How does the effect on the population mean differ between directional and stabilising selection?

In directional selection, the mean shifts toward one extreme phenotype. In stabilising selection, the mean remains constant as the intermediate phenotype is favored.

When comparing population graphs, what visual cue indicates stabilising selection has occurred?

The bell curve becomes narrower and taller, indicating that individuals at the extremes have been eliminated and the population is more uniform around the mean.

Under what environmental conditions is directional selection most likely to occur?

It occurs when there is a change in environmental conditions or when a population migrates to a new habitat with different selection pressures.

Why is it incorrect to say that a selection pressure 'causes' a beneficial mutation to appear?

Mutations occur randomly due to errors in DNA replication. Selection pressures only determine which of those pre-existing or randomly occurring mutations are advantageous for survival.

What is a common mistake when describing the outcome of stabilising selection on genetic diversity?

Students often think it maintains diversity, but it actually reduces it by selecting against extreme alleles and narrowing the range of phenotypes.

Does directional selection always increase the complexity of an organism?

No, it simply favors the trait that increases reproductive success in the current environment, which could involve the loss of a complex trait if it is no longer beneficial.

Define 'Selection Pressure' in the context of natural selection.

An external factor that influences the reproductive success of individuals in a population based on their phenotypic traits.

What is meant by 'Allele Frequency'?

The proportion of a specific allele among all allele copies at a given locus in a population, usually expressed as a fraction or percentage.

What is an 'Intermediate Phenotype'?

A trait value that falls in the middle of the range of variation, often representing the average or mean characteristic of the population.

How does directional selection lead to the emergence of antibiotic resistance in bacteria?

The application of antibiotics acts as a selection pressure that kills susceptible bacteria, allowing the rare individuals with resistance alleles to survive and multiply, shifting the population mean toward resistance.

Directional & Stabilising Selection | AQA AS-Level Biology

Genetics, Variation & Interdependence

Directional & Stabilising Selection

Summary

Natural selection acts on the phenotypic variation within a population to alter allele frequencies over time. Directional selection shifts the population mean toward one extreme phenotype in response to environmental change, while stabilising selection preserves the intermediate phenotype by selecting against extremes in a stable environment.

1. Definition & Core Concepts

Natural Selection is the process by which individuals with advantageous phenotypes are more likely to survive and reproduce, passing their alleles to the next generation.
Selection Pressures are external factors (such as predation, disease, or climate) that influence which individuals in a population are most likely to survive.
Directional Selection occurs when environmental conditions change, favoring individuals at one extreme of the phenotypic range.
Stabilising Selection occurs in stable environments, favoring the average or intermediate individuals and acting against both phenotypic extremes.

Comparison of Directional and Stabilising Selection graphs. The directional graph shows the curve shifting to the right. The stabilising graph shows the curve becoming narrower and taller around the mean.

2. Underlying Principles

Phenotypic Variation: Populations naturally exhibit a range of traits (often following a normal distribution) due to genetic diversity and environmental factors.
Differential Reproductive Success: Individuals with traits better suited to the current selection pressures contribute more alleles to the gene pool of the next generation.
Allele Frequency Shift: Over time, the proportion of advantageous alleles increases, leading to a change in the observable characteristics of the population.
Normal Distribution: In both types of selection, the population is often modeled as a bell curve where the x-axis represents the trait value and the y-axis represents the number of individuals.

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions

Mutation vs. Selection: A common error is stating that selection causes mutations. Mutations are random; selection only acts on the variation that already exists or arises by chance.
Purposeful Evolution: Avoid language suggesting organisms 'evolve to survive'. Evolution is a passive result of differential survival, not a conscious or directed effort by the species.
Stabilising vs. No Selection: Students often think a stable population has no selection. In reality, stabilising selection is actively 'weeding out' extremes to keep the population adapted to its stable niche.