Types of Selection

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.

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.

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.

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.

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.

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.

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.