Directional Selection

Definition

Directional selection is a type of natural selection that favors one extreme phenotype over other phenotypes in a population, leading to a shift in the population's traits in a specific direction. This process can result from environmental changes or shifts in resource availability, causing a selective pressure that enhances the reproductive success of individuals with advantageous traits. As a result, these traits become more common in the population over time, illustrating the dynamic nature of evolution.

5 Must Know Facts For Your Next Test

1. Directional selection can lead to rapid evolutionary changes, particularly when environmental conditions change quickly.
2. This type of selection often occurs during events like mass extinctions or significant habitat changes, where certain traits become more advantageous for survival.
3. Examples of directional selection can be observed in peppered moths, where dark-colored moths became more common in polluted areas due to better camouflage.
4. Directional selection can also contribute to speciation when populations adapt to different environmental pressures over time.
5. In agriculture, directional selection is often used in selective breeding to enhance desirable traits in crops and livestock.

Review Questions

• How does directional selection differ from other forms of natural selection?
  • Directional selection specifically favors one extreme phenotype over others, leading to a shift in the trait distribution within a population. In contrast, other forms such as stabilizing selection favor intermediate phenotypes and disruptive selection favors both extremes while reducing intermediates. This distinction highlights how different environmental pressures can shape the evolutionary paths of populations.
• Discuss an example of directional selection observed in nature and its implications for the affected species.
  • A classic example of directional selection is seen in the case of the peppered moth. During the Industrial Revolution, soot from factories darkened trees, making dark-colored moths less visible to predators compared to lighter-colored moths. As a result, the frequency of dark-colored moths increased significantly, demonstrating how environmental changes can favor specific phenotypes and alter population dynamics.
• Evaluate how directional selection could influence genetic diversity within a population over time.
  • Directional selection tends to reduce genetic diversity by favoring specific traits and causing certain alleles to become more common while others diminish or disappear. This shift can lead to a homogenization of genetic traits within the population as advantageous alleles proliferate. However, if environmental conditions change again or if new selective pressures arise, this reduced diversity may make the population more vulnerable to extinction due to its limited ability to adapt to new challenges.

Related terms

Natural Selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring, thereby influencing the genetic makeup of future generations.

Phenotype: The observable physical or biochemical characteristics of an organism, determined by both genetic makeup and environmental influences.

Stabilizing Selection: A form of natural selection that favors intermediate phenotypes, reducing variation in a population and promoting traits that are advantageous for survival in stable environments.