Directional Selection occurs when environmental changes favor individuals at one extreme of a trait's range. This results in a shift of the population's mean phenotype toward that extreme over several generations.
Stabilizing Selection favors intermediate phenotypes and acts against extreme variations. This process reduces genetic diversity and maintains the status quo for a particular trait, often occurring in stable environments.
Disruptive Selection (or Diversifying Selection) favors individuals at both extremes of the phenotypic range over those with intermediate traits. This can lead to a bimodal distribution and is often a precursor to speciation.
| Selection Type | Phenotype Favored | Effect on Variation | Example Scenario |
|---|---|---|---|
| Directional | One extreme | Shifts mean | Increase in average body size |
| Stabilizing | Intermediate | Reduces variation | Maintaining constant birth weight |
| Disruptive | Both extremes | Increases variation | Small and large beaks favored over medium |
Natural Selection vs. Evolution: Natural selection is the mechanism, while evolution is the outcome. Evolution is the change in allele frequencies in a population over time; natural selection is one of the primary processes that causes that change.
Natural vs. Artificial Selection: In natural selection, the environment determines which traits are favorable. In artificial selection (selective breeding), humans consciously choose which individuals breed based on desired characteristics, often resulting in much faster phenotypic changes.
Acclimatization vs. Adaptation: Acclimatization is a reversible, physiological change in an individual (like tanning in the sun). Adaptation is a genetic change in a population that occurs over generations and is not reversible within a single lifetime.
Identify the Unit of Selection: Always remember that natural selection acts on individuals (their phenotypes), but only populations evolve. If an exam question asks if an individual 'evolved' a new trait to survive, the answer is almost always no.
Analyze the Graph: When presented with phenotypic distribution curves, look at the 'after' curve. If it has moved, it is directional; if it is narrower, it is stabilizing; if it has two peaks, it is disruptive.
Check for the Four Conditions: To prove natural selection is occurring in a scenario, you must verify there is variation, the trait is heritable, there is a struggle for resources, and there is differential reproductive success.
Avoid Teleological Language: Avoid saying a species evolved a trait 'in order to' or 'so that' it could survive. Evolution has no goal or foresight; it is a reactive process based on current environmental conditions.
Survival of the Fittest: This phrase is often misunderstood as 'survival of the strongest' or 'most aggressive.' In biological terms, fitness only refers to reproductive success; a small, timid organism that produces ten offspring is 'fitter' than a strong, dominant one that produces none.
The 'Need' Fallacy: Organisms do not develop traits because they 'need' them. Mutations occur randomly; if a mutation happens to be beneficial in a specific environment, natural selection will increase its frequency. If the necessary mutation never occurs, the population may simply go extinct.
Perfection: Natural selection does not produce 'perfect' organisms. It is limited by genetic constraints, historical baggage (vestigial structures), and trade-offs (e.g., a peacock's tail attracts mates but also predators).
