Stabilising Selection: This occurs in stable environments where the average phenotype is favored. Selection pressures act against extreme phenotypes, reducing variation but keeping the mean trait value constant over time.
Directional Selection: This occurs when environmental conditions change, favoring one extreme phenotype over the mean. This results in the entire population's mean trait value shifting toward that extreme over generations.
Selection Identification: To determine the type of selection, compare the population's trait distribution before and after the selection pressure. A narrowing of the curve suggests stabilising selection, while a horizontal shift suggests directional selection.
| Type of Adaptation | Description | Example Concept |
|---|---|---|
| Anatomical | Structural or physical features of an organism's body. | Specialized beak shapes for specific food sources. |
| Physiological | Internal biological processes or chemical pathways. | Production of concentrated urine to conserve water. |
| Behavioural | The way an organism acts to enhance survival. | Seasonal migration to avoid harsh climates. |
Natural Selection vs. Evolution: Natural selection is the mechanism or process, whereas evolution is the outcome—the cumulative change in the population over time.
Fitness vs. Survival: Survival is necessary for fitness, but fitness specifically requires reproduction. An organism that lives a long time but never reproduces has a biological fitness of zero.
The Standard Sequence: When answering 'how' questions, always follow this logical chain: Mutation Variation Selection Pressure Survival/Reproduction Allele Inheritance Increased Allele Frequency.
Identify the Pressure: Always explicitly state what the selection pressure is in a given scenario (e.g., 'The presence of a specific toxin acts as a selection pressure').
Use Precise Terminology: Use the term alleles rather than 'genes' when discussing variation, and phenotype when discussing the physical expression of those alleles.
Check the Graph: If provided with a distribution curve, look for whether the peak is moving (Directional) or getting taller/narrower (Stabilising) to identify the selection type.
Intentionality: Avoid saying organisms 'adapt to' their environment as if it is a choice. Adaptation is a passive result of individuals with certain traits surviving better than others.
Environment-Induced Mutation: A common error is suggesting the environment causes a specific mutation to happen. Mutations are random; the environment simply determines if a pre-existing mutation is beneficial.
Individual Evolution: Individuals do not evolve; they either survive and reproduce or they do not. Evolution is a process that occurs at the population level over time.
