Directional selection

Definition

Directional selection is a mode of natural selection where one extreme phenotype is favored over others, leading to a shift in the population's traits over time. This process drives evolutionary change by promoting the prevalence of beneficial traits that enhance survival and reproduction in a specific environment. Understanding this concept is crucial for grasping how populations adapt to changing conditions, interact with their environments, and deviate from genetic equilibrium.

5 Must Know Facts For Your Next Test

1. Directional selection can lead to rapid evolutionary change as advantageous traits become more common in the population.
2. An example of directional selection is the increase in size of the peppered moth during the Industrial Revolution when darker-colored moths were favored due to pollution.
3. This type of selection can occur in response to environmental changes, such as climate shifts or new predators, influencing which traits are beneficial.
4. Directional selection contrasts with stabilizing and disruptive selection, which focus on maintaining existing traits or favoring multiple extremes, respectively.
5. It often results in a reduction of genetic diversity as certain alleles become fixed in the population due to consistent selection pressure.

Review Questions

• How does directional selection contribute to evolutionary change in populations?
  • Directional selection contributes to evolutionary change by consistently favoring one extreme phenotype over others, which leads to an increase in that particular trait within the population. This process can cause shifts in the overall distribution of traits, making them more common as they provide advantages in survival and reproduction. Over generations, this can result in significant changes to the genetic makeup of a population, ultimately leading to adaptation to changing environments.
• Compare and contrast directional selection with stabilizing and disruptive selection in terms of their effects on phenotypic variation within a population.
  • Directional selection favors one extreme phenotype, leading to an increase in that trait and potentially reducing overall phenotypic variation as specific traits become more pronounced. In contrast, stabilizing selection favors intermediate phenotypes and reduces extremes, maintaining phenotypic diversity. Disruptive selection, on the other hand, favors multiple extreme phenotypes while selecting against intermediates, increasing variation. Together, these modes of selection highlight different pathways through which populations can evolve based on environmental pressures.
• Evaluate how directional selection might interact with other factors like genetic drift or gene flow to affect a population's evolution.
  • Directional selection can interact with genetic drift and gene flow in complex ways that shape a population's evolution. For example, if a population experiences strong directional selection while also undergoing genetic drift due to small size, beneficial alleles may be lost entirely from the gene pool. Alternatively, gene flow from neighboring populations could introduce new alleles that counteract or enhance the effects of directional selection. The interplay between these mechanisms can either accelerate adaptation or hinder it, showcasing the dynamic nature of evolutionary processes.

Related terms

Natural selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring, influencing the 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 involves random changes in allele frequencies within a population, which can lead to significant changes over time, especially in small populations.