Types of Natural Selection and Their Effects on Allele Frequency
1. Directional Selection: Shifting Traits in One Direction
Definition
Directional selection
Favors one extreme phenotype, shifting the population in that direction.
- Directional selection increases the frequency of alleles associated with the favored trait.
- It occurs when environmental pressures or changes favor individuals at one end of the phenotypic range, such as size, speed, or coloration.
- On Gough Island, house mice evolved larger body sizes to survive harsh conditions.
- These larger mice, twice the size of mainland populations, were better adapted to compete for food and avoid predation.
2. Stabilizing Selection: Reducing Variation
Definition
Stabilizing selection
Favors intermediate phenotypes, reducing variation.
- In stabilizing selection, alleles for extreme traits decrease in frequency, while intermediate alleles dominate.
- This maintains genetic stability within a population and is common in stable environments where extremes are disadvantageous.
- In humans, babies with average birth weights have higher survival rates.
- Babies who are very small often cannot maintain body temperature or resist infections, while very large babies face complications during birth.
3. Disruptive Selection
Definition
Disruptive selection
Favors extreme phenotypes at both ends, increasing variation.
- Here, alleles for intermediate traits decrease, while alleles for both extremes increase in frequency.
- This can lead to population divergence, potentially resulting in the formation of new species over time.
- In coho salmon, small males (sneakers) avoid detection by sneaking into spawning grounds, while large males (fighters) use their size to dominate other males and access mates.
- Medium-sized males, however, lack the advantages of either strategy and are less successful in reproduction.
