Directional, stabilizing and disruptive selection Knowledge-Based Study

Study notes

1. Patterns of Natural Selection

   Natural selection acts directly on phenotypes and indirectly on the underlying genotypes. Most observable physical characteristics, such as body size, pigmentation, and beak depth, are polygenic traits controlled by multiple genes. In an undisturbed population, continuous variation for these traits forms a normal distribution, commonly visualized as a symmetric bell-shaped curve.

   Depending on environmental conditions and selective pressures, natural selection alters this distribution in different ways. The three primary modes of selection acting on continuous variation are:

   • Stabilizing selection
   • Directional selection
   • Disruptive selection

   Each mode influences the population's mean phenotype and its overall phenotypic variance differently.

   Maintaining the Intermediate Phenotype

   Stabilizing selection occurs when selective pressures favor intermediate (average) phenotypes while acting against individuals with extreme phenotypes at both ends of the spectrum.

   Key Characteristics:

   • Effect on the mean: The population mean remains largely unchanged.
   • Effect on variation: Phenotypic variance decreases as extremes are eliminated, resulting in a narrower and taller distribution curve.
   • Environmental context: Commonly observed in stable, unchanging environments where a population is already well adapted.

   Classic Biological Example:

   A classic example of stabilizing selection is human birth weight. Infants born with very low birth weights face higher risks of hypothermia, infection, and underdeveloped organs. Conversely, infants with excessively high birth weights experience complications during delivery. Consequently, infants of intermediate birth weight show the highest survival rates, maintaining the average weight across generations.

   Shifting Toward an Extreme

   Directional selection occurs when environmental pressures systematically favor individuals exhibiting one extreme phenotype over intermediate individuals and individuals at the opposite extreme.

   Key Characteristics:

   • Effect on the mean: The population mean shifts toward the favored extreme phenotype over generations.
   • Effect on variation: Phenotypic variation may temporarily decrease or remain steady as allele frequencies shift toward the advantageous trait.
   • Environmental context: Typically triggered by sustained environmental change or when a population colonizes a new habitat with novel selective pressures.

   Classic Biological Example:

   A well-documented example is the change in beak depth among medium ground finches (Geospiza fortis) in the Galápagos Islands. During severe drought conditions, small, soft seeds become scarce, leaving predominantly large, hard seeds. Finches with larger, deeper beaks can crack these tough seeds and survive, shifting the average beak depth of the population toward larger sizes in subsequent generations.

   Favoring Both Extremes

   Disruptive selection (also known as diversifying selection) occurs when environmental conditions favor individuals at both extremes of the phenotypic range while selecting against intermediate phenotypes.

   Key Characteristics:

   • Effect on the mean: The original intermediate mean is selected against, often creating a bimodal (two-peaked) distribution.
   • Effect on variation: Overall phenotypic variation increases within the population.
   • Evolutionary significance: If accompanied by reproductive isolation or assortative mating, disruptive selection can serve as an evolutionary mechanism driving sympatric speciation.

   Classic Biological Example:

   An example occurs in the black-bellied seedcracker (Pyrenestes ostrinus). In their habitat, the two abundant food resources are either very hard seeds from sedges or very soft seeds from a different plant species. Birds with large beaks crack hard seeds efficiently, while birds with small beaks handle small, soft seeds adeptly. Birds with intermediate beak sizes are inefficient at both food types and experience lower fitness, causing a bimodal beak distribution.

   Comparative Overview of Selection Types

   Understanding how each mode of selection modifies population traits is essential for predicting evolutionary outcomes:

   • Stabilizing Selection: Favors intermediate forms; selects against both extremes; mean stays constant; variance decreases.
   • Directional Selection: Favors one extreme form; selects against the opposite extreme and average; mean shifts toward the favored extreme; variance often remains similar or narrows.
   • Disruptive Selection: Favors both extreme forms; selects against intermediate forms; distribution becomes bimodal; variance increases.

   By assessing how a population's phenotypic curve changes over successive generations, biologists can deduce the nature of the environmental pressures shaping that population.