Explain how environmental factors can act as stabilising, disruptive and directional forces of natural selection

Sourav PanNovember 1, 2024

Answered

Environmental factors play a crucial role in shaping the dynamics of natural selection, acting as stabilizing, disruptive, and directional forces. Each type of selection influences populations differently based on the traits favored under specific environmental conditions.

Stabilizing Selection

Stabilizing selection occurs when environmental pressures favor the average or intermediate phenotypes, reducing variation within a population. This type of selection is common in stable environments where extreme traits may be disadvantageous.

• Mechanism: In stabilizing selection, individuals with traits that deviate significantly from the average are less likely to survive and reproduce. For instance, in a population of mice living in a forest, those with fur colors that closely match the ground are more likely to avoid predation. As a result, the population’s fur color distribution becomes narrower over generations, concentrating around the average color
• Examples:
  • Human Birth Weight: Infants with average birth weights have higher survival rates than those who are either too small or too large
  • Plant Height: Medium-height plants may be favored because they can compete effectively for sunlight without being too susceptible to wind damage

Disruptive Selection

Disruptive selection occurs when environmental factors favor extreme phenotypes at both ends of the trait spectrum while selecting against intermediate forms. This can lead to increased variation within a population.

• Mechanism: In this scenario, individuals with extreme traits may have advantages in specific niches or under certain conditions. For example, in a habitat with varied coloration (like light sand and dark grass), both light and dark mice may thrive while medium-colored mice are more easily spotted by predators
• Examples:
  • Gall-forming Insects: Insects that induce galls on plants may experience selection for both very large and very small galls, as intermediate-sized galls are more vulnerable to predation
  • Flower Color: Plants with extreme flower colors might attract specific pollinators better than those with intermediate colors, leading to greater reproductive success for these extremes

Directional Selection

Directional selection favors one extreme phenotype over others, causing a shift in the population’s trait distribution over time.

• Mechanism: This type of selection typically occurs in response to changes in environmental conditions or when populations migrate to new habitats where different traits confer advantages. For example, if a species faces increased predation pressure from larger predators, individuals with larger body sizes may be favored for their ability to escape or defend themselves
• Examples:
  • Peppered Moth: During the Industrial Revolution in England, darker moths became more common as they were better camouflaged against soot-darkened trees compared to their lighter counterparts
  • Size Evolution: In some species of bears, directional selection has favored larger body sizes over generations as climate changes have altered food availability and predation pressures