How can directional, stabilizing, and disruptive selection be identified in populations?
How can directional, stabilizing, and disruptive selection be identified in populations?
Answered
Identifying the types of natural selection—directional, stabilizing, and disruptive (or diversifying)—in populations involves observing specific patterns in phenotypic traits and allele frequencies over time. Each type of selection influences the genetic structure of a population differently. Here’s how each can be recognized:
Directional Selection
Definition: Directional selection occurs when one extreme phenotype is favored over others, causing a shift in allele frequencies toward that phenotype.
Identification:
- Phenotypic Shift: Look for a consistent change in the population’s traits over generations. For example, if a population of birds shows an increase in beak size due to changes in available food sources, this indicates directional selection favoring larger beaks.
- Environmental Changes: This type of selection often correlates with environmental changes. For instance, the classic case of the peppered moth demonstrates how industrial pollution favored darker moths as they blended better with soot-covered trees.
- Allele Frequency Changes: Genetic analysis can reveal shifts in allele frequencies that align with the favored phenotype. Over time, the alleles associated with the advantageous trait will increase in frequency.
Stabilizing Selection
Definition: Stabilizing selection favors intermediate phenotypes over extremes, reducing variation within a population.
Identification:
- Reduced Variation: Observe a decrease in phenotypic variance around an optimal trait value. For example, in human birth weights, very low or very high weights are less common than those around an average weight.
- Fitness of Intermediate Traits: Measure survival and reproductive success; individuals with intermediate traits should show higher fitness compared to those with extreme traits. In plants, for instance, flowers that are neither too large nor too small may attract more pollinators.
- Genetic Diversity: Genetic studies may reveal decreased genetic variance as alleles associated with extreme phenotypes are selected against.
Disruptive (Diversifying) Selection
Definition: Disruptive selection favors extreme phenotypes at both ends of the spectrum while selecting against intermediate phenotypes.
Identification:
- Increased Variation: Look for an increase in genetic variance as two or more distinct phenotypes become more common. For example, in a population of rabbits, both very light and very dark fur colors may be favored in different environments (e.g., snowy vs. dark soil).
- Fitness of Extreme Traits: Assess survival and reproductive success; individuals with extreme traits should outperform those with intermediate traits. In African seedcracker birds, individuals with either very large or very small beaks may be more successful at exploiting different food sources.
- Bimodal Distribution: Phenotypic distributions may show two peaks corresponding to the favored extremes, indicating that intermediate forms are less fit.
BNO Team
