Evolution Through Natural Selection
Directional Selection occurs when environmental changes favor one extreme phenotype over the average or the opposite extreme. This results in a shift of the population's trait distribution in a specific direction.
Stabilizing Selection acts against extreme phenotypes and favors intermediate variants. This process reduces variation and maintains the status quo for a particular trait, often seen in environments that remain constant over long periods.
Disruptive Selection occurs when environmental conditions favor individuals at both extremes of a phenotypic range over individuals with intermediate phenotypes. This can lead to a bimodal distribution and is a potential precursor to speciation.
| Feature | Natural Selection | Artificial Selection |
|---|---|---|
| Driving Force | Environmental pressures (predation, climate) | Human choice and intervention |
| Goal/Purpose | None; it is a reactive process | Specific desired traits (e.g., crop yield) |
| Speed | Generally slow (geological timescales) | Can be very rapid (few generations) |
| Outcome | Increased fitness in the wild | Traits beneficial to humans, often detrimental in wild |
Identify the Selection Pressure: When analyzing a scenario, always ask: 'What environmental factor is making it harder for certain individuals to survive or reproduce?' This is the key to determining the direction of evolution.
Focus on Populations: Avoid language that suggests individuals 'try' to adapt or 'evolve' to survive. Always frame your answer in terms of population-level changes in allele frequency over generations.
Check for Heritability: If a question describes a trait that is not genetically determined (e.g., a learned behavior not coded in DNA), natural selection cannot act on it to produce evolutionary change.
Verify Reproductive Success: Survival is only half the battle; if an organism survives for 100 years but never reproduces, its 'fitness' is zero in evolutionary terms.
The 'Goal-Oriented' Fallacy: Evolution does not have a goal or a 'ladder of progress.' It does not produce 'perfect' organisms, only organisms that are 'good enough' to survive and reproduce in their current environment.
Survival of the Fittest vs. Strongest: 'Fitness' is often misinterpreted as physical prowess. In many environments, being small, quiet, or inconspicuous provides higher fitness than being large and aggressive.
Acquired Characteristics: A common mistake is believing that traits developed during a lifetime (like a giraffe stretching its neck) are passed to offspring. Only changes in the germline DNA contribute to natural selection.