Directional selection

Definition

Directional selection is a mode of natural selection in which an extreme phenotype is favored over other phenotypes, leading to a shift in the population's traits in one direction. This type of selection often occurs in response to environmental changes or specific pressures, such as predation or competition, that make certain traits more advantageous for survival and reproduction. Over time, directional selection can significantly alter the genetic makeup of a population.

5 Must Know Facts For Your Next Test

1. Directional selection can lead to rapid evolutionary changes in a population as it favors individuals with specific advantageous traits.
2. This type of selection often occurs in environments that are changing, such as when new predators are introduced or when climate shifts happen.
3. An example of directional selection is the increase in size of the beaks of finches during droughts, where larger beaks allowed for better access to food resources.
4. Directional selection can result in the loss of genetic variation within a population as it drives the frequency of certain alleles higher while decreasing others.
5. This process can contribute to speciation if populations become so different due to directional selection that they can no longer interbreed.

Review Questions

• How does directional selection differ from other modes of natural selection?
  • Directional selection differs from stabilizing and disruptive selection primarily in its focus on favoring one extreme phenotype over others. In stabilizing selection, the intermediate phenotype is favored, reducing variation, while disruptive selection favors both extremes at the cost of intermediate traits. Directional selection results in a shift toward one end of the spectrum, which can lead to significant changes in the population's characteristics over time.
• Discuss how environmental changes can trigger directional selection and provide an example.
  • Environmental changes often create new challenges for organisms, prompting directional selection to favor those with traits better suited for survival. For example, during a drought that affects seed availability on islands, finches with larger beaks may thrive because they can crack open tougher seeds. This selective pressure drives the population's average beak size upward over generations as smaller-beaked individuals fail to compete effectively for food.
• Evaluate the long-term effects of directional selection on genetic diversity within a population.
  • The long-term effects of directional selection on genetic diversity can be quite significant. As certain phenotypes become favored and proliferate within a population, the genetic variation associated with less advantageous traits tends to diminish. Over time, this reduction in genetic diversity can make populations more vulnerable to environmental changes and diseases since there may be fewer adaptive traits left for future survival. Consequently, while directional selection can lead to rapid adaptation to specific pressures, it also raises concerns about the resilience and sustainability of populations in changing environments.

Related terms

Natural Selection: The process by which individuals with favorable traits are more likely to survive and reproduce, leading to the gradual evolution of species.

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

Genetic Drift: A mechanism of evolution that refers to random changes in allele frequencies within a population, which can lead to significant changes over time, especially in small populations.