Directional Selection

Definition

Directional selection is a mode of natural selection that favors one extreme phenotype over others, leading to a shift in the population's traits in a specific direction. This type of selection can result from environmental changes or shifts in resource availability, pushing a population to adapt to new conditions. Over time, this can lead to significant evolutionary changes as the favored traits become more prevalent in the gene pool.

5 Must Know Facts For Your Next Test

1. Directional selection often occurs in response to environmental pressures such as climate change, predation, or competition for resources.
2. One classic example of directional selection is the peppered moth, where darker-colored moths became more common during the Industrial Revolution due to pollution darkening tree bark.
3. Directional selection can lead to rapid changes in allele frequencies within a population, sometimes observable within just a few generations.
4. This type of selection can also be influenced by human activities, such as selective breeding in agriculture and animal husbandry.
5. When directional selection is strong, it can result in traits becoming fixed within a population, where almost all individuals exhibit the selected phenotype.

Review Questions

• How does directional selection differ from stabilizing and disruptive selection in terms of its impact on population traits?
  • Directional selection specifically favors one extreme phenotype, causing a shift towards that trait within the population. In contrast, stabilizing selection favors average phenotypes and reduces variation, while disruptive selection promotes extreme phenotypes and can increase diversity by eliminating intermediates. Each type influences how traits are distributed across generations but does so in different ways based on environmental pressures.
• Discuss an example of directional selection in nature and the factors that contributed to this phenomenon.
  • A well-known example of directional selection is observed in the case of the peppered moth during the Industrial Revolution. As pollution darkened tree bark, darker moths had a survival advantage over lighter ones due to better camouflage from predators. This environmental change favored the darker phenotype, leading to an increase in their frequency within the population, demonstrating how shifts in habitat conditions can drive directional selection.
• Evaluate the implications of directional selection on biodiversity and ecosystem dynamics over time.
  • Directional selection can significantly impact biodiversity by favoring certain traits that may lead to the decline or extinction of less advantageous phenotypes. Over time, this can reduce genetic variation within populations and create more homogenous species. In ecosystem dynamics, as certain species adapt and dominate through directional selection, they may alter resource availability or interactions with other species, potentially reshaping community structures and influencing ecological balance.

Related terms

Stabilizing Selection: A type of natural selection that favors average phenotypes and reduces variation within a population.

Disruptive Selection: A form of selection that favors extreme values of a trait over intermediate values, potentially leading to speciation.

Adaptive Radiation: The process by which a single ancestral species rapidly diversifies into a wide variety of forms to adapt to different environments.