Natural selection occurs when individuals with certain inherited traits survive or reproduce more successfully than others in the same population. Over many generations, this can change how common different traits are. A useful way to picture this change is with a trait-distribution graph, where the x-axis shows a trait value and the y-axis shows how many individuals have that trait.
Directional, stabilizing, and disruptive selection describe three common patterns of change in these distributions.
Directional selection shifts the average trait value toward one extreme, stabilizing selection favors intermediate traits, and disruptive selection favors both extremes over the middle. These patterns depend on the environment, predators, mates, disease, competition, and other pressures. Real populations can experience more than one type over time, especially when conditions change.
Understanding these patterns helps explain antibiotic resistance, birth weight trends, beak size in birds, and the formation of new species.
Key Facts
- Natural selection requires variation, heritability, and differential survival or reproduction.
- Directional selection: one extreme phenotype has the highest fitness, so the population mean shifts.
- Stabilizing selection: intermediate phenotypes have the highest fitness, so variation decreases around the mean.
- Disruptive selection: both extreme phenotypes have higher fitness than intermediate phenotypes, so the distribution can split into two peaks.
- Fitness means reproductive success, not physical strength or speed alone.
- Change in allele frequency over generations is evolution: p + q = 1 for two alleles in a simple population model.
Vocabulary
- Natural selection
- A process in which inherited traits become more or less common because they affect survival or reproduction.
- Phenotype
- The observable traits of an organism, such as size, color, behavior, or enzyme function.
- Fitness
- The ability of an organism to survive and produce fertile offspring compared with others in the population.
- Trait distribution
- A graph showing how common different values of a trait are within a population.
- Allele frequency
- The proportion of a specific version of a gene in a population's gene pool.
Common Mistakes to Avoid
- Saying individuals evolve during their lifetime is wrong because evolution is a change in a population's inherited traits across generations.
- Assuming natural selection always makes organisms stronger is wrong because fitness depends on the environment and on reproductive success, not one universal measure of strength.
- Confusing stabilizing selection with no selection is wrong because stabilizing selection actively removes extreme phenotypes and reduces variation.
- Thinking disruptive selection always creates two new species immediately is wrong because it can split a trait distribution, but speciation requires additional processes such as reduced gene flow.
Practice Questions
- 1 A population of beetles has shell thickness values centered at 4 mm. After many generations, the average thickness is 6 mm because predators more easily crush thin shells. Which type of selection is occurring, and how does the distribution change?
- 2 Human birth weights often show lower survival at very low and very high weights, with the highest survival near 3.5 kg. Identify the type of selection and describe what happens to variation in birth weight over time.
- 3 In a bird population, small beaks are best for eating tiny seeds and large beaks are best for cracking hard seeds, but medium beaks are inefficient for both food sources. Explain which type of selection this represents and why the graph might develop two peaks.