Directional selection

Definition

Directional selection is a type of natural selection that favors one extreme phenotype over other phenotypes, causing a shift in the population's genetic distribution. This process leads to a change in the traits of a population over time, often as a response to environmental changes or pressures. It is a key mechanism of evolution that can drive significant changes in species, influencing their adaptability and survival.

5 Must Know Facts For Your Next Test

1. Directional selection can lead to rapid evolutionary changes, particularly when environmental pressures are strong or sudden.
2. An example of directional selection is the increase in beak size among Galápagos finches during drought conditions when larger seeds became the primary food source.
3. This form of selection can result in the fixation of advantageous alleles within a population, decreasing genetic diversity over time.
4. Directional selection is often associated with adaptations to new environments or changes in resource availability, driving species evolution.
5. It can also result from anthropogenic influences, such as selective breeding in agriculture or habitat destruction leading to survival challenges for certain traits.

Review Questions

• How does directional selection differ from stabilizing and disruptive selection in terms of its impact on population traits?
  • Directional selection differs from stabilizing and disruptive selection by favoring one extreme phenotype, leading to a shift in traits toward that extreme over time. In contrast, stabilizing selection favors intermediate phenotypes and reduces variation, while disruptive selection promotes extremes and increases trait variance. Understanding these differences helps explain how populations adapt to changing environments and the resulting evolutionary patterns.
• Discuss a real-world example of directional selection and analyze how it influenced the traits of a specific population.
  • A classic example of directional selection is seen in the peppered moth during the Industrial Revolution. The prevalence of darker moths increased as pollution darkened tree bark, giving them a survival advantage against predation. This shift in color demonstrates how environmental changes can favor specific traits within a population, ultimately leading to evolutionary changes that enhance survival.
• Evaluate the long-term implications of directional selection on genetic diversity within populations and how this relates to their resilience to environmental changes.
  • Long-term directional selection can reduce genetic diversity within populations as certain alleles become fixed due to selective pressures favoring specific traits. While this can lead to short-term adaptability to particular conditions, it may ultimately compromise a population's resilience to future environmental changes. Reduced genetic diversity limits the range of responses available for adaptation, making populations more vulnerable to extinction if conditions change rapidly or unpredictably.

Related terms

natural selection:

The process by which organisms better adapted to their environment tend to survive and produce more offspring.

stabilizing selection: A type of natural selection that favors intermediate phenotypes, reducing variation in traits within a population.

disruptive selection: A form of natural selection that favors individuals at both extremes of the phenotypic range, leading to increased variance in traits.