Directional selection

Definition

Directional selection is a mode of natural selection where an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift in one direction. This type of selection often leads to evolutionary changes by favoring traits that provide a competitive advantage in a specific environment, which can be reflected at the molecular level through changes in gene frequencies and associated traits.

5 Must Know Facts For Your Next Test

1. Directional selection can occur rapidly when environmental changes favor certain traits, leading to quick shifts in population genetics.
2. One classic example is the peppered moth, where dark-colored individuals became more common during the Industrial Revolution due to pollution darkening tree bark.
3. At the molecular level, directional selection can result in specific alleles becoming more prevalent, which can be tracked through genetic analysis.
4. This selection can drive speciation by promoting divergence among populations adapting to different environments.
5. Directional selection is often studied using models that predict how trait distributions will change over time based on selective pressures.

Review Questions

• How does directional selection differ from stabilizing and disruptive selection in terms of its effects on phenotypic variation within a population?
  • Directional selection promotes one extreme phenotype, leading to a shift in the population's average trait value and reducing variation towards that extreme. In contrast, stabilizing selection favors intermediate phenotypes and reduces overall variation by selecting against extremes. Disruptive selection increases variation by favoring both extreme phenotypes over intermediates. Understanding these differences helps in predicting how populations will evolve under different environmental pressures.
• Discuss the implications of directional selection for adaptive evolution and how it contributes to the molecular basis of evolution.
  • Directional selection plays a significant role in adaptive evolution by favoring specific traits that enhance survival and reproduction in changing environments. This selective pressure results in an increase in the frequency of advantageous alleles at the molecular level, shaping the genetic landscape of populations. Over time, these changes can lead to significant evolutionary transformations as populations adapt to their niches, demonstrating the interplay between natural selection and molecular genetics.
• Evaluate the role of directional selection in speciation events and its impact on genetic diversity within evolving populations.
  • Directional selection can contribute to speciation by driving populations apart as different traits become advantageous in varying environments. As certain alleles become favored in isolated populations, genetic divergence occurs, reducing gene flow between them. This process not only affects genetic diversity within those populations by potentially increasing fixation of specific alleles but also creates distinct species adapted to their respective niches. Understanding this dynamic is crucial for grasping how complex ecosystems evolve over time.

Related terms

Stabilizing selection: A form of natural selection that favors intermediate phenotypes over extreme ones, reducing variation and maintaining the status quo in a population.

Disruptive selection: A type of natural selection that favors extreme values of a trait over intermediate values, potentially leading to the emergence of two or more distinct phenotypes.

Adaptive evolution: The process through which populations become better suited to their environment through natural selection acting on heritable traits.