Types of Natural Selection

Why This Matters

Natural selection is the core mechanism driving evolution, and understanding its different modes is essential for explaining how populations change over time. On the AP Biology exam, you need 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.

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.

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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's peak shifts toward that extreme as individuals with the advantageous trait survive and reproduce more successfully.
• Triggered by environmental change or new selective pressures, such as pollution, climate shifts, or the introduction of new predators.
• Classic example: peppered moths. Before the Industrial Revolution, light-colored moths were camouflaged against pale, lichen-covered tree bark. As soot darkened the trees, darker (melanic) moths had better camouflage and survived at higher rates. The population shifted from mostly light to mostly dark. After pollution controls cleaned the air, the trend reversed.

On a graph, you'll see the bell curve slide left or right, with the peak moving toward whichever extreme is favored.

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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 at either extreme have lower fitness, so the trait distribution narrows over time.
• Common in stable environments where existing traits are already well-suited for survival and reproduction.
• Human birth weight is the textbook example. Infants with intermediate weights (around 3.0–4.0 kg) historically had the highest survival rates. Very low birth weights are associated with developmental vulnerability, while very high birth weights increase the risk of delivery complications.

On a graph, the bell curve gets taller and narrower, but the mean stays in the same place.

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Selection That Creates Diversity

Sometimes the environment rewards being different from the average. 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 (two peaks) as intermediate individuals are selected against.
• Occurs in heterogeneous environments where different niches or resources favor different trait values.
• African seedcracker finches demonstrate this. Birds with very large beaks crack hard seeds efficiently. Birds with very small beaks handle soft seeds well. But medium-beaked birds can't do either task effectively, so they have lower fitness.

On a graph, the single bell curve splits into two humps with a valley in the middle.

Disruptive selection is the type most closely linked to speciation. If the two extreme groups stop interbreeding (through geographic separation, behavioral differences, or other isolating mechanisms), they can eventually diverge into separate species.

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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 or compete for mating opportunities, even if those traits don't improve (or actively harm) survival.

Sexual Selection

• Traits that increase mating success are favored, even at a cost to survival. Individuals with preferred characteristics reproduce more and pass those traits to offspring.
• Leads to secondary sexual characteristics like elaborate plumage, courtship displays, large antlers, or bright coloration that signal genetic quality to potential mates.
• Often produces sexual dimorphism, meaning males and females of the same species look noticeably different. Peacocks' extravagant tails evolved because peahens preferentially mate with males displaying larger, more colorful feathers.

Sexual selection operates through two main mechanisms:

1. Intersexual selection (mate choice): One sex (often females) chooses mates based on specific traits. The peacock tail is a classic example.
2. Intrasexual selection (competition): Members of one sex (often males) compete directly with each other for access to mates. Think of elk locking antlers during mating season.

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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, preventing any single allele from being eliminated or reaching 100% frequency.
• Heterozygote advantage is the most commonly tested form. It occurs when individuals heterozygous at a locus (carrying two different alleles) have higher fitness than either homozygote.
• Sickle cell trait is the go-to example. Individuals homozygous for the sickle cell allele ($HbS/HbS$) develop sickle cell disease. Individuals homozygous for the normal allele ($HbA/HbA$) have no protection against malaria. But heterozygotes ($HbA/HbS$) get the best of both worlds: they don't develop severe sickle cell disease, and their red blood cells are inhospitable to the malaria parasite. This is why the sickle cell allele persists at high frequencies in regions where malaria is endemic, like sub-Saharan Africa and parts of Southeast Asia.

Frequency-Dependent Selection

• Fitness depends on how common or rare a phenotype is in the population.
• Negative frequency-dependent selection favors rare phenotypes. As a phenotype becomes more common, its fitness decreases; as it becomes rarer, its fitness increases. This creates a self-correcting cycle that prevents any single type from dominating.
• Predator-prey dynamics illustrate this well. Predators tend to form "search images" for the most common prey appearance. Rare color morphs get overlooked, giving them a survival advantage. As the rare morph increases in frequency, predators start noticing it more, and its advantage fades.

Another example: scale-eating cichlid fish in Lake Tanganyika attack prey from either the left or right side, depending on which way their mouths are oriented. Whichever mouth type is rarer has more success because prey are less vigilant against the uncommon attack direction.

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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.