Lecture Notes on Selection and Evolutionary Traits

Introduction to Selection

• Selection refers to the influence that traits or genotypes have on reproduction.
  • Can increase or decrease probability of reproduction.
  • Alleles controlling traits may increase or decrease in frequency in the population.

Quantitative Traits

• Traits measured on a continuum (e.g., height, milk production, pigment amount).
• X-axis: Trait measurement.
• Y-axis: Frequency of that trait in the population.

Types of Selection

1. Directional Selection

• Definition: Selection towards one extreme.
• Example 1: Bristle number in Drosophila.
  • Artificially selecting for more bristles.
  • Over 90 generations, increase in bristle number.
  • Natural selection eventually reduces excessive bristles.
• Example 2: Bill size in ground finches (Galapagos Islands).
  • Larger bills favored during drought due to changed seed availability.

2. Stabilizing Selection

• Definition: Selection for mid-range values.
• Characteristics:
  • Extremes are selected against.
  • Curve narrows around the best fit value.
• Example: Human birth weight.
  • Low mortality for mid-range weights (5-9 pounds).
  • Higher mortality for very large or very small babies.

3. Diversifying (Disruptive) Selection

• Definition: Selection against mid-range values, favoring extremes.
• Example: Mandible width in black-bellied seedcrackers.
  • Adaptation for either small or large seeds.
  • Mid-range bills are less efficient, selected against.

Summary

• Different types of selection impact the distribution of traits in a population.
• Directional selection shifts population traits towards an extreme.
• Stabilizing selection favors average traits, reducing variation.
• Diversifying selection encourages extremes, increasing diversity.