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.