31.2
Types of Selection
Natural selection influences the frequencies of particular alleles and phenotypes within populations in several different ways. Primarily, natural sel…
Natural selection changes the frequencies of particular alleles and phenotypes within a population through directional, stabilizing, and disruptive selection.
In a typical population, most individuals have intermediate traits, while a few individuals have extreme traits. In directional selection, environmental or sexual pressures favor one extreme phenotype, causing the population mean to shift toward that trait.
Imagine a population of lizards living on an island with no competitors or predators.
In this population, females prefer bright, colorful, and fit males. This preference is called sexual selection.
As females mate more often with bright, colorful males, the frequency of this phenotype increases. Over time, the population shifts toward the bright phenotype. This is directional selection.
Now, consider a different situation. A predator arrives on the island, and bright, colorful males become easy prey.
At the same time, dull males survive but attract few females.
Because both extremes face disadvantages, males with intermediate color survive and reproduce more successfully. This is called stabilizing selection.
The last type of selection is disruptive selection, in which both extreme phenotypes are favored, and the intermediate phenotype is selected against.
In this lizard population, females prefer bright males, so these males mate more often.
At the same time, dull males that look similar to females avoid aggression from dominant males and gain mating opportunities without direct competition. Intermediate males do not have either advantage.
Because both bright and dull males reproduce successfully, their frequencies increase, while the frequency of intermediate males decrease.
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Q1: How does directional selection change a population's traits over time?
Directional selection occurs when environmental or sexual pressures favor one extreme phenotype, causing the population mean to shift toward that trait. For example, in lizard populations where females prefer bright, colorful males, mating success increases the frequency of this phenotype across generations. Over time, the population becomes dominated by individuals displaying the favored extreme trait.
Q2: What is stabilizing selection and when does it occur?
Stabilizing selection favors intermediate phenotypes while selecting against both extremes. This occurs when extreme traits are disadvantageous. In lizard populations, when predators target bright males and dull males attract few mates, intermediate-colored males survive and reproduce most successfully. Similarly, bird clutch sizes remain optimal because too few or too many eggs reduces reproductive success.
Q3: How does disruptive selection differ from other selection types?
Disruptive selection favors both extreme phenotypes while selecting against intermediate phenotypes. In African black-bellied seedcrackers, birds with small or large bills can crack different seed types, but intermediate-billed birds cannot eat either efficiently and are rarely seen. This creates two distinct morphs in the population as both extremes reproduce successfully.
Q4: Why do intermediate phenotypes have different fitness outcomes across selection types?
Intermediate phenotypes experience different selective pressures depending on environmental conditions. In stabilizing selection, intermediate traits are optimal and have high fitness. In disruptive selection, intermediate phenotypes lack specialized advantages and have low fitness. This variation demonstrates how the same trait can be beneficial or harmful depending on ecological context and what extremes offer.
Q5: What role does sexual selection play in directional selection?
Sexual selection occurs when mating preferences favor particular phenotypes, driving directional selection. In the lizard example, female preference for bright, colorful males increases the frequency of this phenotype through increased mating success. This demonstrates how mate choice can be as powerful as environmental pressures in shifting allele frequencies and phenotype distributions within populations.
Q6: How do allele frequencies change under different types of selection?
Each selection type alters allele frequencies differently. Directional selection increases frequency of alleles coding for the favored extreme phenotype. Stabilizing selection maintains intermediate allele combinations at high frequency. Disruptive selection increases frequencies of alleles producing both extremes while reducing those producing intermediate phenotypes. These changes accumulate across generations, reshaping population genetic structure.
Q7: Can you provide an example of disruptive selection in nature?
The African black-bellied seedcracker displays disruptive selection for beak size. Small-billed birds eat soft seeds while large-billed birds crack hard seeds, but intermediate-billed birds cannot efficiently eat either type. This single autosomal locus trait creates two distinct morphs, with intermediate phenotypes selected against because they lack the specialized feeding advantages of either extreme.