Stabilizing Selection

Definition

Stabilizing selection is a type of natural selection that favors the average or intermediate phenotypes in a population, reducing variation and maintaining the status quo. This process occurs when extreme traits are selected against, leading to a decrease in the frequency of those traits over time. It highlights how certain traits can be advantageous for survival and reproduction, making them more common within a stable environment.

5 Must Know Facts For Your Next Test

1. Stabilizing selection often occurs in stable environments where extreme traits may be disadvantageous, such as in birth weight in humans, where both low and high weights have higher mortality rates.
2. This form of selection helps maintain genetic diversity at a lower level by reducing the occurrence of extreme phenotypes while preserving the average traits that contribute to fitness.
3. Stabilizing selection can also be observed in plant populations where intermediate flower sizes are preferred by pollinators, ensuring successful reproduction.
4. In contrast to directional selection, which shifts the frequency of traits toward one extreme, stabilizing selection acts to keep traits within a narrow range.
5. Stabilizing selection contributes to evolutionary stability by ensuring that well-adapted phenotypes remain prevalent within a population across generations.

Review Questions

• How does stabilizing selection differ from directional selection in terms of its effects on phenotypic variation within a population?
  • Stabilizing selection focuses on preserving the average or intermediate phenotypes within a population by reducing the occurrence of extreme traits, while directional selection shifts the trait distribution toward one extreme. For example, in stabilizing selection, traits that are too light or too heavy might be selected against, leading to a concentration of average-sized individuals. This means stabilizing selection maintains genetic diversity at a lower level compared to directional selection, which promotes change and adaptation in one direction.
• Discuss an example of stabilizing selection in human populations and explain its implications for health and reproduction.
  • An example of stabilizing selection in humans can be seen in birth weights. Infants with very low or very high birth weights tend to have higher mortality rates compared to those with average weights. This has led to the predominance of babies born at average weights, as these individuals are more likely to survive and thrive. The implications for health are significant, as maintaining this range helps ensure better outcomes for both infants and mothers during childbirth, contributing to overall reproductive success.
• Evaluate the role of stabilizing selection in shaping evolutionary outcomes and its relationship with genetic variation over time.
  • Stabilizing selection plays a critical role in shaping evolutionary outcomes by promoting the maintenance of successful average traits while limiting the impact of extreme variations. Over time, this can lead to reduced genetic variation within populations since only certain phenotypes persist. However, this stability may also create opportunities for adaptation if environmental conditions change. In such cases, genetic variation becomes essential for future evolutionary responses, as populations with greater diversity are better equipped to adapt to new challenges presented by changes in their environment.

Related terms

Phenotype: The observable physical or behavioral characteristics of an organism, determined by its genotype and environmental influences.

Natural Selection: The process by which organisms better adapted to their environment tend to survive and produce more offspring, influencing the evolution of species.

Genetic Variation: The diversity in gene frequencies within a population, which is crucial for natural selection to occur and can lead to adaptations over time.