How does the environmental state differ between stabilising and directional selection?

Stabilising selection typically occurs in a stable, unchanging environment where the current mean is already well-adapted. Directional selection occurs when the environment changes or a new selection pressure is introduced, favoring a different phenotype.

What is the difference in the effect on the population mean between these two types of selection?

In stabilising selection, the population mean remains constant as the extremes are removed. In directional selection, the mean shifts toward the extreme phenotype that provides the survival advantage.

Compare the impact on phenotypic variance in stabilising vs. directional selection.

Stabilising selection significantly reduces phenotypic variance by narrowing the distribution around the mean. Directional selection shifts the entire distribution, often maintaining similar variance but moving the range of values.

What is the common misconception regarding the origin of advantageous alleles during selection?

A common error is believing that selection pressures cause mutations to occur. In reality, mutations are random and occur independently of the environment; selection only increases the frequency of alleles that happen to be beneficial.

Why is it incorrect to say an individual 'evolves' during directional selection?

Evolution is a population-level process defined by changes in allele frequencies over generations. Individuals possess a fixed set of alleles and can only survive or die; they do not change their genetic makeup in response to selection.

What mistake do students often make when describing the 'survival of the fittest'?

Students often focus only on survival, but biological fitness requires both survival and successful reproduction. An organism that survives for a long time but fails to pass on its alleles has a fitness of zero.

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

Selection pressures are environmental factors (biotic or abiotic) that limit the survival and reproduction of certain phenotypes. Examples include predation, disease, competition for food, and extreme temperatures.

What is meant by 'Allele Frequency'?

Allele frequency refers to how common a specific version of a gene is within a population's gene pool. It is expressed as a fraction or percentage of the total number of gene loci for that trait.

Define 'Stabilising Selection'.

Stabilising selection is a type of natural selection that favors intermediate phenotypes and acts against extreme variations. It results in a population that is more uniform and less genetically diverse for that specific trait.

Define 'Directional Selection'.

Directional selection is a mode of natural selection where a single extreme phenotype is favored, causing the allele frequency to continuously shift in one direction. It is the primary driver of evolutionary change in response to environmental shifts.

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Directional & Stabilising Selection

Summary

Natural selection operates through distinct modes that determine how allele frequencies and phenotypic distributions change within a population over time. Stabilising selection preserves the status quo by favoring intermediate traits in stable environments, whereas directional selection drives evolutionary shifts by favoring extreme phenotypes in response to environmental changes.

1. Definition & Core Concepts

Natural Selection is the process by which individuals with characteristics better suited to their environment are more likely to survive and reproduce. This differential reproductive success leads to changes in the allele frequencies of a population over successive generations.
Selection Pressures are external factors, such as resource competition, predation, or climatic shifts, that influence which phenotypes provide a survival advantage. These pressures dictate the 'direction' or 'stability' of the evolutionary process within a gene pool.
The Phenotypic Distribution of a polygenic trait typically follows a normal distribution (bell curve). Selection acts on this distribution by either narrowing it around the mean or shifting the mean toward one of the extremes.

2. Stabilising Selection

Stabilising Selection occurs when environmental conditions remain constant over long periods, favoring the average or intermediate phenotypes. Individuals with extreme phenotypes at either end of the spectrum are at a selective disadvantage and are less likely to pass on their alleles.
This mode of selection results in the maintenance of the intermediate phenotype and a reduction in the phenotypic variance of the population. While the mean trait value remains the same, the 'bell curve' becomes narrower and taller as extremes are eliminated.
A classic biological example is found in mammalian birth weights, where very low and very high weights correlate with higher mortality rates. Consequently, selection pressures favor a medium birth weight that maximizes infant survival and maternal health.

Graph showing stabilising selection where the distribution narrows around the mean, selecting against extreme phenotypes.

3. Directional Selection

Graph showing directional selection where the entire distribution curve shifts toward one extreme phenotype.

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions