7.4 Population Genetics
- Syllabus
- 2025
- Topic
- 7.4
- Level
- —
Learning objectives
7.4A—Explain how random occurrences affect the genetic makeup of a populationExplain how random occurrences affect the genetic makeup of a population.• Evolution is also driven by random occurrences.- i. Mutation is a random process that adds new genetic variation t o a population.- ii. Genetic drift is a change in allele frequencies attributable to a nons elective process occurring in small populations. iii.The bottleneck effect is a type of genetic drift that occur s when a population size is reduced to a small number of individuals for at least one generation. iv.The founder effect is a type of genetic drift that occur s when a population is separated from other members of the population. The frequency of genes and traits will shift based on the genes in this new founder population.- v. Migration can result in gene flow (the addition or remov al of alleles from a population).7.4B—Describe the role of random processes in the evolution of specific populationsDescribe the role of random processes in the evolution of specific populations.• Random processes can lead to changes in allele frequencies in a population.- i. Mutations result in genetic variation, which provides phenotypes on which natural selection acts.- ii. Genetic drift can allow a small population to diverge from other populations of the same species.- iii. Gene flow between two populations prevents them from diverging into separate species.7.4C—Describe the change in the genetic makeup of a population over timeDescribe the change in the genetic makeup of a population over time.• Changes in allele frequencies provide evidence for the occurrence of evolution in a population.
Random Processes That Change Population Genetics
Population genetic makeup can change through random or nonselective processes. These processes alter which alleles exist or how common they are without necessarily favoring a phenotype because it improves fitness.
| Process | What happens | Genetic effect |
|---|---|---|
| Mutation | A random DNA change occurs | Adds new genetic variation |
| Genetic drift | Chance events alter allele representation, especially in small populations | Allele frequencies change nonselectively |
| Bottleneck effect | Population size falls to very few individuals for at least one generation | Surviving alleles may not represent the original population |
| Founder effect | A small separated group establishes a population | Frequencies reflect the founders' alleles and traits |
| Migration and gene flow | Individuals or alleles move between populations | Alleles are added to or removed from a population |
In drift, bottlenecks, and founder events, which alleles become common can depend on who survives or establishes the population by chance. The outcome can therefore differ from what natural selection alone would predict.
Random does not mean that allele frequencies stay unchanged or that every outcome is equally likely. It means the change is not caused by consistent selection for a higher-fitness phenotype.
How Random Processes Shape Population Divergence
Random processes change allele frequencies and therefore influence whether populations remain genetically similar or diverge from one another.
| Process | Role in population evolution | Expected effect on divergence |
|---|---|---|
| Mutation | Creates new genetic variation and possible phenotypes | Supplies differences on which selection can later act |
| Genetic drift | Changes allele frequencies by chance, strongly in small populations | Can make a small population diverge from others of the same species |
| Gene flow | Transfers alleles between populations | Makes populations more genetically connected and counteracts divergence |
Two isolated small populations can experience different chance frequency shifts, so drift can increase their genetic difference. If migration resumes, transferred alleles make their gene pools more similar, which can prevent divergence into separate species.
Mutation creates variation but does not by itself select the useful variants. Gene flow can add variation within a receiving population while simultaneously reducing genetic differences between populations.
Allele-Frequency Change Is Evidence of Evolution
A population evolves when its allele frequencies change over time. An allele frequency is the proportion of all copies of a gene in the population represented by a particular allele.
Measure the same population at different generations → compare the frequency of the same allele → a reproducible difference shows that the population's genetic makeup changed → this is evidence that evolution occurred.
Mutation, selection, genetic drift, and gene flow can all contribute to frequency change. The defining evidence is the population-level change across time, regardless of which mechanism produced it.
A new mutation in one individual is a source of variation, but population evolution is demonstrated only when the frequency of alleles in the population changes across generations.