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Natural Selection and Its Types Explained

May 5, 2025

Lecture Notes: Natural Selection and Types of Selection

Introduction to Natural Selection

  • Definition: A process where better-adapted organisms survive and reproduce, passing on advantageous traits.
  • Founder: Charles Darwin.
  • Natural selection as a driving force for evolution, acting on differences in traits.
  • Genetic Variability: Slight differences among individuals lead to varying reproductive success, influencing evolutionary processes.
  • Survival and reproduction lead to genetic traits being passed to offspring, while less adapted organisms may eventually cease to exist.
  • Major Driving Force: Natural selection acts on genetic combinations attempting reproduction.

Types of Natural Selection

1. Stabilizing Selection

  • Bell Curve Distribution: Most traits are average, with fewer extremes.
  • Function: Screens against trait outliers, reducing their reproduction compared to regular individuals.
  • Results in distinct species appearances where members look alike.
  • Example: Plant height—medium height is favored over short (less sunlight) or tall (wind damage) plants.

2. Directional Selection

  • Definition: One extreme of a trait is favored over the other.
  • Impact: Population trait distribution shifts towards one extreme.
  • Artificial Directional Selection: Used by humans to breed animals with specific traits, ignoring negative traits.
  • Example: Giraffe necks—long necks favored due to access to leaves, causing a shift in neck length distribution.

3. Diversifying (Disruptive) Selection

  • Definition: Favors multiple extremes of a trait, potentially leading to speciation.
  • Function: Selection pressures act against average traits, resulting in a bimodal distribution (two peaks).
  • Example: Plant pollination—if medium height pollinators disappear, the population favors short and tall plants.
  • Polymorphic Population: Multiple distinct forms or morphs exist.

Key Points Summary

  • Stabilizing Selection: Decreases genetic variance, favoring average phenotypes.
  • Directional Selection: Shifts genetic variance towards new phenotypes due to environmental changes.
  • Diversifying Selection: Increases genetic variance, selecting for multiple extreme phenotypes.
  • In disruptive selection, average phenotypes often less fit than extremes.