Types of Natural Selection

Why This Matters

Natural selection is the engine driving evolution, and understanding its different modes is essential for explaining how populations change over time. On the AP Biology exam, you're being tested on your ability to recognize how selective pressures shape trait distributions, why certain phenotypes increase or decrease in frequency, and what environmental conditions favor each type of selection. These concepts connect directly to population genetics, Hardy-Weinberg equilibrium, and speciation—all major exam topics.

Don't just memorize the names of selection types—know what each one does to a population's phenotype distribution and when you'd expect to see it in nature. FRQs often ask you to interpret graphs showing trait distributions before and after selection, or to predict which selection type would operate in a given scenario. Master the underlying mechanisms, and you'll be ready for anything the exam throws at you.

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Selection That Shifts the Mean

When environmental conditions change or new pressures emerge, populations often respond by shifting toward one phenotypic extreme. Directional selection moves the population's trait distribution in one direction, favoring individuals at one end of the phenotypic spectrum.

Directional Selection

• Favors one extreme phenotype—the trait distribution shifts toward that extreme as individuals with advantageous traits survive and reproduce more successfully
• Triggered by environmental change or new selective pressures, such as pollution, climate shifts, or introduction of new predators
• Classic example: peppered moths—during the Industrial Revolution, darker moths gained a survival advantage on soot-covered trees, shifting the population toward the melanic phenotype

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Selection That Maintains the Average

In stable environments, being "average" often pays off. Stabilizing selection reduces phenotypic variation by selecting against extreme traits, keeping the population clustered around an optimal intermediate value.

Stabilizing Selection

• Favors intermediate phenotypes—individuals with extreme traits have lower fitness, narrowing the trait distribution over time
• Common in stable environments where well-established traits are already optimized for survival and reproduction
• Human birth weight is the textbook example—infants with intermediate weights have the highest survival rates, while very low or very high birth weights correlate with increased mortality

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Selection That Creates Diversity

Sometimes the environment rewards being different. Disruptive selection favors phenotypes at both extremes while selecting against intermediate forms, potentially splitting a population into distinct groups.

Disruptive Selection

• Favors both extreme phenotypes—the trait distribution becomes bimodal as intermediate individuals are selected against
• Occurs in heterogeneous environments where different niches or resources favor different trait combinations
• African seedcracker birds demonstrate this—birds with very large beaks crack hard seeds efficiently, birds with very small beaks handle soft seeds well, but medium-beaked birds struggle with both

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Selection Driven by Mate Choice

Not all selection is about survival—reproductive success matters just as much. Sexual selection operates when certain traits increase an individual's ability to attract mates, even if those traits don't improve survival.

Sexual Selection

• Traits that attract mates are favored—individuals with preferred characteristics reproduce more, passing those traits to offspring
• Leads to secondary sexual characteristics like elaborate plumage, courtship displays, or exaggerated features that signal fitness to potential mates
• Often produces sexual dimorphism—peacocks' extravagant tails evolved because peahens preferentially mate with males displaying larger, more colorful feathers

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Selection That Preserves Variation

Some selection mechanisms actively maintain multiple alleles in a population rather than driving one to fixation. Balancing selection encompasses several processes that preserve genetic diversity, giving populations flexibility to adapt to changing conditions.

Balancing Selection

• Maintains multiple alleles in the population through mechanisms like heterozygote advantage or frequency-dependent selection
• Heterozygote advantage occurs when individuals with two different alleles have higher fitness than either homozygote
• Sickle cell trait exemplifies this—heterozygotes gain malaria resistance without the severe anemia that affects homozygotes for the sickle cell allele

Frequency-Dependent Selection

• Fitness depends on phenotype frequency—how common or rare a trait is determines its selective advantage
• Negative frequency-dependent selection favors rare phenotypes, preventing any single type from dominating the population
• Predator-prey dynamics illustrate this—predators form search images for common prey types, giving rare color morphs a survival advantage

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Quick Reference Table

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Self-Check Questions

1. A population of rabbits lives in an environment where medium-brown fur provides the best camouflage. Over several generations, the population shows less variation in fur color. Which type of selection is operating, and what would the trait distribution graph look like?

2. Compare and contrast disruptive selection and stabilizing selection in terms of their effects on phenotypic variation and the shape of the trait distribution curve.

3. A scientist observes that heterozygous individuals for a particular gene have higher survival rates than either homozygote. Which type of selection is this, and what long-term effect would you predict on allele frequencies?

4. Why might sexual selection lead to traits that actually decrease an organism's survival? Use a specific example to support your answer.

5. An FRQ describes a prey species where rare color morphs have higher survival rates than common ones. Identify the selection type, explain the mechanism, and predict what would happen to allele frequencies if one morph became very common.