must be able to do
- Explain how random events (mutation, genetic drift, the bottleneck and founder effects, and migration causing gene flow) change the genetic make-up of a population"Identify the process that explains the change and justify your answer" (2 points); MCQ on which population is most affected by drift
- Describe the part random processes play in the evolution of particular populations: mutation supplies variation, drift lets small populations diverge, gene flow keeps populations alike"Predict the effect of building a corridor between the two populations" (1–2 points)
- Describe how a population's genetic make-up changes over time, using changing allele frequencies as evidence of evolutionA table of allele frequencies over several generations; "State the null hypothesis" and "Describe the evidence that the population is evolving"
Once you have read the note, tick off each one you can do in Check your understanding.
In the exam. Section I multiple choice: scenarios (a storm, a few colonists, a new road, migrants arriving) where you name the process and predict its effect on allele frequencies, and tables of allele frequencies across generations or between populations to interpret. The skill most tested is predicting results or stating a null hypothesis. In Section II these ideas sit in Conceptual analysis (Q4), Scientific investigation (Q3: "state the null hypothesis"), and Analyze data (Q6) whenever two populations' allele frequencies are compared.
1Allele frequency: the measure of evolution
Count copies of the gene, not individuals. In a diploid organism, each individual carries two copies of each autosomal gene.
An invented population of 50 snails, with a shell-colour gene that has alleles B and b:
| Genotype | BB | Bb | bb |
|---|---|---|---|
| Number of snails | 18 | 24 | 8 |
If, five generations later, the frequency of B is 0.75, the population has evolved, whatever the reason. A change in allele frequencies is the evidence that evolution has happened; topic 7.5 turns that into a test.
Evolution is a change in allele frequencies in a population over generations, and selection is only one of the things that can cause it.
2Mutation: new alleles, at random
A mutation is a change in the DNA sequence. Mutations arise at random with respect to need: a bacterium does not mutate towards resistance because an antibiotic is present. Most mutations are neutral or harmful; a few happen to help in a particular environment.
Mutation is the only source of genuinely new alleles. Meiosis and fertilisation shuffle existing alleles into new combinations, but only mutation makes an allele that did not exist before. That is why the CED pairs it with selection: mutations produce the genetic variation, and so the phenotypes, that natural selection then acts on.
3Genetic drift: chance in small populations
Genetic drift is a change in allele frequencies caused by chance, not by any difference in fitness. Which individuals happen to die in a flood, which happen to find a mate, which of a parent's two alleles happens to end up in the one egg that is fertilised: all of these are random, and they change allele frequencies a little each generation.
In a large population these chance effects mostly cancel out. In a small one they do not.
Figure 1 shows a computer model: eight populations, each starting with allele A at a frequency of 0.5, with no selection, no mutation and no migration. Each generation, the alleles of the next generation are drawn at random from the current one. In the four populations of 10 individuals (panel a), the frequency swings widely; in one, A reached 1 and every other allele was lost (A is fixed), and in another A was lost within five generations. In the four populations of 5,000 (panel b), the frequency stays close to 0.5 for all 40 generations.
Three things to take from it.
- Drift is strongest in small populations.
- Drift is random in direction. Nothing about allele A made it rise in one population and vanish in another.
- Drift reduces genetic variation. Once an allele is lost, drift cannot bring it back; only mutation or migration can.
4Two special cases: bottlenecks and founders
The bottleneck effect. When a population is cut to a small number of individuals for at least one generation, by a disaster, a disease or hunting, the survivors carry only a random sample of the original gene pool. Some alleles are lost by chance, and others become much more common than before, regardless of their usefulness. Even if the population later grows large again, the lost variation does not return.
A real case: northern elephant seals were hunted until only a small remnant population was left, probably no more than a few tens to a hundred animals, by the 1890s. Protected, they have recovered to well over a hundred thousand animals, but they have far less genetic variation than related seal species that were never reduced so far.
The founder effect. When a few individuals leave a population and start a new one somewhere else, the new population's gene pool is set by the alleles those founders happened to carry. Allele frequencies in the new population can be very different from the parent population's.
A real case: among the Old Order Amish of Lancaster County, Pennsylvania, descended from a small group of settlers, a rare recessive condition called Ellis–van Creveld syndrome (short stature and extra fingers) is far more common than in the population at large, because one or more founders happened to carry the allele.
The difference between the two: a bottleneck shrinks a population where it is; a founder event starts a new population from a few individuals that have left.
5Gene flow: migration moves alleles
Gene flow is the transfer of alleles from one population to another, when individuals migrate and breed, or when gametes such as pollen are carried between populations. Immigrants can bring alleles the population lacked, and emigrants take alleles away.
Gene flow has a clear effect on two populations: it makes them more alike. Alleles that become common in one population by drift or selection are carried into the other. That is why the CED says gene flow prevents populations from diverging into separate species. Cut the gene flow, by a new river channel, a road or a stretch of farmland, and the populations can begin to drift and be selected apart. Topic 7.10 picks this up as the first step of speciation.
6Putting the processes together
| Process | Random with respect to fitness? | Strongest when | Effect on variation within the population | Effect on differences between populations |
|---|---|---|---|---|
| Natural selection | no | strong selective pressure | often reduces it | increases them if environments differ |
| Mutation | yes | always present, slow | adds new alleles | adds small differences |
| Genetic drift (bottleneck, founder) | yes | population is small | reduces it | increases them |
| Gene flow | yes, as to which alleles move | much migration | can add alleles | reduces them |
7Predicting results and stating a null hypothesis
Science practice 3.B asks for a null hypothesis: the prediction that there is no effect, no difference or no change. In population genetics the standard null hypothesis is:
"Allele frequencies in the population do not change from one generation to the next" (the population is not evolving).
A table that shows them changing, by more than chance in sampling would explain, rejects that null hypothesis. Then the question is which process did it, and the scenario usually tells you.
- A sudden, catastrophic loss of most individuals, with the survivors random with respect to the trait: bottleneck.
- A few individuals colonising a new place: founder effect.
- A small population with no obvious selective pressure, frequencies wandering up and down: genetic drift.
- Individuals arriving from elsewhere, and the population becoming more like the source: gene flow.
- A change in a consistent direction, linked to an environmental pressure that favours one phenotype: natural selection.
Practice questions
7 questions on this note, from easy to hard. Each one you get right lights a star of this topic's constellation.